NullReferenceException
Class
Definition
The exception that is thrown when there is an attempt to dereference a null object reference.
[System.Runtime.InteropServices.ComVisible(true)]
public class NullReferenceException : SystemException
- Inheritance
- Attributes
Inherited Members
System.Exception
System.Object
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,
namesis 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<String> names; if (value > 0) names = new List<String>(); names.Add("Major Major Major"); } } // 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()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<String> names = new List<String>(); 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 ModuleYou've forgotten to dimension an array before initializing it. In the following example,
valuesis 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()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 // 18Module 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 ' 18For 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
Personobject whoseFirstNamefield 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()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
nullbefore 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 foundModule 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 foundYou'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 returnsnull. As a result, your rest fornullis never evaluated.The following example defines a
Pagesobject that caches information about web pages, which are presented byPageobjects. TheExample.Mainmethod 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()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.CurrentPagereturnsnullif no page information is stored in the cache. This exception can be corrected by testing the value of theCurrentPageproperty before retrieving the currentPageobject'sTitleproperty, 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.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
forstatement 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()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 isnullbefore 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, , twoModule 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, , twoA 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 isnull, the method throws an 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<String> names = GetData(); PopulateNames(names); } private static void PopulateNames(List<String> names) { String[] arrNames = { "Dakota", "Samuel", "Nikita", "Koani", "Saya", "Yiska", "Yumaevsky" }; foreach (var arrName in arrNames) names.Add(arrName); } private static List<String> 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()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 atry…catch…finallyblock. For more information, see Exceptions.
The following Microsoft intermediate language (MSIL) instructions throw NullReferenceException: callvirt, cpblk, cpobj, initblk, ldelem.<type>, ldelema, ldfld, ldflda, ldind.<type>, ldlen, stelem.<type>, stfld, stind.<type>, 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 him 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
| NullReferenceException() |
Initializes a new instance of the NullReferenceException class, setting the Message property of the new instance to a system-supplied message that describes the error, such as "The value 'null' was found where an instance of an object was required." This message takes into account the current system culture. |
| NullReferenceException(String) |
Initializes a new instance of the NullReferenceException class with a specified error message. |
| NullReferenceException(SerializationInfo, StreamingContext) |
Initializes a new instance of the NullReferenceException class with serialized data. |
| NullReferenceException(String, Exception) |
Initializes a new instance of the NullReferenceException class with a specified error message and a reference to the inner exception that is the cause of this exception. |