Canceling Threads Cooperatively

Canceling Threads Cooperatively

.NET Framework 4.6 and 4.5

Prior to the .NET Framework 4, the .NET Framework provided no built-in way to cancel a thread cooperatively after it was started. However, in .NET Framework 4, you can use cancellation tokens to cancel threads, just as you can use them to cancel System.Threading.Tasks.Task objects or PLINQ queries. Although the System.Threading.Thread class does not offer built-in support for cancellation tokens, you can pass a token to a thread procedure by using the Thread constructor that takes a ParameterizedThreadStart delegate. The following example demonstrates how to do this.

using System;
using System.Threading;

public class ServerClass
   public static void StaticMethod(object obj)
      CancellationToken ct = (CancellationToken)obj;
      Console.WriteLine("ServerClass.StaticMethod is running on another thread.");

      // Simulate work that can be canceled.
      while (!ct.IsCancellationRequested) {
      Console.WriteLine("The worker thread has been canceled. Press any key to exit.");

public class Simple
   public static void Main()
      // The Simple class controls access to the token source.
      CancellationTokenSource cts = new CancellationTokenSource();

      Console.WriteLine("Press 'C' to terminate the application...\n");
      // Allow the UI thread to capture the token source, so that it
      // can issue the cancel command.
      Thread t1 = new Thread(() => { if (Console.ReadKey(true).KeyChar.ToString().ToUpperInvariant() == "C")
                                     cts.Cancel(); } );

      // ServerClass sees only the token, not the token source.
      Thread t2 = new Thread(new ParameterizedThreadStart(ServerClass.StaticMethod));
      // Start the UI thread.


      // Start the worker thread and pass it the token.

// The example displays the following output:
//       Press 'C' to terminate the application...
//       ServerClass.StaticMethod is running on another thread.
//       The worker thread has been canceled. Press any key to exit.
This snippet ID 12 is used in multiple files: /vb/cancellation_vb.vb, /vb/cancelthreads.vb. Rename the duplicate ID to make it unique, and update the reference to the snippet ID in the topic(s).
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