Ошибка null object reference

Причина

Вкратце

Вы пытаетесь воспользоваться чем-то, что равно null (или Nothing в VB.NET). Это означает, что либо вы присвоили это значение, либо вы ничего не присваивали.

Как и любое другое значение, null может передаваться от объекта к объекту, от метода к методу. Если нечто равно null в методе «А», вполне может быть, что метод «В» передал это значение в метод «А».

Остальная часть статьи описывает происходящее в деталях и перечисляет распространённые ошибки, которые могут привести к исключению NullReferenceException.

Более подробно

Если среда выполнения выбрасывает исключение NullReferenceException, то это всегда означает одно: вы пытаетесь воспользоваться ссылкой. И эта ссылка не инициализирована (или была инициализирована, но уже не инициализирована).

Это означает, что ссылка равна null, а вы не сможете вызвать методы через ссылку, равную null. В простейшем случае:

string foo = null;
foo.ToUpper();

Этот код выбросит исключение NullReferenceException на второй строке, потому что вы не можете вызвать метод ToUpper() у ссылки на string, равной null.

Отладка

Как определить источник ошибки? Кроме изучения, собственно, исключения, которое будет выброшено именно там, где оно произошло, вы можете воспользоваться общими рекомендациями по отладке в Visual Studio: поставьте точки останова в ключевых точках, изучите значения переменных, либо расположив курсор мыши над переменной, либо открыв панели для отладки: Watch, Locals, Autos.

Если вы хотите определить место, где значение ссылки устанавливается или не устанавливается, нажмите правой кнопкой на её имени и выберите «Find All References». Затем вы можете поставить точки останова на каждой найденной строке и запустить приложение в режиме отладки. Каждый раз, когда отладчик остановится на точке останова, вы можете удостовериться, что значение верное.

Следя за ходом выполнения программы, вы придёте к месту, где значение ссылки не должно быть null, и определите, почему не присвоено верное значение.

Примеры

Несколько общих примеров, в которых возникает исключение.

Цепочка

ref1.ref2.ref3.member

Если ref1, ref2 или ref3 равно null, вы получите NullReferenceException. Для решения проблемы и определения, что именно равно null, вы можете переписать выражение более простым способом:

var r1 = ref1;
var r2 = r1.ref2;
var r3 = r2.ref3;
r3.member

Например, в цепочке HttpContext.Current.User.Identity.Name, значение может отсутствовать и у HttpContext.Current, и у User, и у Identity.

Неявно

public class Person {
    public int Age { get; set; }
}
public class Book {
    public Person Author { get; set; }
}
public class Example {
    public void Foo() {
        Book b1 = new Book();
        int authorAge = b1.Author.Age; // Свойство Author не было инициализировано
                                       // нет Person, у которого можно вычислить Age.
    }
}

То же верно для вложенных инициализаторов:

Book b1 = new Book { Author = { Age = 45 } };

Несмотря на использование ключевого слова new, создаётся только экземпляр класса Book, но экземпляр Person не создаётся, поэтому свойство Author остаётся null.

Массив

int[] numbers = null;
int n = numbers[0]; // numbers = null. Нет массива, чтобы получить элемент по индексу

Элементы массива

Person[] people = new Person[5];
people[0].Age = 20; // people[0] = null. Массив создаётся, но не
                    // инициализируется. Нет Person, у которого можно задать Age.

Массив массивов

long[][] array = new long[1][];
array[0][0] = 3; // = null, потому что инициализировано только первое измерение.
                 // Сначала выполните array[0] = new long[2].

Collection/List/Dictionary

Dictionary<string, int> agesForNames = null;
int age = agesForNames["Bob"]; // agesForNames = null.
                               // Экземпляр словаря не создан.

LINQ

public class Person {
    public string Name { get; set; }
}
var people = new List<Person>();
people.Add(null);
var names = from p in people select p.Name;
string firstName = names.First(); // Исключение бросается здесь, хотя создаётся
                                  // строкой выше. p = null, потому что
                                  // первый добавленный элемент = null.

События

public class Demo
{
    public event EventHandler StateChanged;

    protected virtual void OnStateChanged(EventArgs e)
    {        
        StateChanged(this, e); // Здесь бросится исключение, если на
                               // событие StateChanged никто не подписался
    }
}

Неудачное именование переменных

Если бы в коде ниже у локальных переменных и полей были разные имена, вы бы обнаружили, что поле не было инициализировано:

public class Form1 {
    private Customer customer;

    private void Form1_Load(object sender, EventArgs e) {
        Customer customer = new Customer();
        customer.Name = "John";
    }

    private void Button_Click(object sender, EventArgs e) {
        MessageBox.Show(customer.Name);
    }
}

Можно избежать проблемы, если использовать префикс для полей:

private Customer _customer;

Цикл жизни страницы ASP.NET

public partial class Issues_Edit : System.Web.UI.Page
{
    protected TestIssue myIssue;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            // Выполняется только на первой загрузке, но не когда нажата кнопка
            myIssue = new TestIssue(); 
        }
    }
    
    protected void SaveButton_Click(object sender, EventArgs e)
    {
        myIssue.Entry = "NullReferenceException здесь!";
    }
}

Сессии ASP.NET

// Если сессионная переменная "FirstName" ещё не была задана,
// то эта строка бросит NullReferenceException.
string firstName = Session["FirstName"].ToString();

Пустые вью-модели ASP.NET MVC

Если вы возвращаете пустую модель (или свойство модели) в контроллере, то вью бросит исключение при попытке доступа к ней:

// Controller
public class Restaurant:Controller
{
    public ActionResult Search()
    {
         return View();  // Модель не задана.
    }
}

// Razor view 
@foreach (var restaurantSearch in Model.RestaurantSearch)  // Исключение.
{
}

Способы избежать

Явно проверять на null, пропускать код

Если вы ожидаете, что ссылка в некоторых случаях будет равна null, вы можете явно проверить на это значение перед доступом к членам экземпляра:

void PrintName(Person p) {
    if (p != null) {
        Console.WriteLine(p.Name);
    }
}

Явно проверять на null, использовать значение по умолчанию

Методы могут возвращать null, например, если не найден требуемый экземпляр. В этом случае вы можете вернуть значение по умолчанию:

string GetCategory(Book b) {
    if (b == null)
        return "Unknown";
    return b.Category;
}

Явно проверять на null, выбрасывать своё исключение

Вы также можете бросать своё исключение, чтобы позже его поймать:

string GetCategory(string bookTitle) {
    var book = library.FindBook(bookTitle);  // Может вернуть null
    if (book == null)
        throw new BookNotFoundException(bookTitle);  // Ваше исключение
    return book.Category;
}

Использовать Debug.Assert для проверки на null для обнаружения ошибки до бросания исключения

Если во время разработки вы знаете, что метод может, но вообще-то не должен возвращать null, вы можете воспользоваться Debug.Assert для быстрого обнаружения ошибки:

string GetTitle(int knownBookID) {
    // Вы знаете, что метод не должен возвращать null
    var book = library.GetBook(knownBookID);  

    // Исключение будет выброшено сейчас, а не в конце метода.
    Debug.Assert(book != null, "Library didn't return a book for known book ID.");

    // Остальной код...

    return book.Title; // Не выбросит NullReferenceException в режиме отладки.
}

Однако эта проверка не будет работать в релизной сборке, и вы снова получите NullReferenceException, если book == null.

Использовать GetValueOrDefault() для Nullable типов

DateTime? appointment = null;
Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now));
// Отобразит значение по умолчанию, потому что appointment = null.

appointment = new DateTime(2022, 10, 20);
Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now));
// Отобразит дату, а не значение по умолчанию.

