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Thread.ApartmentState Property

NOTE: This API is now obsolete. The non-obsolete alternative is GetApartmentState(). The non-obsolete alternative is SetApartmentState(ApartmentState).

Gets or sets the apartment state of this thread.

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

[ObsoleteAttribute("The ApartmentState property has been deprecated.  Use GetApartmentState, SetApartmentState or TrySetApartmentState instead.", 
public ApartmentState ApartmentState { get; set; }

Property Value

Type: System.Threading.ApartmentState
One of the ApartmentState values. The initial value is Unknown.


An attempt is made to set this property to a state that is not a valid apartment state (a state other than single-threaded apartment (STA) or multithreaded apartment (MTA)).

In the .NET Framework versions 1.0 and 1.1, the ApartmentState property marks a thread to indicate that it will execute in a single-threaded or multithreaded apartment. This property can be set when the thread is in the Unstarted or Running thread state; however, it can be set only once for a thread. If the property has not been set, it returns Unknown.

Important noteImportant Note:

In the .NET Framework version 2.0, new threads are initialized as ApartmentState.MTA if their apartment state has not been set before they are started. The main application thread is initialized to ApartmentState.MTA by default. You can no longer set the main application thread to ApartmentState.STA by setting the System.Threading.ApartmentState property on the first line of code. Use the STAThreadAttribute instead.

In the .NET Framework version 2.0, you can specify the COM threading model for a C++ application using the /CLRTHREADATTRIBUTE (Set CLR Thread Attribute) linker option.

The following code example demonstrates how to set the apartment state of a thread.

using System;
using System.Threading;

class ApartmentTest
    static void Main()
        Thread newThread = 
            new Thread(new ThreadStart(ThreadMethod));

        // The following line is ignored since  
        // ApartmentState can only be set once.

        Console.WriteLine("ThreadState: {0}, ApartmentState: {1}", 
            newThread.ThreadState, newThread.ApartmentState);


        // Wait for newThread to start and go to sleep.
            // This causes an exception since newThread is sleeping.
        catch(ThreadStateException stateException)
            Console.WriteLine("\n{0} caught:\n" +
                "Thread is not in the Unstarted or Running state.", 
            Console.WriteLine("ThreadState: {0}, ApartmentState: {1}",
                newThread.ThreadState, newThread.GetApartmentState());

    static void ThreadMethod()

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

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

.NET Framework

Supported in: 1.1, 1.0
Obsolete (compiler warning) in 3.5
Obsolete (compiler warning) in 3.5 SP1
Obsolete (compiler warning) in 3.0
Obsolete (compiler warning) in 3.0 SP1
Obsolete (compiler warning) in 3.0 SP2
Obsolete (compiler warning) in 2.0
Obsolete (compiler warning) in 2.0 SP1
Obsolete (compiler warning) in 2.0 SP2