Pausing and Resuming Threads

The most common ways to synchronize the activities of threads are to block and release threads, or to lock objects or regions of code. For more information on these locking and blocking mechanisms, see Overview of Synchronization Primitives.

You can also have threads put themselves to sleep. When threads are blocked or sleeping, you can use a ThreadInterruptedException to break them out of their wait states.

The Thread.Sleep Method

Calling the Thread.Sleep method causes the current thread to immediately block for the number of milliseconds or the time interval you pass to the method, and yields the remainder of its time slice to another thread. Once that interval elapses, the sleeping thread resumes execution.

One thread cannot call Thread.Sleep on another thread. Thread.Sleep is a static method that always causes the current thread to sleep.

Calling Thread.Sleep with a value of Timeout.Infinite causes a thread to sleep until it is interrupted by another thread that calls the Thread.Interrupt method on the sleeping thread, or until it is terminated by a call to its Thread.Abort method. The following example illustrates both methods of interrupting a sleeping thread.

using System;
using System.Threading;

public class Example
{
   public static void Main()
   {
      // Interrupt a sleeping thread. 
      var sleepingThread = new Thread(Example.SleepIndefinitely);
      sleepingThread.Name = "Sleeping";
      sleepingThread.Start();
      Thread.Sleep(2000);
      sleepingThread.Interrupt();
      
      Thread.Sleep(1000);
      
      sleepingThread = new Thread(Example.SleepIndefinitely);
      sleepingThread.Name = "Sleeping2";
      sleepingThread.Start();
      Thread.Sleep(2000);
      sleepingThread.Abort();
   }

   private static void SleepIndefinitely()
   {
      Console.WriteLine("Thread '{0}' about to sleep indefinitely.",
                        Thread.CurrentThread.Name);
      try {
         Thread.Sleep(Timeout.Infinite);
      }
      catch (ThreadInterruptedException) {
         Console.WriteLine("Thread '{0}' awoken.",
                           Thread.CurrentThread.Name);
      }
      catch (ThreadAbortException) {
         Console.WriteLine("Thread '{0}' aborted.",
                           Thread.CurrentThread.Name);
      }
      finally 
      {
         Console.WriteLine("Thread '{0}' executing finally block.", 
                           Thread.CurrentThread.Name);
      }
      Console.WriteLine("Thread '{0} finishing normal execution.", 
                        Thread.CurrentThread.Name);
      Console.WriteLine();
   }
}
// The example displays the following output:
//       Thread 'Sleeping' about to sleep indefinitely.
//       Thread 'Sleeping' awoken.
//       Thread 'Sleeping' executing finally block.
//       Thread 'Sleeping finishing normal execution.
//       
//       Thread 'Sleeping2' about to sleep indefinitely.
//       Thread 'Sleeping2' aborted.
//       Thread 'Sleeping2' executing finally block.
Imports System.Threading

Module Example
   Public Sub Main()
      ' Interrupt a sleeping thread. 
      Dim sleepingThread = New Thread(AddressOf Example.SleepIndefinitely)
      sleepingThread.Name = "Sleeping"
      sleepingThread.Start()
      Thread.Sleep(2000)
      sleepingThread.Interrupt()
      
      Thread.Sleep(1000)
      
      sleepingThread = New Thread(AddressOf Example.SleepIndefinitely)
      sleepingThread.Name = "Sleeping2"
      sleepingThread.Start()
      Thread.Sleep(2000)
      sleepingThread.Abort()
   End Sub

   Private Sub SleepIndefinitely()
      Console.WriteLine("Thread '{0}' about to sleep indefinitely.",
                        Thread.CurrentThread.Name)
      Try 
         Thread.Sleep(Timeout.Infinite)
      Catch ex As ThreadInterruptedException
         Console.WriteLine("Thread '{0}' awoken.",
                           Thread.CurrentThread.Name)
      Catch ex As ThreadAbortException
         Console.WriteLine("Thread '{0}' aborted.",
                           Thread.CurrentThread.Name)
      Finally 
         Console.WriteLine("Thread '{0}' executing finally block.", 
                           Thread.CurrentThread.Name)
      End Try
      Console.WriteLine("Thread '{0} finishing normal execution.", 
                        Thread.CurrentThread.Name)
      Console.WriteLine()
   End Sub
End Module
' The example displays the following output:
'       Thread 'Sleeping' about to sleep indefinitely.
'       Thread 'Sleeping' awoken.
'       Thread 'Sleeping' executing finally block.
'       Thread 'Sleeping finishing normal execution.
'       
'       Thread 'Sleeping2' about to sleep indefinitely.
'       Thread 'Sleeping2' aborted.
'       Thread 'Sleeping2' executing finally block.

Interrupting Threads

You can interrupt a waiting thread by calling the Thread.Interrupt method on the blocked thread to throw a ThreadInterruptedException, which breaks the thread out of the blocking call. The thread should catch the ThreadInterruptedException and do whatever is appropriate to continue working. If the thread ignores the exception, the runtime catches the exception and stops the thread.

Note

If the target thread is not blocked when Thread.Interrupt is called, the thread is not interrupted until it blocks. If the thread never blocks, it could complete without ever being interrupted.

If a wait is a managed wait, then Thread.Interrupt and Thread.Abort both wake the thread immediately. If a wait is an unmanaged wait (for example, a platform invoke call to the Win32 WaitForSingleObject function), neither Thread.Interrupt nor Thread.Abort can take control of the thread until it returns to or calls into managed code. In managed code, the behavior is as follows:

See Also

Thread
ThreadInterruptedException
ThreadAbortException
Threading
Using Threads and Threading
Overview of Synchronization Primitives