Использовать оператор ?? (C#) или If() (VB)

Краткая запись для задания значения по умолчанию:

IService CreateService(ILogger log, Int32? frobPowerLevel)
{
    var serviceImpl = new MyService(log ?? NullLog.Instance);
    serviceImpl.FrobPowerLevel = frobPowerLevel ?? 5;
}

Использовать операторы ?. и ?[ (C# 6+, VB.NET 14+):

Это оператор безопасного доступа к членам, также известный как оператор Элвиса за специфическую форму. Если выражение слева от оператора равно null, то правая часть игнорируется, и результатом считается null. Например:

var title = person.Title.ToUpper();

Если свойство Title равно null, то будет брошено исключение, потому что это попытка вызвать метод ToUpper на значении, равном null. В C# 5 и ниже можно добавить проверку:

var title = person.Title == null ? null : person.Title.ToUpper();

Теперь вместо бросания исключения переменной title будет присвоено null. В C# 6 был добавлен более короткий синтаксис:

var title = person.Title?.ToUpper();

Разумеется, если переменная person может быть равна null, то надо проверять и её. Также можно использовать операторы ?. и ?? вместе, чтобы предоставить значение по умолчанию:

// обычная проверка на null
int titleLength = 0;
if (title != null)
    titleLength = title.Length;

// совмещаем операторы `?.` и `??`
int titleLength = title?.Length ?? 0;

Если любой член в цепочке может быть null, то можно полностью обезопасить себя (хотя, конечно, архитектуру стоит поставить под сомнение):

int firstCustomerOrderCount = customers?[0]?.Orders?.Count() ?? 0;

• Из-за чего возникает исключение NullReferenceException?
  • Теория
  • Как в переменную может попасть null-значение
  • Операции с null-значением, приводящие к исключению
• Как исправить исключение NullReferenceException
• Как предотвратить исключения NullReferenceException
  • Используйте nullable-контекст
  • Используйте статический анализ

NullReferenceException (NRE) — тип исключения платформы .NET, возникающий при попытке обращения по нулевой ссылке. В заметке рассмотрим причины, из-за которых возникают исключения этого типа, а также способы их предотвращения и исправления.

Примечание. Эта заметка рассчитана на начинающих программистов. Разработчикам с опытом предлагаю 2 активности:

• посмотреть перечисленные здесь способы столкнуться с NullReferenceException и проверить, все ли из них вы знаете;
• сыграть в игру на поиск ошибок.

Из-за чего возникает исключение NullReferenceException?

Теория

Переменные ссылочных типов в C# хранят ссылки на объекты. Чтобы обозначить, что ссылка не указывает на объект, используют значение null. Стоит также отметить, что null — значение выражений ссылочных типов по умолчанию.

Исключение типа NullReferenceException возникает при попытке обращения по нулевой ссылке. Операции, при которых может возникнуть исключение, мы перечислим ниже.

Рассмотрим пример:

Object notNullRef = new Object();
Object nullRef = default;

int hash;
hash = notNullRef.GetHashCode();
hash = nullRef.GetHashCode(); // NullReferenceException (NRE)

В коде объявляются две переменные ссылочного типа Object — notNullRef и nullRef:

• notNullRef хранит ссылку на объект, созданный в результате вызова конструктора типа Object;
• nullRef содержит default-значение типа Object — null.

Вызов метода GetHashCode через ссылку в notNullRef отработает нормально, так как ссылка указывает на объект. При попытке вызова того же метода для nullRef средой CLR будет выброшено исключение типа NullReferenceException.

Ниже мы рассмотрим, откуда могут прийти null-значения и какие операции могут привести к исключению NullReferenceException.

Как в переменную может попасть null-значение

Рассмотрим примеры того, как в переменную может попасть значение null.

1. Явная запись значения null или default.

String name = null;
var len = name.Length; // NRE

Результатом выражения default и default(T) для ссылочных типов также будет null.

Object obj = default; // or default(Object)
var hash = obj.GetHashCode(); // NRE

2. Инициализация поля ссылочного типа по умолчанию.

class A 
{
  private String _name;
  public void Foo()
  {
    var len = _name.Length; // NRE
  }
}

var obj = new A();
obj.Foo();

В этом примере поле _name инициализируется значением по умолчанию. На момент вызова Foo поле _name равно null, поэтому при обращении к свойству Length будет выброшено исключение.

3. Результат работы null-conditional оператора (?.).

String name = user?.Name;
var len = name.Length; // Potential NRE

Если значение user или user.Name будет равно null, в переменную name также будет записано значение null. В таком случае при обращении к свойству Length без проверки на null возникнет исключение.

4. Результат приведения с использованием оператора as.

Object obj = new Object();
String name = obj as String; // unsuccessful cast, name is null
var len = name.Length; // NRE

Результатом преобразования с помощью оператора as будет значение null, если преобразование выполнить не удалось.

В примере выше переменная obj хранит ссылку на экземпляр типа Object. Попытка приведения obj к типу String закончится неудачей, в результате чего в name будет записано значение null.

5. Результат работы *OrDefault метода.

Методы вида *OrDefault (FirstOrDefault, LastOrDefault и т. п.) из стандартной библиотеки возвращают значение по умолчанию, если значение предиката не подходит ни для одного элемента или коллекция пустая.

String[] strArr = ....;
String firstStr = strArr.FirstOrDefault();
var len = firstStr.Length; // Potential NRE

Если в массиве strArr нет элементов, метод FirstOrDefault вернёт значение default(String) — null. При разыменовании нулевой ссылки возникнет исключение.

6. Упаковка default значения типа Nullable<T>.

Результатом упаковки экземпляров Nullable<T> с default-значением будет null.

long? nullableLong1 = default;
long? nullableLong2 = null;

Nullable<long> nullableLong3 = default;
Nullable<long> nullableLong4 = null;
Nullable<long> nullableLong5 = new Nullable<long>();


var nullableToBox = ....; // nullableLong1 — nullableLong5

object boxedValue = (Object)nullableToBox; // null
_ = boxedValue.GetHashCode(); // NRE

При записи в переменную nullableToBox любого из значений nullableLong1 – nullableLong5 и последующей упаковки результатом будет null. При использовании такого значения без проверки на null будет выброшено исключение.

Подробности упаковки значений типа Nullable<T> описаны в статье «Хорошо ли вы помните nullable value types?».

Операции с null-значением, приводящие к исключению

В этом разделе перечислены операции, выполнение которых с null-значением приведёт к исключению NullReferenceException.

1. Явное обращение к члену объекта.

class A
{
  public String _name;
  public String Name => _name;
  public String GetName() { return _name; }
}

A aObj = null;
_ = aObj._name; // NRE
_ = aObj.Name; // NRE
_ = aObj.GetName(); // NRE

То же самое — при разыменовании внутри метода:

void Foo(A obj)
{
  _ = obj.Name; 
}

A aObj = null;
Foo(aObj); // NRE inside method

2. Обращение по индексу.

int[] arr = null;
int val = arr[0]; // NRE

3. Вызов делегата.

Action fooAct = null;
fooAct(); // NRE

4. Итерирование в foreach.

List<long> list = null;
foreach (var item in list) // NRE
{ .... }

Обратите внимание, что оператор ‘?.’ здесь не поможет:

foreach (var item in wrapper?.List) // Potential NRE
{ .... }

Если wrapper или wrapper.List равны null, всё так же будет выброшено исключение. Подробнее ситуацию разобрали в статье «Использование оператора ?. в foreach: защита от NullReferenceException, которая не работает».

5. Использование null-значения в качестве операнда для await.

Task GetPotentialNull()
{
  return _condition ? .... : null;
}
await GetPotentialNull(); // Potential NRE

6. Распаковка null-значения.

object obj = null;
int intVal = (int)obj; // NRE

7. Выброс исключения с null-значением.

InvalidOperationException invalidOpException 
  = flag ? new InvalidOperationException() 
         : null;

throw invalidOpException; // Potential NRE

В переменную invalidOpException может быть записано значение null. В этом случае оператор throw выбросит исключение типа NullReferenceException.

