Which exception is thrown when an instance method of a null object gets called access or modifies the field of a null object?

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NullReferenceException Class

  • Reference

Definition

The exception that is thrown when there is an attempt to dereference a null object reference.

In this article

public ref class NullReferenceException : Exceptionpublic ref class NullReferenceException : SystemExceptionpublic class NullReferenceException : Exceptionpublic class NullReferenceException : SystemException[System.Serializable] public class NullReferenceException : SystemException[System.Serializable] [System.Runtime.InteropServices.ComVisible(true)] public class NullReferenceException : SystemExceptiontype NullReferenceException = class inherit Exceptiontype NullReferenceException = class inherit SystemException[] type NullReferenceException = class inherit SystemException[] [] type NullReferenceException = class inherit SystemExceptionPublic Class NullReferenceException Inherits ExceptionPublic Class NullReferenceException Inherits SystemExceptionInheritanceInheritanceAttributes

Remarks

A NullReferenceException exception is thrown when you try to access a member on a type whose value is null. A NullReferenceException exception typically reflects developer error and is thrown in the following scenarios:

  • You've forgotten to instantiate a reference type. In the following example, names is declared but never instantiated:

    using System; using System.Collections.Generic; public class Example { public static void Main(string[] args) { int value = Int32.Parse(args[0]); List names; if (value > 0) names = new List(); names.Add("Major Major Major"); } } // Compilation displays a warning like the following: // Example1.cs(10) : warning BC42104: Variable //names// is used before it // has been assigned a value. A null reference exception could result // at runtime. // // names.Add("Major Major Major") // ~~~~~ // The example displays output like the following output: // Unhandled Exception: System.NullReferenceException: Object reference // not set to an instance of an object. // at Example.Main() open System [] let main args = let value = Int32.Parse args[0] // Set names to null, don't initialize it. let mutable names = Unchecked.defaultof> if value > 0 then names <- ResizeArray() names.Add "Major Major Major" 0 // Compilation does not display a warning as this is an extremely rare occurance in F#. // Creating a value without initalizing either requires using 'null' (not possible // on types defined in F# without []) or Unchecked.defaultof. // // The example displays output like the following output: // Unhandled Exception: System.NullReferenceException: Object reference // not set to an instance of an object. // at Example.main() Imports System.Collections.Generic Module Example Public Sub Main() Dim names As List(Of String) names.Add("Major Major Major") End Sub End Module ' Compilation displays a warning like the following: ' Example1.vb(10) : warning BC42104: Variable 'names' is used before it ' has been assigned a value. A null reference exception could result ' at runtime. ' ' names.Add("Major Major Major") ' ~~~~~ ' The example displays output like the following output: ' Unhandled Exception: System.NullReferenceException: Object reference ' not set to an instance of an object. ' at Example.Main()

    Some compilers issue a warning when they compile this code. Others issue an error, and the compilation fails. To address this problem, instantiate the object so that its value is no longer null. The following example does this by calling a type's class constructor.

    using System; using System.Collections.Generic; public class Example { public static void Main() { List names = new List(); names.Add("Major Major Major"); } } let names = ResizeArray() names.Add "Major Major Major" Imports System.Collections.Generic Module Example Public Sub Main() Dim names As New List(Of String)() names.Add("Major Major Major") End Sub End Module
  • You've forgotten to dimension an array before initializing it. In the following example, values is declared to be an integer array, but the number of elements that it contains is never specified. The attempt to initialize its values therefore thrown a NullReferenceException exception.

    using System; public class Example { public static void Main() { int[] values = null; for (int ctr = 0; ctr <= 9; ctr++) values[ctr] = ctr * 2; foreach (var value in values) Console.WriteLine(value); } } // The example displays the following output: // Unhandled Exception: // System.NullReferenceException: Object reference not set to an instance of an object. // at Example.Main() let values: int[] = null for i = 0 to 9 do values[i] <- i * 2 for value in values do printfn $"{value}" // The example displays the following output: // Unhandled Exception: // System.NullReferenceException: Object reference not set to an instance of an object. // at .main() Module Example Public Sub Main() Dim values() As Integer For ctr As Integer = 0 To 9 values(ctr) = ctr * 2 Next For Each value In values Console.WriteLine(value) Next End Sub End Module ' The example displays the following output: ' Unhandled Exception: ' System.NullReferenceException: Object reference not set to an instance of an object. ' at Example.Main()

