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NullReferenceException.xml
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NullReferenceException.xml
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<Type Name="NullReferenceException" FullName="System.NullReferenceException">
<TypeSignature Language="C#" Value="public class NullReferenceException : Exception" FrameworkAlternate="dotnet-uwp-10.0;netcore-1.0;netcore-1.1;netstandard-1.0;netstandard-1.1;netstandard-1.2;netstandard-1.3;netstandard-1.4;netstandard-1.5;netstandard-1.6" />
<TypeSignature Language="ILAsm" Value=".class public auto ansi beforefieldinit NullReferenceException extends System.Exception" FrameworkAlternate="dotnet-uwp-10.0;netcore-1.0;netcore-1.1;netstandard-1.0;netstandard-1.1;netstandard-1.2;netstandard-1.3;netstandard-1.4;netstandard-1.5;netstandard-1.6" />
<TypeSignature Language="DocId" Value="T:System.NullReferenceException" />
<TypeSignature Language="VB.NET" Value="Public Class NullReferenceException
Inherits Exception" FrameworkAlternate="dotnet-uwp-10.0;netcore-1.0;netcore-1.1;netstandard-1.0;netstandard-1.1;netstandard-1.2;netstandard-1.3;netstandard-1.4;netstandard-1.5;netstandard-1.6" />
<TypeSignature Language="F#" Value="type NullReferenceException = class
 inherit Exception" FrameworkAlternate="dotnet-uwp-10.0;netcore-1.0;netcore-1.1;netstandard-1.0;netstandard-1.1;netstandard-1.2;netstandard-1.3;netstandard-1.4;netstandard-1.5;netstandard-1.6" />
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<TypeSignature Language="C#" Value="public class NullReferenceException : SystemException" FrameworkAlternate="net-5.0;net-6.0;net-7.0;net-8.0;net-9.0;netcore-2.0;netcore-2.1;netcore-2.2;netcore-3.0;netcore-3.1;netframework-1.1;netframework-2.0;netframework-3.0;netframework-3.5;netframework-4.0;netframework-4.5;netframework-4.5.1;netframework-4.5.2;netframework-4.6;netframework-4.6.1;netframework-4.6.2;netframework-4.7;netframework-4.7.1;netframework-4.7.2;netframework-4.8;netframework-4.8.1;netstandard-2.0;netstandard-2.1" />
<TypeSignature Language="ILAsm" Value=".class public auto ansi beforefieldinit NullReferenceException extends System.SystemException" FrameworkAlternate="net-5.0;net-6.0;net-7.0;net-8.0;net-9.0;netcore-2.0;netcore-2.1;netcore-2.2;netcore-3.0;netcore-3.1;netstandard-2.0;netstandard-2.1" />
<TypeSignature Language="VB.NET" Value="Public Class NullReferenceException
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<TypeSignature Language="F#" Value="type NullReferenceException = class
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<summary>The exception that is thrown when there is an attempt to dereference a null object reference.</summary>
<remarks>
<format type="text/markdown">< and [Arrays](/dotnet/visual-basic/programming-guide/language-features/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 <xref:System.Array.Find%2A?displayProperty=nameWithType> method always returns `Person` object whose `FirstName` field matches a search string. Because there is no match, the runtime throws a <xref:System.NullReferenceException> exception.
:::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/nullreturn2.cs" id="Snippet4":::
:::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/nullreturn2.fs" id="Snippet4":::
:::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/nullreturn2.vb" id="Snippet4":::
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.
:::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/nullreturn2a.cs" id="Snippet5":::
:::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/nullreturn2a.fs" id="Snippet5":::
:::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/nullreturn2a.vb" id="Snippet5":::
- 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 <xref:System.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 <xref:System.NullReferenceException> exception.
:::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/Chain1.cs" id="Snippet6":::
:::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/Chain1.fs" id="Snippet6":::
:::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/Chain1.vb" id="Snippet6":::
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:
:::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/Chain2.cs" id="Snippet7":::
:::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/Chain2.fs" id="Snippet7":::
:::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/Chain2.vb" id="Snippet7":::
- You're enumerating the elements of an array that contains reference types, and your attempt to process one of the elements throws a <xref:System.NullReferenceException> exception.