8. Разыменование значения свойства Target у экземпляра типа WeakReference.

void ProcessIfNecessary(WeakReference weakRef)
{
  if (weakRef.IsAlive)
    (weakRef.Target as DataProcessor).Process(); // Potential NRE
}

Ссылка в WeakReference указывает на объект, при этом не защищая его от сборки мусора. Если объект попадёт под сборку мусора после проверки weakRef.IsAlive, но до вызова метода Process, то:

• значением weakRef.Target будет null;
• результатом оператора as также будет null;
• при попытке вызова метода Process будет выброшено исключение NullReferenceException.

9. Использование значения поля ссылочного типа до явной инициализации.

class A
{
  private String _name;
  public A()
  {
    var len = _name.Length; // NRE
  }
}

На момент обращения к свойству Length поле _name проинициализировано значением по умолчанию (null). Результат обращения — исключение.

10. Небезопасный вызов обработчиков события в многопоточном коде.

public event EventHandler MyEvent;

void OnMyEvent(EventArgs e)
{
  if (MyEvent != null)
    MyEvent(this, e); // Potential NRE
}

Если между проверкой MyEvent != null и вызовом обработчиков события MyEvent у него не останется подписчиков, при вызове будет выброшено исключение типа NullRefernceException.

Как исправить исключение NullReferenceException

Чтобы избежать исключений типа NullReferenceException, исключите ситуацию разыменования нулевых ссылок. Для этого:

• определите, откуда в выражение попадает нулевая ссылка;
• измените логику работы приложения, чтобы доступа по нулевой ссылке не происходило.

Рассмотрим пример:

foreach (var item in potentialNullCollection?.Where(....))
{ .... }

Если значением potentialNullCollection будет null, оператор ‘?.’ также вернёт значение null. При попытке обхода коллекции в цикле foreach возникнет исключение.

Если potentialNullCollection в данном фрагменте кода никогда не равен null, стоит убрать оператор ‘?.’, чтобы не запутать разработчиков и инструменты анализа кода:

foreach (var item in potentialNullCollection.Where(....))
{ .... }

Если potentialNullCollection может принимать значение null, стоит добавить явную проверку или использовать оператор ‘??’.

// 1
if (potentialNullCollection != null)
{
  foreach (var item in potentialNullCollection.Where(....))
  { .... }
}

// 2
foreach (var item in    potentialNullCollection?.Where(....) 
                     ?? Enumerable.Empty<T>)
{ .... }

Примечание. Добавить проверку на неравенство null — самый простой способ защититься от NullReferenceException. Однако иногда такая правка будет не решать исходную проблему, а только маскировать её. Поэтому при исправлении кода полезно думать о том, достаточно ли будет добавить проверку или нужно исправить в коде что-то ещё.

Как предотвратить исключения NullReferenceException

Кроме достаточно очевидного совета «не разыменовывать нулевые ссылки» есть несколько практик, которые помогут избежать возникновения исключений NRE.

Используйте nullable-контекст

Без использования nullable-контекста значение null считается допустимым для ссылочных типов:

String str = null; // No warnings

Начиная с C# 8, в языке появилась возможность использовать nullable-контекст. Он вводит понятие nullable reference types. В nullable-контексте ссылочные типы считаются не допускающими значения null. Например, при использовании nullable-контекста на код, который мы только что рассмотрели, компилятор выдаст предупреждение:

String str = null; // CS8600

Предупреждение: CS8600 Converting null literal or possible null value to non-nullable type.

Аналогичная ситуация при вызове методов:

void ProcessUserName(String userName)
{
  var len = userName.Length;
  ....
}
....
ProcessUserName(null); // CS8625

Предупреждение компилятора: CS8625 Cannot convert null literal to non-nullable reference type.

Чтобы указать компилятору, что переменная ссылочного типа может принимать значение null, используется символ ‘?’:

String firstName = null; // CS8600
String? lastName = null; // No warning

При попытке разыменовать nullable-переменную без проверки на null компилятор также выдаст предупреждение:

void ProcessUserName(String? userName)
{
  var len = userName.Length; // CS8602
}

Предупреждение компилятора: CS8602 — Dereference of a possibly null reference.

Если нужно указать компилятору, что в конкретном месте кода выражение точно не имеет значения null, можно использовать null-forgiving оператор — ‘!’. Пример:

void ProcessUserName(String? userName)
{
  int len = default;
  if (_flag)
    len = userName.Length; // CS8602
  else
    len = userName!.Length; // No warnings
}

Таким образом, nullable-контекст помогает писать код так, чтобы минимизировать возможность разыменования нулевых ссылок.

Включить nullable-контекст можно несколькими способами:

• изменить соответствующую опцию в настройках проекта («Nullable» в Visual Studio или «Nullable reference types» в JetBrains Rider);
• самостоятельно прописать настройку в проектном файле (.csproj): <Nullable>enable</Nullable>;
• с помощью директив #nullable enable / #nullable disable в коде.

У nullable-контекста куда больше возможностей для настройки. Подробнее о них мы писали в отдельной статье.

Примечание. Обратите внимание, что nullable-context влияет на выдачу предупреждений компилятором, но не на логику исполнения приложения.

String? str = null;
var len = str!.Length;

Компилятор не выдаст предупреждения на этот код, так как в нём используется null-forgiving оператор. Однако на этапе исполнения в коде возникнет исключение типа NullReferenceException.

Используйте статический анализ

Статические анализаторы помогают находить дефекты безопасности и ошибки в коде. В том числе анализаторы помогают находить места возникновения исключений типа NullReferenceException.

Пример такого статического анализатора — PVS-Studio.

Рассмотрим пример C# кода, в котором может возникнуть NullReferenceException.

private ImmutableArray<char>
GetExcludedCommitCharacters(ImmutableArray<CompletionItem> items)
{
  var hashSet = new HashSet<char>();
  foreach (var item in items)
  {
    foreach (var rule in item.Rules?.FilterCharacterRules)
    {
      if (rule.Kind == CharacterSetModificationKind.Add)
      {
        foreach (var c in rule.Characters)
        {
          hashSet.Add(c);
        }
      }
    }
  }

  return hashSet.ToImmutableArray();
}

Во втором цикле foreach разработчики выполняют обход коллекции FilterCharacterRules, для получения которой используют выражение roslynItem.Rules?.FilterCharacterRules. Оператор ‘?.’ предполагает, что свойство Rules может иметь значение null. Однако если результатом выражения будет null, при попытке перебора null-значения в foreach всё равно возникнет NullReferenceException.

PVS-Studio находит эту проблему и выдаёт предупреждение V3153.

Если items.Rules действительно может иметь значение null, защититься от NullReferenceException можно дополнительной проверкой:

foreach (var item in items)
{
  if (item.Rules == null)
    continue;

  foreach (var rule in item.Rules.FilterCharacterRules)
  {
    ....
  }
}

Анализатор не будет выдавать предупреждение на такой код.

PVS-Studio ищет и другие ситуации в коде, при которых может возникнуть исключение NullReferenceException:

• V3080. Possible null dereference.
• V3083. Unsafe invocation of event, NullReferenceException is possible.
• V3095. The object was used before it was verified against null.
• и т. д.