    You can eliminate the exception by declaring the number of elements in the array before initializing it, as the following example does.

    using System; public class Example { public static void Main() { int[] values = new int[10]; for (int ctr = 0; ctr <= 9; ctr++) values[ctr] = ctr * 2; foreach (var value in values) Console.WriteLine(value); } } // The example displays the following output: // 0 // 2 // 4 // 6 // 8 // 10 // 12 // 14 // 16 // 18 let values = Array.zeroCreate 10 for i = 0 to 9 do values[i] <- i * 2 for value in values do printfn $"{value}" // The example displays the following output: // 0 // 2 // 4 // 6 // 8 // 10 // 12 // 14 // 16 // 18 Module Example Public Sub Main() Dim values(9) As Integer For ctr As Integer = 0 To 9 values(ctr) = ctr * 2 Next For Each value In values Console.WriteLine(value) Next End Sub End Module ' The example displays the following output: ' 0 ' 2 ' 4 ' 6 ' 8 ' 10 ' 12 ' 14 ' 16 ' 18

    For more information on declaring and initializing arrays, see Arrays and Arrays.

  • You get a null return value from a method, and then call a method on the returned type. This sometimes is the result of a documentation error; the documentation fails to note that a method call can return null. In other cases, your code erroneously assumes that the method will always return a non-null value.

    The code in the following example assumes that the Array.Find method always returns Person object whose FirstName field matches a search string. Because there is no match, the runtime throws a NullReferenceException exception.

    using System; public class Example { public static void Main() { Person[] persons = Person.AddRange( new String[] { "Abigail", "Abra", "Abraham", "Adrian", "Ariella", "Arnold", "Aston", "Astor" } ); String nameToFind = "Robert"; Person found = Array.Find(persons, p => p.FirstName == nameToFind); Console.WriteLine(found.FirstName); } } public class Person { public static Person[] AddRange(String[] firstNames) { Person[] p = new Person[firstNames.Length]; for (int ctr = 0; ctr < firstNames.Length; ctr++) p[ctr] = new Person(firstNames[ctr]); return p; } public Person(String firstName) { this.FirstName = firstName; } public String FirstName; } // The example displays the following output: // Unhandled Exception: System.NullReferenceException: // Object reference not set to an instance of an object. // at Example.Main() open System type Person(firstName) = member _.FirstName = firstName static member AddRange(firstNames) = Array.map Person firstNames let persons = [| "Abigail"; "Abra"; "Abraham"; "Adrian" "Ariella"; "Arnold"; "Aston"; "Astor" |] |> Person.AddRange let nameToFind = "Robert" let found = Array.Find(persons, fun p -> p.FirstName = nameToFind) printfn $"{found.FirstName}" // The example displays the following output: // Unhandled Exception: System.NullReferenceException: // Object reference not set to an instance of an object. // at .main() Module Example Public Sub Main() Dim persons() As Person = Person.AddRange( { "Abigail", "Abra", "Abraham", "Adrian", "Ariella", "Arnold", "Aston", "Astor" } ) Dim nameToFind As String = "Robert" Dim found As Person = Array.Find(persons, Function(p) p.FirstName = nameToFind) Console.WriteLine(found.FirstName) End Sub End Module Public Class Person Public Shared Function AddRange(firstNames() As String) As Person() Dim p(firstNames.Length - 1) As Person For ctr As Integer = 0 To firstNames.Length - 1 p(ctr) = New Person(firstNames(ctr)) Next Return p End Function Public Sub New(firstName As String) Me.FirstName = firstName End Sub Public FirstName As String End Class ' The example displays the following output: ' Unhandled Exception: System.NullReferenceException: ' Object reference not set to an instance of an object. ' at Example.Main()