The following example defines a string array. A `for` statement enumerates the elements in the array and calls each string's <xref:System.String.Trim%2A> method before displaying the string.
:::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/Array1.cs" id="Snippet8":::
:::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/Array1.fs" id="Snippet8":::
:::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/Array1.vb" id="Snippet8":::
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.
:::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/Array2.cs" id="Snippet9":::
:::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/Array2.fs" id="Snippet9":::
:::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/Array2.vb" id="Snippet9":::
- A <xref:System.NullReferenceException> exception can be thrown by a method when it accesses a member of one of its arguments, but that argument is `null`. The `PopulateNames` method in the following example throws the exception at the line `names.Add(arrName);`.
:::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/example2.cs" id="Snippet3":::
:::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/example2.fs" id="Snippet3":::
:::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/example2.vb" id="Snippet3":::
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](/dotnet/standard/exceptions/).
The following Microsoft intermediate language (MSIL) instructions throw <xref:System.NullReferenceException>: `callvirt`, `cpblk`, `cpobj`, `initblk`, `ldelem.<type>`, `ldelema`, `ldfld`, `ldflda`, `ldind.<type>`, `ldlen`, `stelem.<type>`, `stfld`, `stind.<type>`, `throw`, and `unbox`.
<xref:System.NullReferenceException> uses the HRESULT COR_E_NULLREFERENCE, which has the value 0x80004003.
For a list of initial property values for an instance of <xref:System.NullReferenceException>, see the <xref:System.NullReferenceException.%23ctor%2A> 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.
]]></format>
</remarks>
<altmember cref="T:System.Exception" />
<related type="Article" href="/dotnet/standard/exceptions/">Handling and Throwing Exceptions</related>
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<summary>Initializes a new instance of the <see cref="T:System.NullReferenceException" /> class.</summary>
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<MemberSignature Language="C#" Value="public NullReferenceException ();" />
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 NullReferenceException();" />
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<Docs>
<summary>Initializes a new instance of the <see cref="T:System.NullReferenceException" /> class, setting the <see cref="P:System.Exception.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.</summary>
<remarks>
<format type="text/markdown"><![CDATA[
## Remarks
The following table shows the initial property values for an instance of <xref:System.NullReferenceException>.
|Property|Value|
|--------------|-----------|
|<xref:System.Exception.InnerException%2A>|`null`.|
|<xref:System.Exception.Message%2A>|The localized error message string.|
]]></format>
</remarks>
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<param name="message">A <see cref="T:System.String" /> that describes the error. The content of <paramref name="message" /> is intended to be understood by humans. The caller of this constructor is required to ensure that this string has been localized for the current system culture.</param>
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<format type="text/markdown"><![CDATA[
## Remarks
The following table shows the initial property values for an instance of <xref:System.NullReferenceException>.
|Property|Value|
|--------------|-----------|
|<xref:System.Exception.InnerException%2A>|`null`.|
|<xref:System.Exception.Message%2A>|The error message string.|
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<param name="info">The object that holds the serialized object data.</param>
<param name="context">The contextual information about the source or destination.</param>
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<related type="Article" href="/dotnet/standard/serialization/xml-and-soap-serialization">XML and SOAP Serialization</related>
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<param name="message">The error message that explains the reason for the exception.</param>
<param name="innerException">The exception that is the cause of the current exception. If the <paramref name="innerException" /> parameter is not <see langword="null" />, the current exception is raised in a <see langword="catch" /> block that handles the inner exception.</param>
<summary>Initializes a new instance of the <see cref="T:System.NullReferenceException" /> class with a specified error message and a reference to the inner exception that is the cause of this exception.</summary>
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## Remarks
An exception that is thrown as a direct result of a previous exception should include a reference to the previous exception in the <xref:System.Exception.InnerException%2A> property. The <xref:System.Exception.InnerException%2A> property returns the same value that is passed into the constructor, or `null` if the <xref:System.Exception.InnerException%2A> property does not supply the inner exception value to the constructor.
The following table shows the initial property values for an instance of <xref:System.NullReferenceException>.
|Property|Value|
|--------------|-----------|
|<xref:System.Exception.InnerException%2A>|The inner exception reference.|
|<xref:System.Exception.Message%2A>|The error message string.|
]]></format>
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<altmember cref="T:System.Exception" />
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