Привет, столкнулся с ошибкой, уже все просмотрел, все перешерстил, ничего не могу поделать.
Выдает ошибку :

java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.google.firebase.firestore.QuerySnapshot.getDocumentChanges()' on a null object reference

Логи с ошибкой :

Process: com.vuradoapps.mycity, PID: 8801
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.google.firebase.firestore.QuerySnapshot.getDocumentChanges()' on a null object reference
        at com.vuradoapps.mycity.Tasks_mini_app_Activity$3.onEvent(Tasks_mini_app_Activity.java:86)
        at com.vuradoapps.mycity.Tasks_mini_app_Activity$3.onEvent(Tasks_mini_app_Activity.java:82)
        at com.google.firebase.firestore.Query.lambda$addSnapshotListenerInternal$2$com-google-firebase-firestore-Query(Query.java:1131)
        at com.google.firebase.firestore.Query$$ExternalSyntheticLambda2.onEvent(Unknown Source:6)
        at com.google.firebase.firestore.core.AsyncEventListener.lambda$onEvent$0$com-google-firebase-firestore-core-AsyncEventListener(AsyncEventListener.java:42)
        at com.google.firebase.firestore.core.AsyncEventListener$$ExternalSyntheticLambda0.run(Unknown Source:6)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:246)
        at android.app.ActivityThread.main(ActivityThread.java:8653)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)

Код с активити на которую указывают логи :

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.recyclerview.widget.ItemTouchHelper;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;

import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.firebase.firestore.DocumentChange;
import com.google.firebase.firestore.EventListener;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.FirebaseFirestoreException;
import com.google.firebase.firestore.ListenerRegistration;
import com.google.firebase.firestore.Query;
import com.google.firebase.firestore.QuerySnapshot;
import com.vuradoapps.mycity.Adapter.ToDoAdapter;
import com.vuradoapps.mycity.Model.ToDoModel;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;


public class Tasks_mini_app_Activity extends AppCompatActivity implements OnDialogChooce{

    private RecyclerView recyclerView;
    private FloatingActionButton mFab;
    private FirebaseFirestore firebaseFirestore;
    private ToDoAdapter adapter;
    private List<ToDoModel> mList;
    private Query query;
    private ListenerRegistration listenerRegistration;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_tasks_mini_app);

        Toolbar toolbar = findViewById(R.id.app_bar);
        setSupportActionBar(toolbar);
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                onBackPressed();
            }
        });

        recyclerView = findViewById(R.id.recycleView);
        mFab = findViewById(R.id.floatingActionButton);
        firebaseFirestore = FirebaseFirestore.getInstance();

        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(Tasks_mini_app_Activity.this));

        mFab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Add_new_task.newInstance().show(getSupportFragmentManager(), Add_new_task.TAG);
            }
        });

        mList = new ArrayList<>();
        adapter = new ToDoAdapter(Tasks_mini_app_Activity.this, mList);

        ItemTouchHelper itemTouchHelper = new ItemTouchHelper(new TouchHelper(adapter));
        itemTouchHelper.attachToRecyclerView(recyclerView);

        recyclerView.setAdapter(adapter);
        showData();
    }

    private  void showData(){
        query = firebaseFirestore.collection("task").orderBy("time" , Query.Direction.DESCENDING);

        listenerRegistration = query.addSnapshotListener(new EventListener<QuerySnapshot>() {
            @Override
            public void onEvent(@Nullable QuerySnapshot value, @Nullable FirebaseFirestoreException error) {

                for (DocumentChange documentChange : value.getDocumentChanges()){
                    if (documentChange.getType() == DocumentChange.Type.ADDED){
                        String id = documentChange.getDocument().getId();
                        ToDoModel toDoModel = documentChange.getDocument().toObject(ToDoModel.class).withId(id);

                        mList.add(toDoModel);
                        adapter.notifyDataSetChanged();
                    }
                }
                listenerRegistration.remove();


            }
        });
    }

    @Override
    public void onDialogClose(DialogInterface dialogInterface) {
        mList.clear();
        showData();
        adapter.notifyDataSetChanged();
    }
}

Я проверил строки, но ничего не нашел, строки на которые указывают логи, они обе тут :

private  void showData(){
        query = firebaseFirestore.collection("task").orderBy("time" , Query.Direction.DESCENDING);

        listenerRegistration = query.addSnapshotListener(new EventListener<QuerySnapshot>() {
            @Override
            public void onEvent(@Nullable QuerySnapshot value, @Nullable FirebaseFirestoreException error) {

                for (DocumentChange documentChange : value.getDocumentChanges()){
                    if (documentChange.getType() == DocumentChange.Type.ADDED){
                        String id = documentChange.getDocument().getId();
                        ToDoModel toDoModel = documentChange.getDocument().toObject(ToDoModel.class).withId(id);

                        mList.add(toDoModel);
                        adapter.notifyDataSetChanged();
                    }
                }
                listenerRegistration.remove();


            }
        });
    }

Time for another post in the series Debugging common .NET exceptions. Today’s exception is, without a doubt, the error most people have experienced: System.NullReferenceException. The exception happens when you try to invoke a reference that you were expecting to point to an object but in fact, points to null. Let’s get started!

Handling the error

There are some clever ways to avoid a NullReferenceException, but before we start looking into those, let us see how the exception can be caught. Being a plain old C# exception, NullReferenceException can be caught using a try/catch:

try
{
    string s = null;
    s.ToString();
}
catch (NullReferenceException e)
{
    // Do something with e, please.
}

Running the code above will produce the following error:

System.NullReferenceException: Object reference not set to an instance of an object.

Debugging the error

We already know why the exception is happening. Something is null. When looking at the code above, it’s clear that s is null and the stack trace even tells us that:

Sometimes spotting what is null can be hard. Take a look at the following example:

var street = service.GetUser().Address.Street;

If the code above throws a NullReferenceException, what is null? service? The result of GetUser()? Address? At first glance, Visual Studio isn’t exactly helpful either:

There is a range of different ways to find out what is going on. Let’s look at the most commonly used ones.

Splitting chained method-calls to multiple lines

Spotting which call that caused an error is a lot easier if the calls are split into multiple lines:

var service = new Service();
var user = service.GetUser();
var address = user.Address;
var street = address.Street;

Running the code reveals the actual call causing the exception:

In the example above user.Address returns null, why address.Street causes the NullReferenceException.

While splitting code into atoms like this can help debug what is going wrong, it’s not preferable in terms of readability (IMO).

Using Null Reference Analysis in Visual Studio

If you are on Visual Studio 2017 or newer (if not, now is the time to upgrade), you will have the Null Reference Analysis feature available. With this in place, Visual Studio can show you exactly what is null. Let’s change the example back to method-chaining:

var street = service.GetUser().Address.Street;

To enable the analysis go to Debug | Windows | Exception Settings. Check Common Language Runtime Exceptions (if not already checked) or extend the node and check the exceptions you are interested in. In this case, you can check System.NullReferenceException. When running the code, the debugger breaks on the NullReferenceException and you now see the Exception Thrown window:

Voila! The window says «ConsoleApp18.User.Address.get returned null». Exactly what we wanted to see. This will require you to run the code locally, though. If you are experiencing the exception on your production website, the Null Reference Analysis will not be available, since this is a feature belonging to Visual Studio (unfortunately). With that said, you can attach a debugger to a remote site running on Azure as explained here: Introduction to Remote Debugging on Azure Web Sites.

Fixing the error

There are various ways to fix NullReferenceException. We’ll start with the simple (but dirty) approach.

Using null checks

If null is an allowed value of an object, you will need to check for it. The most simple solution is to include a bunch of if-statements.

if (service != null)
{
    var user = service.GetUser();
    if (user != null)
    {
        var address = user.Address;
        if (address != null)
        {
            var street = address.Street;
        }
    }
}

The previous code will only reach address.Street if everything else is not null. We can probably agree that the code isn’t exactly pretty. Having multiple nested steps is harder to read. We can reverse the if-statements:

if (service == null) return;
var user = service.GetUser();
if (user == null) return;
var address = user.Address;
if (address == null) return;
var street = address.Street;

Simpler, but still a lot of code to get a street name.

Using null-conditional operator

C# 6 introduced a piece of syntactic sugar to check for null: null-conditional operator. Let’s change the method-chain example from before to use the «new» operator:

var user = service?.GetUser()?.Address?.Street;

The ? to the right of each variable, corresponds the nested if-statements from previously. But with much less code.

Use Debug.Assert during development

When getting a NullReferenceException it can be hard to spot the intent with the code from the original developer. Rather than including if-statements, it can be clearer for future authors of your code to use the Debug.Assert-method. Much like in a xUnit or NUnit test, you use Assert to verify the desired state on your objects. In the example from above, the service object could have come through a parameter or a constructor injected dependency:

class MyClass
{
    Service service;

    public MyClass(Service service)
    {
        this.service = service;
    }

    public string UserStreet()
    {
        return service.GetUser().Address.Street;
    }
}

To make a statement in your code that service should never be allowed to have the value of null, extend the constructor:

public MyClass(Service service)
{
    Debug.Assert(service != null);
    this.service = service;
}

In the case MyClass is constructed with null, the following error is shown when running locally:

Use nullable reference types in C# 8.0

When designing code you often end up expecting parameters to be not null but end up checking for null to avoid a NullReferenceException. As you already know, all reference types in C# can take the value of null. Value types like int and boolean cannot take a value of null unless explicitly specified using the nullable value type (int? or Nullable<int>). Maybe it should have been the other way around with reference types all along?