    To address this problem, test the method's return value to ensure that it is not null before calling any of its members, as the following example does.

    using System; public class Example { public static void Main() { Person[] persons = Person.AddRange( new String[] { "Abigail", "Abra", "Abraham", "Adrian", "Ariella", "Arnold", "Aston", "Astor" } ); String nameToFind = "Robert"; Person found = Array.Find(persons, p => p.FirstName == nameToFind); if (found != null) Console.WriteLine(found.FirstName); else Console.WriteLine("{0} not found.", nameToFind); } } public class Person { public static Person[] AddRange(String[] firstNames) { Person[] p = new Person[firstNames.Length]; for (int ctr = 0; ctr < firstNames.Length; ctr++) p[ctr] = new Person(firstNames[ctr]); return p; } public Person(String firstName) { this.FirstName = firstName; } public String FirstName; } // The example displays the following output: // Robert not found open System [] type Person(firstName) = member _.FirstName = firstName static member AddRange(firstNames) = Array.map Person firstNames let persons = [| "Abigail"; "Abra"; "Abraham"; "Adrian" "Ariella"; "Arnold"; "Aston"; "Astor" |] |> Person.AddRange let nameToFind = "Robert" let found = Array.Find(persons, fun p -> p.FirstName = nameToFind) if found <> null then printfn $"{found.FirstName}" else printfn $"{nameToFind} not found." // Using F#'s Array.tryFind function // This does not require a null check or [] let found2 = persons |> Array.tryFind (fun p -> p.FirstName = nameToFind) match found2 with | Some firstName -> printfn $"{firstName}" | None -> printfn $"{nameToFind} not found." // The example displays the following output: // Robert not found. // Robert not found. Module Example Public Sub Main() Dim persons() As Person = Person.AddRange( { "Abigail", "Abra", "Abraham", "Adrian", "Ariella", "Arnold", "Aston", "Astor" } ) Dim nameToFind As String = "Robert" Dim found As Person = Array.Find(persons, Function(p) p.FirstName = nameToFind) If found IsNot Nothing Then Console.WriteLine(found.FirstName) Else Console.WriteLine("{0} not found.", nameToFind) End If End Sub End Module Public Class Person Public Shared Function AddRange(firstNames() As String) As Person() Dim p(firstNames.Length - 1) As Person For ctr As Integer = 0 To firstNames.Length - 1 p(ctr) = New Person(firstNames(ctr)) Next Return p End Function Public Sub New(firstName As String) Me.FirstName = firstName End Sub Public FirstName As String End Class ' The example displays the following output: ' Robert not found
  • You're using an expression (for example, you're chaining a list of methods or properties together) to retrieve a value and, although you're checking whether the value is null, the runtime still throws a NullReferenceException exception. This occurs because one of the intermediate values in the expression returns null. As a result, your test for null is never evaluated.

    The following example defines a Pages object that caches information about web pages, which are presented by Page objects. The Example.Main method checks whether the current web page has a non-null title and, if it does, displays the title. Despite this check, however, the method throws a NullReferenceException exception.

