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STAThreadAttribute Class

Indicates that the COM threading model for an application is single-threaded apartment (STA).

System.Object
  System.Attribute
    System.STAThreadAttribute

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

[ComVisibleAttribute(true)]
[AttributeUsageAttribute(AttributeTargets.Method)]
public sealed class STAThreadAttribute : Attribute

The STAThreadAttribute type exposes the following members.

  NameDescription
Public methodSupported in .NET for Windows Store appsSTAThreadAttributeInitializes a new instance of the STAThreadAttribute class.
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  NameDescription
Public propertyTypeIdWhen implemented in a derived class, gets a unique identifier for this Attribute. (Inherited from Attribute.)
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  NameDescription
Public methodSupported in .NET for Windows Store appsEqualsInfrastructure. Returns a value that indicates whether this instance is equal to a specified object. (Inherited from Attribute.)
Public methodSupported in .NET for Windows Store appsGetHashCodeReturns the hash code for this instance. (Inherited from Attribute.)
Public methodSupported in .NET for Windows Store appsGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodIsDefaultAttributeWhen overridden in a derived class, indicates whether the value of this instance is the default value for the derived class. (Inherited from Attribute.)
Public methodMatchWhen overridden in a derived class, returns a value that indicates whether this instance equals a specified object. (Inherited from Attribute.)
Public methodSupported in .NET for Windows Store appsToStringReturns a string that represents the current object. (Inherited from Object.)
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  NameDescription
Explicit interface implemetationPrivate method_Attribute.GetIDsOfNamesMaps a set of names to a corresponding set of dispatch identifiers. (Inherited from Attribute.)
Explicit interface implemetationPrivate method_Attribute.GetTypeInfoRetrieves the type information for an object, which can be used to get the type information for an interface. (Inherited from Attribute.)
Explicit interface implemetationPrivate method_Attribute.GetTypeInfoCountRetrieves the number of type information interfaces that an object provides (either 0 or 1). (Inherited from Attribute.)
Explicit interface implemetationPrivate method_Attribute.InvokeProvides access to properties and methods exposed by an object. (Inherited from Attribute.)
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Apply this attribute to the entry point method (the Main() method in C# and Visual Basic). It has no effect on other methods. To set the apartment state of threads you start in your code, use the Thread.SetApartmentState or Thread.TrySetApartmentState method before starting the thread.

NoteNote

For an overview of COM threading models, see Understanding and Using COM Threading Models.

COM threading models only apply to applications that use COM interop. The COM threading model can be set to single-threaded apartment or multithreaded apartment. The application thread is only initialized for COM interop if the thread actually makes a call to a COM component. If COM interop is not used, then the thread is not initialized, and the STAThreadAttribute attribute, if it is present, has no effect.

Starting with the .NET Framework version 2.0, the default threading model for COM interop depends on the language in which you are developing your application, as the following table shows.

Language

COM apartment model

C#

Multithreaded apartment

C++

Multithreaded apartment

Visual Basic

Single-threaded apartment

To change these defaults, you use the STAThreadAttribute attribute to set the threading model for the application, or call the Thread.SetApartmentState or Thread.TrySetApartmentState method before starting the thread to set the threading model for a particular thread. In C++, you can also use the /CLRTHREADATTRIBUTE linker option to specify the apartment model.

ASP.NET applications should set the ASPCompat attribute of the @ Page directive to true to force the page to be serviced by the STA thread pool.

Here are some of the cases in which you’ll want to use the STAThreadAttribute attribute to explicitly set the threading model to single-threaded apartment:

  • You're developing a Windows Forms app. Windows Forms apps must be single-threaded if they communicate with Windows system components such as the Clipboard or Windows common dialog boxes, or if they use system features such as drag-and-drop functionality. The Windows Forms Application template for C# automatically adds the STAThreadAttribute attribute to C# projects. Because the single-threaded apartment model is the default for Visual Basic, there is no need for the attribute.

  • You're developing a C# app that calls a Visual Basic library, which, in turn, relies on COM interop. Because the single-threaded apartment model is the default for Visual Basic, you should change your app's threading model to single-threaded by using the STAThreadAttribute attribute.

  • Your application makes calls to COM components that use the single-threaded apartment model.

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
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