C# 8 can fix this with nullable reference types (maybe NOT nullable reference types are a better name). Since this is a breaking change, it is launched as an opt-in feature. Nullable reference types are a great way to avoid NullReferenceException since you are very explicit about where you expect null and where not.

To enable not nullable reference types, create a new .NET Core 3 project and add the following to the csproj file:

<LangVersion>8.0</LangVersion>
<Nullable>enable</Nullable>

You should be on C# 8 already, but to make it explicit, I’ve added the LangVersion element. The Nullable element enables nullable reference types. Out of the box, C# 8 creates a warning if it identifies the use of null where a value is expected. Let’s see how that looks:

class Program
{
    static void Main()
    {
        new Program().SayHello(null);
    }

    public void SayHello(string msg)
    {
        Console.WriteLine(msg);
    }
}

When compiling we see the following warning:

Program.cs(9,36): warning CS8625: Cannot convert null literal to non-nullable reference type. [C:projectscore3core3.csproj]

I know you are not one of them, but some people developed a warning resistance which means that warnings are simply ignored. To overcome this, make sure that errors like this causes build errors over warnings by adding the following to csproj:

<WarningsAsErrors>CS8602,CS8603,CS8618,CS8625</WarningsAsErrors>

This will tell the C# compiler to treat these four nullable reference type warnings as errors instead.

Just to make it clear, allowing null in the msg parameter, you use the ? characters as with value types:

public void SayHello(string? msg)
{
    ...
}

Logging and monitoring

Logging and monitoring for null reference exceptions are a must. While some developers tempt to create control flow from exceptions (no one should), null reference exceptions should never happen. This means that a System.NullReferenceException is a type of exception that should always be logged and fixed. A null check through either an if statement or the null-conditional operator is always the preferred way of handling potential null values. But make sure to implement a logging strategy that logs all uncaught exceptions, including the System.NullReferenceException.

When logging a System.NullReferenceException in a log file, database, elmah.io, or similar, it can be hard to spot what is null. You typically only see the method-name that causes the NullReferenceException and the Null Reference Analysis feature is only available while debugging inside Visual Studio. I will recommend you to always Include filename and line number in stack traces. This will pinpoint the exact line where the error happens.

Would your users appreciate fewer errors?

elmah.io is the easy error logging and uptime monitoring service for .NET. Take back control of your errors with support for all .NET web and logging frameworks.

➡️ Error Monitoring for .NET Web Applications ⬅️

This article first appeared on the elmah.io blog at https://blog.elmah.io/debugging-system-nullreferenceexception-object-reference-not-set-to-an-instance-of-an-object/

NullReference Exception — Visual Basic

The NullReference Exception for Visual Basic is no different from the one in C#. After all, they are both reporting the same exception defined in the .NET Framework which they both use. Causes unique to Visual Basic are rare (perhaps only one).

This answer will use Visual Basic terms, syntax, and context. The examples used come from a large number of past Stack  Overflow questions. This is to maximize relevance by using the kinds of situations often seen in posts. A bit more explanation is also provided for those who might need it. An example similar to yours is very likely listed here.

Note:

1. This is concept-based: there is no code for you to paste into your project. It is intended to help you understand what causes a NullReferenceException (NRE), how to find it, how to fix it, and how to avoid it. An NRE can be caused many ways so this is unlikely to be your sole encounter.
2. The examples (from Stack  Overflow posts) do not always show the best way to do something in the first place.
3. Typically, the simplest remedy is used.

Basic Meaning

The message «Object not set to an instance of Object» means you are trying to use an object which has not been initialized. This boils down to one of these:

• Your code declared an object variable, but it did not initialize it (create an instance or ‘instantiate‘ it)
• Something which your code assumed would initialize an object, did not
• Possibly, other code prematurely invalidated an object still in use

Finding The Cause

Since the problem is an object reference which is Nothing, the answer is to examine them to find out which one. Then determine why it is not initialized. Hold the mouse over the various variables and Visual Studio (VS) will show their values — the culprit will be Nothing.

You should also remove any Try/Catch blocks from the relevant code, especially ones where there is nothing in the Catch block. This will cause your code to crash when it tries to use an object which is Nothing. This is what you want because it will identify the exact location of the problem, and allow you to identify the object causing it.

A MsgBox in the Catch which displays Error while... will be of little help. This method also leads to very bad Stack  Overflow questions, because you can’t describe the actual exception, the object involved or even the line of code where it happens.

You can also use the Locals Window (Debug -> Windows -> Locals) to examine your objects.

Once you know what and where the problem is, it is usually fairly easy to fix and faster than posting a new question.

See also:

• Breakpoints
• MSDN: How to: Use the Try/Catch Block to Catch Exceptions
• MSDN: Best Practices for Exceptions

Examples and Remedies

Class Objects / Creating an Instance

Dim reg As CashRegister
...
TextBox1.Text = reg.Amount         ' NRE

The problem is that Dim does not create a CashRegister object; it only declares a variable named reg of that Type. Declaring an object variable and creating an instance are two different things.

Remedy

The New operator can often be used to create the instance when you declare it:

Dim reg As New CashRegister        ' [New] creates instance, invokes the constructor

' Longer, more explicit form:
Dim reg As CashRegister = New CashRegister

When it is only appropriate to create the instance later:

Private reg As CashRegister         ' Declare
  ...
reg = New CashRegister()            ' Create instance

Note: Do not use Dim again in a procedure, including the constructor (Sub New):

Private reg As CashRegister
'...

Public Sub New()
   '...
   Dim reg As New CashRegister
End Sub

This will create a local variable, reg, which exists only in that context (sub). The reg variable with module level Scope which you will use everywhere else remains Nothing.

Missing the New operator is the #1 cause of NullReference Exceptions seen in the Stack  Overflow questions reviewed.

Visual Basic tries to make the process clear repeatedly using New: Using the New Operator creates a new object and calls Sub New — the constructor — where your object can perform any other initialization.

To be clear, Dim (or Private) only declares a variable and its Type. The Scope of the variable — whether it exists for the entire module/class or is local to a procedure — is determined by where it is declared. Private | Friend | Public defines the access level, not Scope.

For more information, see:

• New Operator
• Scope in Visual Basic
• Access Levels in Visual Basic
• Value Types and Reference Types

Arrays

Arrays must also be instantiated:

Private arr as String()

This array has only been declared, not created. There are several ways to initialize an array:

Private arr as String() = New String(10){}
' or
Private arr() As String = New String(10){}

' For a local array (in a procedure) and using 'Option Infer':
Dim arr = New String(10) {}

Note: Beginning with VS 2010, when initializing a local array using a literal and Option Infer, the As <Type> and New elements are optional:

Dim myDbl As Double() = {1.5, 2, 9.9, 18, 3.14}
Dim myDbl = New Double() {1.5, 2, 9.9, 18, 3.14}
Dim myDbl() = {1.5, 2, 9.9, 18, 3.14}

The data Type and array size are inferred from the data being assigned. Class/Module level declarations still require As <Type> with Option Strict:

Private myDoubles As Double() = {1.5, 2, 9.9, 18, 3.14}

Example: Array of class objects

Dim arrFoo(5) As Foo

For i As Integer = 0 To arrFoo.Count - 1
   arrFoo(i).Bar = i * 10       ' Exception
Next

The array has been created, but the Foo objects in it have not.