    using System; public class Example { public static void Main() { var pages = new Pages(); if (! String.IsNullOrEmpty(pages.CurrentPage.Title)) { String title = pages.CurrentPage.Title; Console.WriteLine("Current title: '{0}'", title); } } } public class Pages { Page[] page = new Page[10]; int ctr = 0; public Page CurrentPage { get { return page[ctr]; } set { // Move all the page objects down to accommodate the new one. if (ctr > page.GetUpperBound(0)) { for (int ndx = 1; ndx <= page.GetUpperBound(0); ndx++) page[ndx - 1] = page[ndx]; } page[ctr] = value; if (ctr < page.GetUpperBound(0)) ctr++; } } public Page PreviousPage { get { if (ctr == 0) { if (page[0] == null) return null; else return page[0]; } else { ctr--; return page[ctr + 1]; } } } } public class Page { public Uri URL; public String Title; } // The example displays the following output: // Unhandled Exception: // System.NullReferenceException: Object reference not set to an instance of an object. // at Example.Main() open System type Page() = [] val mutable public URL: Uri [] val mutable public Title: string type Pages() = let pages = Array.zeroCreate 10 let mutable i = 0 member _.CurrentPage with get () = pages[i] and set (value) = // Move all the page objects down to accommodate the new one. if i > pages.GetUpperBound 0 then for ndx = 1 to pages.GetUpperBound 0 do pages[ndx - 1] <- pages[ndx] pages[i] <- value if i < pages.GetUpperBound 0 then i <- i + 1 member _.PreviousPage = if i = 0 then if box pages[0] = null then Unchecked.defaultof else pages[0] else i <- i - 1 pages[i + 1] let pages = Pages() if String.IsNullOrEmpty pages.CurrentPage.Title |> not then let title = pages.CurrentPage.Title printfn $"Current title: '{title}'" // The example displays the following output: // Unhandled Exception: // System.NullReferenceException: Object reference not set to an instance of an object. // at .main() Module Example Public Sub Main() Dim pages As New Pages() Dim title As String = pages.CurrentPage.Title End Sub End Module Public Class Pages Dim page(9) As Page Dim ctr As Integer = 0 Public Property CurrentPage As Page Get Return page(ctr) End Get Set ' Move all the page objects down to accommodate the new one. If ctr > page.GetUpperBound(0) Then For ndx As Integer = 1 To page.GetUpperBound(0) page(ndx - 1) = page(ndx) Next End If page(ctr) = value If ctr < page.GetUpperBound(0) Then ctr += 1 End Set End Property Public ReadOnly Property PreviousPage As Page Get If ctr = 0 Then If page(0) Is Nothing Then Return Nothing Else Return page(0) End If Else ctr -= 1 Return page(ctr + 1) End If End Get End Property End Class Public Class Page Public URL As Uri Public Title As String End Class ' The example displays the following output: ' Unhandled Exception: ' System.NullReferenceException: Object reference not set to an instance of an object. ' at Example.Main()

    The exception is thrown because pages.CurrentPage returns null if no page information is stored in the cache. This exception can be corrected by testing the value of the CurrentPage property before retrieving the current Page object's Title property, as the following example does:

    using System; public class Example { public static void Main() { var pages = new Pages(); Page current = pages.CurrentPage; if (current != null) { String title = current.Title; Console.WriteLine("Current title: '{0}'", title); } else { Console.WriteLine("There is no page information in the cache."); } } } // The example displays the following output: // There is no page information in the cache. let pages = Pages() let current = pages.CurrentPage if box current <> null then let title = current.Title printfn $"Current title: '{title}'" else printfn "There is no page information in the cache." // The example displays the following output: // There is no page information in the cache. Module Example Public Sub Main() Dim pages As New Pages() Dim current As Page = pages.CurrentPage If current IsNot Nothing Then Dim title As String = current.Title Console.WriteLine("Current title: '{0}'", title) Else Console.WriteLine("There is no page information in the cache.") End If End Sub End Module ' The example displays the following output: ' There is no page information in the cache.
  • You're enumerating the elements of an array that contains reference types, and your attempt to process one of the elements throws a NullReferenceException exception.

    The following example defines a string array. A for statement enumerates the elements in the array and calls each string's Trim method before displaying the string.

    using System; public class Example { public static void Main() { String[] values = { "one", null, "two" }; for (int ctr = 0; ctr <= values.GetUpperBound(0); ctr++) Console.Write("{0}{1}", values[ctr].Trim(), ctr == values.GetUpperBound(0) ? "" : ", "); Console.WriteLine(); } } // The example displays the following output: // Unhandled Exception: // System.NullReferenceException: Object reference not set to an instance of an object. // at Example.Main() open System let values = [| "one"; null; "two" |] for i = 0 to values.GetUpperBound 0 do printfn $"""{values[i].Trim()}{if i = values.GetUpperBound 0 then "" else ", "}""" printfn "" // The example displays the following output: // Unhandled Exception: // System.NullReferenceException: Object reference not set to an instance of an object. // at .main() Module Example Public Sub Main() Dim values() As String = { "one", Nothing, "two" } For ctr As Integer = 0 To values.GetUpperBound(0) Console.Write("{0}{1}", values(ctr).Trim(), If(ctr = values.GetUpperBound(0), "", ", ")) Next Console.WriteLine() End Sub End Module ' The example displays the following output: ' Unhandled Exception: System.NullReferenceException: ' Object reference not set to an instance of an object. ' at Example.Main()

