STAThreadAttribute Class
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STAThreadAttribute Class


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

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


type STAThreadAttribute = 
        inherit Attribute


Initializes a new instance of the STAThreadAttribute class.


When implemented in a derived class, gets a unique identifier for this Attribute.(Inherited from Attribute.)


This API supports the product infrastructure and is not intended to be used directly from your code. Returns a value that indicates whether this instance is equal to a specified object.(Inherited from Attribute.)


Returns the hash code for this instance.(Inherited from Attribute.)


Gets the Type of the current instance.(Inherited from Object.)


When overridden in a derived class, indicates whether the value of this instance is the default value for the derived class.(Inherited from Attribute.)


When overridden in a derived class, returns a value that indicates whether this instance equals a specified object.(Inherited from Attribute.)


Returns a string that represents the current object.(Inherited from Object.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.GetIDsOfNames(Guid, IntPtr, UInt32, UInt32, IntPtr)

Maps a set of names to a corresponding set of dispatch identifiers.(Inherited from Attribute.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.GetTypeInfo(UInt32, UInt32, IntPtr)

Retrieves the type information for an object, which can be used to get the type information for an interface.(Inherited from Attribute.)


Retrieves the number of type information interfaces that an object provides (either 0 or 1).(Inherited from Attribute.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.Invoke(UInt32, Guid, UInt32, Int16, IntPtr, IntPtr, IntPtr, IntPtr)

Provides access to properties and methods exposed by an object.(Inherited from Attribute.)

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.


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.


COM apartment model


Multithreaded apartment


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 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.

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1

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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