Remedy

For i As Integer = 0 To arrFoo.Count - 1
    arrFoo(i) = New Foo()         ' Create Foo instance
    arrFoo(i).Bar = i * 10
Next

Using a List(Of T) will make it quite difficult to have an element without a valid object:

Dim FooList As New List(Of Foo)     ' List created, but it is empty
Dim f As Foo                        ' Temporary variable for the loop

For i As Integer = 0 To 5
    f = New Foo()                    ' Foo instance created
    f.Bar =  i * 10
    FooList.Add(f)                   ' Foo object added to list
Next

For more information, see:

• Option Infer Statement
• Scope in Visual Basic
• Arrays in Visual Basic

Lists and Collections

.NET collections (of which there are many varieties — Lists, Dictionary, etc.) must also be instantiated or created.

Private myList As List(Of String)
..
myList.Add("ziggy")           ' NullReference

You get the same exception for the same reason — myList was only declared, but no instance created. The remedy is the same:

myList = New List(Of String)

' Or create an instance when declared:
Private myList As New List(Of String)

A common oversight is a class which uses a collection Type:

Public Class Foo
    Private barList As List(Of Bar)

    Friend Function BarCount As Integer
        Return barList.Count
    End Function

    Friend Sub AddItem(newBar As Bar)
        If barList.Contains(newBar) = False Then
            barList.Add(newBar)
        End If
    End Function

Either procedure will result in an NRE, because barList is only declared, not instantiated. Creating an instance of Foo will not also create an instance of the internal barList. It may have been the intent to do this in the constructor:

Public Sub New         ' Constructor
    ' Stuff to do when a new Foo is created...
    barList = New List(Of Bar)
End Sub

As before, this is incorrect:

Public Sub New()
    ' Creates another barList local to this procedure
     Dim barList As New List(Of Bar)
End Sub

For more information, see List(Of T) Class.

Data Provider Objects

Working with databases presents many opportunities for a NullReference because there can be many objects (Command, Connection, Transaction, Dataset, DataTable, DataRows….) in use at once. Note: It does not matter which data provider you are using — MySQL, SQL Server, OleDB, etc. — the concepts are the same.

Example 1

Dim da As OleDbDataAdapter
Dim ds As DataSet
Dim MaxRows As Integer

con.Open()
Dim sql = "SELECT * FROM tblfoobar_List"
da = New OleDbDataAdapter(sql, con)
da.Fill(ds, "foobar")
con.Close()

MaxRows = ds.Tables("foobar").Rows.Count      ' Error

As before, the ds Dataset object was declared, but an instance was never created. The DataAdapter will fill an existing DataSet, not create one. In this case, since ds is a local variable, the IDE warns you that this might happen:

When declared as a module/class level variable, as appears to be the case with con, the compiler can’t know if the object was created by an upstream procedure. Do not ignore warnings.

Remedy

Dim ds As New DataSet

Example 2

ds = New DataSet
da = New OleDBDataAdapter(sql, con)
da.Fill(ds, "Employees")

txtID.Text = ds.Tables("Employee").Rows(0).Item(1)
txtID.Name = ds.Tables("Employee").Rows(0).Item(2)

A typo is a problem here: Employees vs Employee. There was no DataTable named «Employee» created, so a NullReferenceException results trying to access it. Another potential problem is assuming there will be Items which may not be so when the SQL includes a WHERE clause.

Remedy

Since this uses one table, using Tables(0) will avoid spelling errors. Examining Rows.Count can also help:

If ds.Tables(0).Rows.Count > 0 Then
    txtID.Text = ds.Tables(0).Rows(0).Item(1)
    txtID.Name = ds.Tables(0).Rows(0).Item(2)
End If

Fill is a function returning the number of Rows affected which can also be tested:

If da.Fill(ds, "Employees") > 0 Then...

Example 3

Dim da As New OleDb.OleDbDataAdapter("SELECT TICKET.TICKET_NO,
        TICKET.CUSTOMER_ID, ... FROM TICKET_RESERVATION AS TICKET INNER JOIN
        FLIGHT_DETAILS AS FLIGHT ... WHERE [TICKET.TICKET_NO]= ...", con)
Dim ds As New DataSet
da.Fill(ds)

If ds.Tables("TICKET_RESERVATION").Rows.Count > 0 Then

The DataAdapter will provide TableNames as shown in the previous example, but it does not parse names from the SQL or database table. As a result, ds.Tables("TICKET_RESERVATION") references a non-existent table.

The Remedy is the same, reference the table by index:

If ds.Tables(0).Rows.Count > 0 Then

See also DataTable Class.

Object Paths / Nested

If myFoo.Bar.Items IsNot Nothing Then
   ...

The code is only testing Items while both myFoo and Bar may also be Nothing. The remedy is to test the entire chain or path of objects one at a time:

If (myFoo IsNot Nothing) AndAlso
    (myFoo.Bar IsNot Nothing) AndAlso
    (myFoo.Bar.Items IsNot Nothing) Then
    ....

AndAlso is important. Subsequent tests will not be performed once the first False condition is encountered. This allows the code to safely ‘drill’ into the object(s) one ‘level’ at a time, evaluating myFoo.Bar only after (and if) myFoo is determined to be valid. Object chains or paths can get quite long when coding complex objects:

myBase.myNodes(3).Layer.SubLayer.Foo.Files.Add("somefilename")

It is not possible to reference anything ‘downstream’ of a null object. This also applies to controls:

myWebBrowser.Document.GetElementById("formfld1").InnerText = "some value"

Here, myWebBrowser or Document could be Nothing or the formfld1 element may not exist.

UI Controls

Dim cmd5 As New SqlCommand("select Cartons, Pieces, Foobar " _
     & "FROM Invoice where invoice_no = '" & _
     Me.ComboBox5.SelectedItem.ToString.Trim & "' And category = '" & _
     Me.ListBox1.SelectedItem.ToString.Trim & "' And item_name = '" & _
     Me.ComboBox2.SelectedValue.ToString.Trim & "' And expiry_date = '" & _
     Me.expiry.Text & "'", con)

Among other things, this code does not anticipate that the user may not have selected something in one or more UI controls. ListBox1.SelectedItem may well be Nothing, so ListBox1.SelectedItem.ToString will result in an NRE.

Remedy

Validate data before using it (also use Option Strict and SQL parameters):

Dim expiry As DateTime         ' for text date validation
If (ComboBox5.SelectedItems.Count > 0) AndAlso
    (ListBox1.SelectedItems.Count > 0) AndAlso
    (ComboBox2.SelectedItems.Count > 0) AndAlso
    (DateTime.TryParse(expiry.Text, expiry) Then

    '... do stuff
Else
    MessageBox.Show(...error message...)
End If

Alternatively, you can use (ComboBox5.SelectedItem IsNot Nothing) AndAlso...

Visual Basic Forms

Public Class Form1

    Private NameBoxes = New TextBox(5) {Controls("TextBox1"), _
                   Controls("TextBox2"), Controls("TextBox3"), _
                   Controls("TextBox4"), Controls("TextBox5"), _
                   Controls("TextBox6")}

    ' same thing in a different format:
    Private boxList As New List(Of TextBox) From {TextBox1, TextBox2, TextBox3 ...}

    ' Immediate NRE:
    Private somevar As String = Me.Controls("TextBox1").Text

This is a fairly common way to get an NRE. In C#, depending on how it is coded, the IDE will report that Controls does not exist in the current context, or «cannot reference non-static member». So, to some extent, this is a VB-only situation. It is also complex because it can result in a failure cascade.

The arrays and collections cannot be initialized this way. This initialization code will run before the constructor creates the Form or the Controls. As a result:

• Lists and Collection will simply be empty
• The Array will contain five elements of Nothing
• The somevar assignment will result in an immediate NRE because Nothing doesn’t have a .Text property

Referencing array elements later will result in an NRE. If you do this in Form_Load, due to an odd bug, the IDE may not report the exception when it happens. The exception will pop up later when your code tries to use the array. This «silent exception» is detailed in this post. For our purposes, the key is that when something catastrophic happens while creating a form (Sub New or Form Load event), exceptions may go unreported, the code exits the procedure and just displays the form.

Since no other code in your Sub New or Form Load event will run after the NRE, a great many other things can be left uninitialized.