    This exception occurs if you assume that each element of the array must contain a non-null value, and the value of the array element is in fact null. The exception can be eliminated by testing whether the element is null before performing any operation on that element, as the following example shows.

    using System; public class Example { public static void Main() { String[] values = { "one", null, "two" }; for (int ctr = 0; ctr <= values.GetUpperBound(0); ctr++) Console.Write("{0}{1}", values[ctr] != null ? values[ctr].Trim() : "", ctr == values.GetUpperBound(0) ? "" : ", "); Console.WriteLine(); } } // The example displays the following output: // one, , two open System let values = [| "one"; null; "two" |] for i = 0 to values.GetUpperBound 0 do printf $"""{if values[i] <> null then values[i].Trim() else ""}{if i = values.GetUpperBound 0 then "" else ", "}""" Console.WriteLine() // The example displays the following output: // one, , two Module Example Public Sub Main() Dim values() As String = { "one", Nothing, "two" } For ctr As Integer = 0 To values.GetUpperBound(0) Console.Write("{0}{1}", If(values(ctr) IsNot Nothing, values(ctr).Trim(), ""), If(ctr = values.GetUpperBound(0), "", ", ")) Next Console.WriteLine() End Sub End Module ' The example displays the following output: ' one, , two
  • A NullReferenceException exception is thrown by a method that is passed null. Some methods validate the arguments that are passed to them. If they do and one of the arguments is null, the method throws an System.ArgumentNullException exception. Otherwise, it throws a NullReferenceException exception. The following example illustrates this scenario.

    using System; using System.Collections.Generic; public class Example { public static void Main() { List names = GetData(); PopulateNames(names); } private static void PopulateNames(List names) { String[] arrNames = { "Dakota", "Samuel", "Nikita", "Koani", "Saya", "Yiska", "Yumaevsky" }; foreach (var arrName in arrNames) names.Add(arrName); } private static List GetData() { return null; } } // The example displays output like the following: // Unhandled Exception: System.NullReferenceException: Object reference // not set to an instance of an object. // at Example.PopulateNames(List`1 names) // at Example.Main() let populateNames (names: ResizeArray) = let arrNames = [ "Dakota"; "Samuel"; "Nikita" "Koani"; "Saya"; "Yiska"; "Yumaevsky" ] for arrName in arrNames do names.Add arrName let getData () : ResizeArray = null let names = getData () populateNames names // The example displays output like the following: // Unhandled Exception: System.NullReferenceException: Object reference // not set to an instance of an object. // at Example.PopulateNames(List`1 names) // at .main() Imports System.Collections.Generic Module Example Public Sub Main() Dim names As List(Of String) = GetData() PopulateNames(names) End Sub Private Sub PopulateNames(names As List(Of String)) Dim arrNames() As String = { "Dakota", "Samuel", "Nikita", "Koani", "Saya", "Yiska", "Yumaevsky" } For Each arrName In arrNames names.Add(arrName) Next End Sub Private Function GetData() As List(Of String) Return Nothing End Function End Module ' The example displays output like the following: ' Unhandled Exception: System.NullReferenceException: Object reference ' not set to an instance of an object. ' at Example.PopulateNames(List`1 names) ' at Example.Main()

    To address this issue, make sure that the argument passed to the method is not null, or handle the thrown exception in a try…catch…finally block. For more information, see Exceptions.

