Thread.Join Method

Blocks the calling thread until a thread terminates, while continuing to perform standard COM and SendMessage pumping.

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

[HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization = true, 
	ExternalThreading = true)]
public void Join()


The caller attempted to join a thread that is in the ThreadState.Unstarted state.


The thread is interrupted while waiting.

Join is a synchronization method that blocks the calling thread (that is, the thread that calls the method) until the thread whose Join method is called has completed. Use this method to ensure that a thread has been terminated. The caller will block indefinitely if the thread does not terminate. In the following example, the Thread1 thread calls the Join() method of Thread2, which causes Thread1 to block until Thread2 has completed.

using System;
using System.Threading;

public class Example
   static Thread mainThread, thread1, thread2;

   public static void Main()
      mainThread = Thread.CurrentThread;
      thread1 = new Thread(ThreadProc);
      thread1.Name = "Thread1";

      thread2 = new Thread(ThreadProc);
      thread2.Name = "Thread2";

   private static void ThreadProc()
      Console.WriteLine("\nCurrent thread: {0}", Thread.CurrentThread.Name);
      if (Thread.CurrentThread.Name == "Thread1" && 
          thread2.ThreadState != ThreadState.Unstarted)

      Console.WriteLine("\nCurrent thread: {0}", Thread.CurrentThread.Name);
      Console.WriteLine("Thread1: {0}", thread1.ThreadState);
      Console.WriteLine("Thread2: {0}\n", thread2.ThreadState);
// The example displays output like the following: 
//       Current thread: Thread1 
//       Current thread: Thread2 
//       Current thread: Thread2 
//       Thread1: WaitSleepJoin 
//       Thread2: Running 
//       Current thread: Thread1 
//       Thread1: Running 
//       Thread2: Stopped

If the thread has already terminated when Join is called, the method returns immediately.

Caution noteCaution

You should never call the Join method of the Thread object that represents the current thread from the current thread. This causes your app to hang because the current thread waits upon itself indefinitely,

This method changes the state of the calling thread to include ThreadState.WaitSleepJoin. You cannot invoke Join on a thread that is in the ThreadState.Unstarted state.


The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: Synchronization | ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

.NET for Windows Phone apps

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