Sub Form_Load(..._
   '...
   Dim name As String = NameBoxes(2).Text        ' NRE
   ' ...
   ' More code (which will likely not be executed)
   ' ...
End Sub

Note this applies to any and all control and component references making these illegal where they are:

Public Class Form1

    Private myFiles() As String = Me.OpenFileDialog1.FileName & ...
    Private dbcon As String = OpenFileDialog1.FileName & ";Jet Oledb..."
    Private studentName As String = TextBox13.Text

Partial Remedy

It is curious that VB does not provide a warning, but the remedy is to declare the containers at the form level, but initialize them in form load event handler when the controls do exist. This can be done in Sub New as long as your code is after the InitializeComponent call:

' Module level declaration
Private NameBoxes as TextBox()
Private studentName As String

' Form Load, Form Shown or Sub New:
'
' Using the OP's approach (illegal using OPTION STRICT)
NameBoxes = New TextBox() {Me.Controls("TextBox1"), Me.Controls("TestBox2"), ...)
studentName = TextBox32.Text           ' For simple control references

The array code may not be out of the woods yet. Any controls which are in a container control (like a GroupBox or Panel) will not be found in Me.Controls; they will be in the Controls collection of that Panel or GroupBox. Nor will a control be returned when the control name is misspelled ("TeStBox2"). In such cases, Nothing will again be stored in those array elements and an NRE will result when you attempt to reference it.

These should be easy to find now that you know what you are looking for:

«Button2» resides on a Panel

Remedy

Rather than indirect references by name using the form’s Controls collection, use the control reference:

' Declaration
Private NameBoxes As TextBox()

' Initialization -  simple and easy to read, hard to botch:
NameBoxes = New TextBox() {TextBox1, TextBox2, ...)

' Initialize a List
NamesList = New List(Of TextBox)({TextBox1, TextBox2, TextBox3...})
' or
NamesList = New List(Of TextBox)
NamesList.AddRange({TextBox1, TextBox2, TextBox3...})

Function Returning Nothing

Private bars As New List(Of Bars)        ' Declared and created

Public Function BarList() As List(Of Bars)
    bars.Clear
    If someCondition Then
        For n As Integer = 0 to someValue
            bars.Add(GetBar(n))
        Next n
    Else
        Exit Function
    End If

    Return bars
End Function

This is a case where the IDE will warn you that ‘not all paths return a value and a NullReferenceException may result‘. You can suppress the warning, by replacing Exit Function with Return Nothing, but that does not solve the problem. Anything which tries to use the return when someCondition = False will result in an NRE:

bList = myFoo.BarList()
For Each b As Bar in bList      ' EXCEPTION
      ...

Remedy

Replace Exit Function in the function with Return bList. Returning an empty List is not the same as returning Nothing. If there is a chance that a returned object can be Nothing, test before using it:

 bList = myFoo.BarList()
 If bList IsNot Nothing Then...

Poorly Implemented Try/Catch

A badly implemented Try/Catch can hide where the problem is and result in new ones:

Dim dr As SqlDataReader
Try
    Dim lnk As LinkButton = TryCast(sender, LinkButton)
    Dim gr As GridViewRow = DirectCast(lnk.NamingContainer, GridViewRow)
    Dim eid As String = GridView1.DataKeys(gr.RowIndex).Value.ToString()
    ViewState("username") = eid
    sqlQry = "select FirstName, Surname, DepartmentName, ExtensionName, jobTitle,
             Pager, mailaddress, from employees1 where username='" & eid & "'"
    If connection.State <> ConnectionState.Open Then
        connection.Open()
    End If
    command = New SqlCommand(sqlQry, connection)

    'More code fooing and barring

    dr = command.ExecuteReader()
    If dr.Read() Then
        lblFirstName.Text = Convert.ToString(dr("FirstName"))
        ...
    End If
    mpe.Show()
Catch

Finally
    command.Dispose()
    dr.Close()             ' <-- NRE
    connection.Close()
End Try

This is a case of an object not being created as expected, but also demonstrates the counter usefulness of an empty Catch.

There is an extra comma in the SQL (after ‘mailaddress’) which results in an exception at .ExecuteReader. After the Catch does nothing, Finally tries to perform clean up, but since you cannot Close a null DataReader object, a brand new NullReferenceException results.

An empty Catch block is the devil’s playground. This OP was baffled why he was getting an NRE in the Finally block. In other situations, an empty Catch may result in something else much further downstream going haywire and cause you to spend time looking at the wrong things in the wrong place for the problem. (The «silent exception» described above provides the same entertainment value.)

Remedy

Don’t use empty Try/Catch blocks — let the code crash so you can a) identify the cause b) identify the location and c) apply a proper remedy. Try/Catch blocks are not intended to hide exceptions from the person uniquely qualified to fix them — the developer.

DBNull is not the same as Nothing

For Each row As DataGridViewRow In dgvPlanning.Rows
    If Not IsDBNull(row.Cells(0).Value) Then
        ...

The IsDBNull function is used to test if a value equals System.DBNull: From MSDN:

The System.DBNull value indicates that the Object represents missing or non-existent data. DBNull is not the same as Nothing, which indicates that a variable has not yet been initialized.

Remedy

If row.Cells(0) IsNot Nothing Then ...

As before, you can test for Nothing, then for a specific value:

If (row.Cells(0) IsNot Nothing) AndAlso (IsDBNull(row.Cells(0).Value) = False) Then

Example 2

Dim getFoo = (From f In dbContext.FooBars
               Where f.something = something
               Select f).FirstOrDefault

If Not IsDBNull(getFoo) Then
    If IsDBNull(getFoo.user_id) Then
        txtFirst.Text = getFoo.first_name
    Else
       ...

FirstOrDefault returns the first item or the default value, which is Nothing for reference types and never DBNull:

If getFoo IsNot Nothing Then...

Controls

Dim chk As CheckBox

chk = CType(Me.Controls(chkName), CheckBox)
If chk.Checked Then
    Return chk
End If

If a CheckBox with chkName can’t be found (or exists in a GroupBox), then chk will be Nothing and be attempting to reference any property will result in an exception.

Remedy

If (chk IsNot Nothing) AndAlso (chk.Checked) Then ...

The DataGridView

The DGV has a few quirks seen periodically:

dgvBooks.DataSource = loan.Books
dgvBooks.Columns("ISBN").Visible = True       ' NullReferenceException
dgvBooks.Columns("Title").DefaultCellStyle.Format = "C"
dgvBooks.Columns("Author").DefaultCellStyle.Format = "C"
dgvBooks.Columns("Price").DefaultCellStyle.Format = "C"

If dgvBooks has AutoGenerateColumns = True, it will create the columns, but it does not name them, so the above code fails when it references them by name.

Remedy

Name the columns manually, or reference by index:

dgvBooks.Columns(0).Visible = True

Example 2 — Beware of the NewRow

xlWorkSheet = xlWorkBook.Sheets("sheet1")

For i = 0 To myDGV.RowCount - 1
    For j = 0 To myDGV.ColumnCount - 1
        For k As Integer = 1 To myDGV.Columns.Count
            xlWorkSheet.Cells(1, k) = myDGV.Columns(k - 1).HeaderText
            xlWorkSheet.Cells(i + 2, j + 1) = myDGV(j, i).Value.ToString()
        Next
    Next
Next

When your DataGridView has AllowUserToAddRows as True (the default), the Cells in the blank/new row at the bottom will all contain Nothing. Most attempts to use the contents (for example, ToString) will result in an NRE.

Remedy

Use a For/Each loop and test the IsNewRow property to determine if it is that last row. This works whether AllowUserToAddRows is true or not:

For Each r As DataGridViewRow in myDGV.Rows
    If r.IsNewRow = False Then
         ' ok to use this row

If you do use a For n loop, modify the row count or use Exit For when IsNewRow is true.