The following Microsoft intermediate language (MSIL) instructions throw NullReferenceException: callvirt, cpblk, cpobj, initblk, ldelem., ldelema, ldfld, ldflda, ldind., ldlen, stelem., stfld, stind., throw, and unbox.

NullReferenceException uses the HRESULT COR_E_NULLREFERENCE, which has the value 0x80004003.

For a list of initial property values for an instance of NullReferenceException, see the NullReferenceException constructors.

Handling NullReferenceException in release code

It's usually better to avoid a NullReferenceException than to handle it after it occurs. Handling an exception can make your code harder to maintain and understand, and can sometimes introduce other bugs. A NullReferenceException is often a non-recoverable error. In these cases, letting the exception stop the app might be the best alternative.

However, there are many situations where handling the error can be useful:

  • Your app can ignore objects that are null. For example, if your app retrieves and processes records in a database, you might be able to ignore some number of bad records that result in null objects. Recording the bad data in a log file or in the application UI might be all you have to do.

  • You can recover from the exception. For example, a call to a web service that returns a reference type might return null if the connection is lost or the connection times out. You can attempt to reestablish the connection and try the call again.

  • You can restore the state of your app to a valid state. For example, you might be performing a multi-step task that requires you to save information to a data store before you call a method that throws a NullReferenceException. If the uninitialized object would corrupt the data record, you can remove the previous data before you close the app.

  • You want to report the exception. For example, if the error was caused by a mistake from the user of your app, you can generate a message to help them supply the correct information. You can also log information about the error to help you fix the problem. Some frameworks, like ASP.NET, have a high-level exception handler that captures all errors to that the app never crashes; in that case, logging the exception might be the only way you can know that it occurs.

Constructors

Properties

Data

Gets a collection of key/value pairs that provide additional user-defined information about the exception.

(Inherited from Exception)
HelpLink

Gets or sets a link to the help file associated with this exception.

(Inherited from Exception)
HResult

Gets or sets HRESULT, a coded numerical value that is assigned to a specific exception.

(Inherited from Exception)
InnerException

Gets the Exception instance that caused the current exception.

(Inherited from Exception)
Message

Gets a message that describes the current exception.

(Inherited from Exception)
Source

Gets or sets the name of the application or the object that causes the error.

(Inherited from Exception)
StackTrace

Gets a string representation of the immediate frames on the call stack.

(Inherited from Exception)
TargetSite

Gets the method that throws the current exception.

(Inherited from Exception)

Methods

Equals(Object)

Determines whether the specified object is equal to the current object.

(Inherited from Object)
GetBaseException()

When overridden in a derived class, returns the Exception that is the root cause of one or more subsequent exceptions.

(Inherited from Exception)
GetHashCode()

Serves as the default hash function.

(Inherited from Object)
GetObjectData(SerializationInfo, StreamingContext)

When overridden in a derived class, sets the SerializationInfo with information about the exception.

(Inherited from Exception)
GetType()

Gets the runtime type of the current instance.

(Inherited from Exception)
MemberwiseClone()

Creates a shallow copy of the current Object.

(Inherited from Object)
ToString()

Creates and returns a string representation of the current exception.

(Inherited from Exception)

Events

SerializeObjectState

Occurs when an exception is serialized to create an exception state object that contains serialized data about the exception.

(Inherited from Exception)

Applies to

See also

  • Exception
  • Handling and Throwing Exceptions

Which exception is thrown when an instance method of a null object gets called access or modifiers the field of a null object?

NullPointerException is thrown when program attempts to use an object reference that has the null value. These can be: Invoking a method from a null object. Accessing or modifying a null object's field.

Which exception is thrown when an instance method of a null object?

NullPointerException is thrown when an application attempts to use an object reference that has the null value. These include: Calling an instance method on the object referred by a null reference.

Which exception is thrown when an instance method?

I suspect Calling the instance method of a null object is the reason for it throwing a null pointer exception.

What causes a NullPointerException?

What Causes NullPointerException. The NullPointerException occurs due to a situation in application code where an uninitialized object is attempted to be accessed or modified. Essentially, this means the object reference does not point anywhere and has a null value.