Thread.SetApartmentState(ApartmentState) Method

Definition

Namespace:

System.Threading

Assembly:

System.Threading.Thread.dll

Source:

Thread.cs

Sets the apartment state of a thread before it is started.

public:
 void SetApartmentState(System::Threading::ApartmentState state);

[System.Runtime.Versioning.SupportedOSPlatform("windows")]
public void SetApartmentState (System.Threading.ApartmentState state);

[<System.Runtime.Versioning.SupportedOSPlatform("windows")>]
member this.SetApartmentState : System.Threading.ApartmentState -> unit

Public Sub SetApartmentState (state As ApartmentState)

Parameters

state

ApartmentState

The new apartment state.

Attributes

SupportedOSPlatformAttribute

Exceptions

PlatformNotSupportedException

.NET Core and .NET 5+ only: In all cases on macOS and Linux.

ArgumentException

state is not a valid apartment state.

ThreadStateException

The thread has already been started.

InvalidOperationException

The apartment state has already been initialized.

Examples

The following code example demonstrates the GetApartmentState, SetApartmentState, and TrySetApartmentState methods. The code example creates a thread. Before the thread is started, GetApartmentState displays the initial ApartmentState.Unknown state and SetApartmentState changes the state to ApartmentState.STA. The TrySetApartmentState method then returns false when attempting to change the state to ApartmentState.MTA because the apartment state is already set. If the same operation had been attempted with SetApartmentState, InvalidOperationException would have been thrown.

After the thread is started, the TrySetApartmentState method is used again. This time it throws ThreadStateException because the thread has already been started.

using namespace System;
using namespace System::Threading;

void ThreadProc()
{
    Thread::Sleep(2000);
};

void main()
{
    Thread^ t = gcnew Thread(gcnew ThreadStart(ThreadProc));
    Console::WriteLine("Before setting apartment state: {0}", 
            t->GetApartmentState());

    t->SetApartmentState(ApartmentState::STA);
    Console::WriteLine("After setting apartment state: {0}", 
        t->GetApartmentState());

    bool result = t->TrySetApartmentState(ApartmentState::MTA);
    Console::WriteLine("Try to change state: {0}", result);

    t->Start();

    Thread::Sleep(500);

    try
    {
        t->TrySetApartmentState(ApartmentState::STA);
    }
    catch (ThreadStateException^)
    {
        Console::WriteLine("ThreadStateException occurs " +
            "if apartment state is set after starting thread.");
    }

    t->Join();
}

/* This code example produces the following output:

Before setting apartment state: Unknown
After setting apartment state: STA
Try to change state: False
ThreadStateException occurs if apartment state is set after starting thread.
 */

using System;
using System.Threading;

class Example
{
    public static void Main()
    {
        Thread t = new Thread(ThreadProc);
        Console.WriteLine("Before setting apartment state: {0}",
            t.GetApartmentState());

        t.SetApartmentState(ApartmentState.STA);
        Console.WriteLine("After setting apartment state: {0}",
            t.GetApartmentState());

        bool result = t.TrySetApartmentState(ApartmentState.MTA);
        Console.WriteLine("Try to change state: {0}", result);

        t.Start();

        Thread.Sleep(500);

        try
        {
            t.TrySetApartmentState(ApartmentState.STA);
        }
        catch (ThreadStateException)
        {
            Console.WriteLine("ThreadStateException occurs " +
                "if apartment state is set after starting thread.");
        }

        t.Join();
    }

    public static void ThreadProc()
    {
        Thread.Sleep(2000);
    }
}

/* This code example produces the following output:

Before setting apartment state: Unknown
After setting apartment state: STA
Try to change state: False
ThreadStateException occurs if apartment state is set after starting thread.
 */

open System.Threading

let threadProc () = Thread.Sleep 2000

let t = Thread threadProc
printfn $"Before setting apartment state: {t.GetApartmentState()}"

t.SetApartmentState ApartmentState.STA
printfn $"After setting apartment state: {t.GetApartmentState()}"

let result = t.TrySetApartmentState ApartmentState.MTA
printfn $"Try to change state: {result}"

t.Start()

Thread.Sleep 500

try
    t.TrySetApartmentState ApartmentState.STA |> ignore
with :? ThreadStateException ->
    printfn "ThreadStateException occurs if apartment state is set after starting thread."

t.Join()

// This code example produces the following output:
//     Before setting apartment state: Unknown
//     After setting apartment state: STA
//     Try to change state: False
//     ThreadStateException occurs if apartment state is set after starting thread.

Imports System.Threading

Module Example

    Sub Main()
 
        Dim t As New Thread(AddressOf ThreadProc)
        Console.WriteLine("Before setting apartment state: {0}", _
            t.GetApartmentState())

        t.SetApartmentState(ApartmentState.STA)
        Console.WriteLine("After setting apartment state: {0}", _
            t.GetApartmentState())

        Dim result As Boolean = _
            t.TrySetApartmentState(ApartmentState.MTA)
        Console.WriteLine("Try to change state: {0}", result)

        t.Start()

        Thread.Sleep(500)

        Try
            t.TrySetApartmentState(ApartmentState.STA)
        Catch ex As ThreadStateException
            Console.WriteLine("ThreadStateException occurs " & _
                "if apartment state is set after starting thread.")
        End Try

        t.Join()
    End Sub

    Sub ThreadProc()
        Thread.Sleep(2000)
    End Sub
End Module

' This code example produces the following output:
'
'Before setting apartment state: Unknown
'After setting apartment state: STA
'Try to change state: False
'ThreadStateException occurs if apartment state is set after starting thread.

Remarks

New threads are initialized as ApartmentState.MTA if their apartment state has not been set before they are started. Apartment state must be set before a thread is started.

Note

The main application thread is initialized to ApartmentState.MTA by default. The only way to set the apartment state of the main application thread to ApartmentState.STA is to apply the STAThreadAttribute attribute to the entry point method.

The SetApartmentState method, along with the GetApartmentState method and the TrySetApartmentState method, replaces the ApartmentState property.