My.Settings (StringCollection)

Under certain circumstances, trying to use an item from My.Settings which is a StringCollection can result in a NullReference the first time you use it. The solution is the same, but not as obvious. Consider:

My.Settings.FooBars.Add("ziggy")         ' foobars is a string collection

Since VB is managing Settings for you, it is reasonable to expect it to initialize the collection. It will, but only if you have previously added an initial entry to the collection (in the Settings editor). Since the collection is (apparently) initialized when an item is added, it remains Nothing when there are no items in the Settings editor to add.

Remedy

Initialize the settings collection in the form’s Load event handler, if/when needed:

If My.Settings.FooBars Is Nothing Then
    My.Settings.FooBars = New System.Collections.Specialized.StringCollection
End If

Typically, the Settings collection will only need to be initialized the first time the application runs. An alternate remedy is to add an initial value to your collection in Project -> Settings | FooBars, save the project, then remove the fake value.

Key Points

You probably forgot the New operator.

or

Something you assumed would perform flawlessly to return an initialized object to your code, did not.

Don’t ignore compiler warnings (ever) and use Option Strict On (always).

MSDN NullReference Exception

Debugging System.NullReferenceException — Object reference not set to an instance of an object

TOC

Time for another post in the series Debugging common .NET exceptions. Today’s exception is, without a doubt, the error most people have experienced: System.NullReferenceException. The exception happens when you try to invoke a reference that you were expecting to point to an object but in fact, points to null. Let’s get started!

Handling the error

There are some clever ways to avoid a NullReferenceException, but before we start looking into those, let us see how the exception can be caught. Being a plain old C# exception, NullReferenceException can be caught using a try/catch:

try
{
    string s = null;
    s.ToString();
}
catch (NullReferenceException e)
{
    // Do something with e, please.
}

Running the code above will produce the following error:

System.NullReferenceException: Object reference not set to an instance of an object.

Debugging the error

We already know why the exception is happening. Something is null. When looking at the code above, it’s clear that s is null and the stack trace even tells us that:

Sometimes spotting what is null can be hard. Take a look at the following example:

var street = service.GetUser().Address.Street;

If the code above throws a NullReferenceException, what is null? service? The result of GetUser()? Address? At first glance, Visual Studio isn’t exactly helpful either:

There is a range of different ways to find out what is going on. Let’s look at the most commonly used ones.

Splitting chained method-calls to multiple lines

Spotting which call that caused an error is a lot easier if the calls are split into multiple lines:

var service = new Service();
var user = service.GetUser();
var address = user.Address;
var street = address.Street;

Running the code reveals the actual call causing the exception:

In the example above user.Address returns null, why address.Street causes the NullReferenceException.

While splitting code into atoms like this can help debug what is going wrong, it’s not preferable in terms of readability (IMO).

Using Null Reference Analysis in Visual Studio

If you are on Visual Studio 2017 or newer (if not, now is the time to upgrade), you will have the Null Reference Analysis feature available. With this in place, Visual Studio can show you exactly what is null. Let’s change the example back to method-chaining:

var street = service.GetUser().Address.Street;

To enable the analysis go to Debug | Windows | Exception Settings. Check Common Language Runtime Exceptions (if not already checked) or extend the node and check the exceptions you are interested in. In this case, you can check System.NullReferenceException. When running the code, the debugger breaks on the NullReferenceException and you now see the Exception Thrown window:

Voila! The window says «ConsoleApp18.User.Address.get returned null». Exactly what we wanted to see. This will require you to run the code locally, though. If you are experiencing the exception on your production website, the Null Reference Analysis will not be available, since this is a feature belonging to Visual Studio (unfortunately). With that said, you can attach a debugger to a remote site running on Azure as explained here: Introduction to Remote Debugging on Azure Web Sites.

Fixing the error

There are various ways to fix NullReferenceException. We’ll start with the simple (but dirty) approach.

Using null checks

If null is an allowed value of an object, you will need to check for it. The most simple solution is to include a bunch of if-statements.

if (service != null)
{
    var user = service.GetUser();
    if (user != null)
    {
        var address = user.Address;
        if (address != null)
        {
            var street = address.Street;
        }
    }
}

The previous code will only reach address.Street if everything else is not null. We can probably agree that the code isn’t exactly pretty. Having multiple nested steps is harder to read. We can reverse the if-statements:

if (service == null) return;
var user = service.GetUser();
if (user == null) return;
var address = user.Address;
if (address == null) return;
var street = address.Street;

Simpler, but still a lot of code to get a street name.

Using null-conditional operator

C# 6 introduced a piece of syntactic sugar to check for null: null-conditional operator. Let’s change the method-chain example from before to use the «new» operator:

var user = service?.GetUser()?.Address?.Street;

The ? to the right of each variable, corresponds the nested if-statements from previously. But with much less code.

Use Debug.Assert during development

When getting a NullReferenceException it can be hard to spot the intent with the code from the original developer. Rather than including if-statements, it can be clearer for future authors of your code to use the Debug.Assert-method. Much like in a xUnit or NUnit test, you use Assert to verify the desired state on your objects. In the example from above, the service object could have come through a parameter or a constructor injected dependency:

class MyClass
{
    Service service;
    
    public MyClass(Service service)
    {
        this.service = service;
    }
    
    public string UserStreet()
    {
        return service.GetUser().Address.Street;
    }
}

To make a statement in your code that service should never be allowed to have the value of null, extend the constructor:

public MyClass(Service service)
{
    Debug.Assert(service != null);
    this.service = service;
}

In the case MyClass is constructed with null, the following error is shown when running locally:

Use nullable reference types in C# 8.0

When designing code you often end up expecting parameters to be not null but end up checking for null to avoid a NullReferenceException. As you already know, all reference types in C# can take the value of null. Value types like int and boolean cannot take a value of null unless explicitely specified using the nullable value type (int? or Nullable<int>). Maybe it should have been the other way around with reference types all along?

C# 8 can fix this with nullable reference types (maybe NOT nullable reference types is a better name). Since this is a breaking change, it is launched as an opt-in feature. Nullable reference types are a great way to avoid NullReferenceExceptions, since you are very explicit about where you expect null and where not.

To enable not nullable reference types, create a new .NET Core 3 project and add the following to the csproj file:

<LangVersion>8.0</LangVersion>
<Nullable>enable</Nullable>

You should be on C# 8 already, but to make it explicit, I’ve added the LangVersion element. The Nullable element enables nullable reference types. Out of the box, C# 8 creates a warning if it identifies the use of null where a value is expected. Let’s see how that looks:

class Program
{
    static void Main()
    {
        new Program().SayHello(null);
    }

    public void SayHello(string msg)
    {
        Console.WriteLine(msg);
    }
}

When compiling we see the following warning:

Program.cs(9,36): warning CS8625: Cannot convert null literal to non-nullable reference type. [C:projectscore3core3.csproj]

I know you are not one of them, but some people developed a warning-resistance which means that warnings are simply ignored. To overcome this, make sure that errors like this causes build errors over warnings by adding the following to csproj:

<WarningsAsErrors>CS8602,CS8603,CS8618,CS8625</WarningsAsErrors>

This will tell the C# compiler to treat these four nullable reference type warnings as errors instead.

Just to make it clear, allowing null in the msg parameter, you use the ? characters as with value types:

public void SayHello(string? msg)
{
    ...
}

Logging and monitoring

Logging and monitoring for null reference exceptions are a must. While some developers tempt to create control flow from exceptions (no-one should), null reference exceptions should never happen. This means that a System.NullReferenceException is a type of exception that should always be logged and fixed. A null check through either an if statement or the null-conditional operator is always the preferred way of handling potential null values. But make sure to implement a logging strategy that logs all uncaught exceptions, including the System.NullReferenceException.

When logging a System.NullReferenceException in a log file, database, elmah.io, or similar, it can be hard to spot what is null. You typically only see the method-name that causes the NullReferenceException and the Null Reference Analysis feature is only available while debugging inside Visual Studio. I will recommend you to always Include filename and line number in stack traces. This will pinpoint the exact line where the error happens.

See how we can help you monitor your website for crashes
Monitor your website