The following example shows one way that user code can poll a cancellation token at regular intervals to see whether cancellation has been requested from the calling thread. This example uses the System.Threading.Tasks.Task type, but the same pattern applies to asynchronous operations created directly by the System.Threading.ThreadPool type or the System.Threading.Thread type.

using System;
using System.Threading;
using System.Threading.Tasks;

public struct Rectangle
{
   public int columns;
   public int rows;
}

class CancelByPolling
{
   static void Main()
   {
      var tokenSource = new CancellationTokenSource();

      // Toy object for demo purposes
      Rectangle rect = new Rectangle() { columns = 1000, rows = 500 };

      // Simple cancellation scenario #1. Calling thread does not wait
      // on the task to complete, and the user delegate simply returns
      // on cancellation request without throwing.
      Task.Run(() => NestedLoops(rect, tokenSource.Token), tokenSource.Token);

      // Simple cancellation scenario #2. Calling thread does not wait
      // on the task to complete, and the user delegate throws
      // OperationCanceledException to shut down task and transition its state.
      // Task.Run(() => PollByTimeSpan(tokenSource.Token), tokenSource.Token);

      Console.WriteLine("Press 'c' to cancel");
      if (Console.ReadKey(true).KeyChar == 'c') {
          tokenSource.Cancel();
          Console.WriteLine("Press any key to exit.");
      }
      Console.ReadKey();
      
      tokenSource.Dispose();
  }

   static void NestedLoops(Rectangle rect, CancellationToken token)
   {
      for (int x = 0; x < rect.columns && !token.IsCancellationRequested; x++) {
         for (int y = 0; y < rect.rows; y++) {
            // Simulating work.
            Thread.SpinWait(5000);
            Console.Write("{0},{1} ", x, y);
         }

         // Assume that we know that the inner loop is very fast.
         // Therefore, checking once per row is sufficient.
         if (token.IsCancellationRequested) {
            // Cleanup or undo here if necessary...
            Console.WriteLine("\r\nCancelling after row {0}.", x);
            Console.WriteLine("Press any key to exit.");
            // then...
            break;
            // ...or, if using Task:
            // token.ThrowIfCancellationRequested();
         }
      }
   }
}