If a method is blocked while it is waiting for an event to be signaled, it cannot check the value of the cancellation token and respond in a timely manner. The first example shows how to solve this problem when you are working with events such as System.Threading.ManualResetEvent that do not natively support the unified cancellation framework. The second example shows a more streamlined approach that uses System.Threading.ManualResetEventSlim, which does support unified cancellation.
Example 1
The following example uses a System.Threading.ManualResetEvent to demonstrate how to unblock wait handles that do not support unified cancellation.
sample · csharpsamples/snippets/csharp/VS_Snippets_Misc/cancellation/cs/cancellationex9.cs9
// How to: Cancel by using a WaitHandle
//<snippet9>
using System;
using System.Threading;
using System.Threading.Tasks;
class CancelOldStyleEvents
{
// Old-style MRE that doesn't support unified cancellation.
static ManualResetEvent mre = new ManualResetEvent(false);
static void Main()
{
var cts = new CancellationTokenSource();
// Pass the same token source to the delegate and to the task instance.
Task.Run(() => DoWork(cts.Token), cts.Token);
Console.WriteLine("Press s to start/restart, p to pause, or c to cancel.");
Console.WriteLine("Or any other key to exit.");
// Old-style UI thread.
bool goAgain = true;
while (goAgain)
{
char ch = Console.ReadKey(true).KeyChar;
switch (ch)
{
case 'c':
cts.Cancel();
break;
case 'p':
mre.Reset();
break;
case 's':
mre.Set();
break;
default:
goAgain = false;
break;
}
Thread.Sleep(100);
}
cts.Dispose();
}
static void DoWork(CancellationToken token)
{
while (true)
{
//<snippet5>
// Wait on the event if it is not signaled.
int eventThatSignaledIndex =
WaitHandle.WaitAny(new WaitHandle[] { mre, token.WaitHandle },
new TimeSpan(0, 0, 20));
//</snippet5>
// Were we canceled while waiting?
if (eventThatSignaledIndex == 1)
{
Console.WriteLine("The wait operation was canceled.");
throw new OperationCanceledException(token);
}
// Were we canceled while running?
else if (token.IsCancellationRequested)
{
Console.WriteLine("I was canceled while running.");
token.ThrowIfCancellationRequested();
}
// Did we time out?
else if (eventThatSignaledIndex == WaitHandle.WaitTimeout)
{
Console.WriteLine("I timed out.");
break;
}
else
{
Console.Write("Working... ");
// Simulating work.
Thread.SpinWait(5000000);
}
}
}
}
//</snippet9>sample · vbsamples/snippets/visualbasic/VS_Snippets_Misc/cancellation/vb/cancellationex9.vb9
' Visual Basic .NET Document
Option Strict On
'<snippet9>
Imports System.Threading
Imports System.Threading.Tasks
Class CancelOldStyleEvents
' Old-style MRE that doesn't support unified cancellation.
Shared mre As New ManualResetEvent(False)
Shared Sub Main9()
Dim cts As New CancellationTokenSource()
' Pass the same token source to the delegate and to the task instance.
Task.Run(Sub() DoWork(cts.Token), cts.Token)
Console.WriteLine("Press c to cancel, p to pause, or s to start/restart.")
Console.WriteLine("Or any other key to exit.")
' Old-style UI thread.
Dim goAgain As Boolean = True
While goAgain
Dim ch As Char = Console.ReadKey(True).KeyChar
Select Case ch
Case "c"c
cts.Cancel()
Case "p"c
mre.Reset()
Case "s"c
mre.Set()
Case Else
goAgain = False
End Select
Thread.Sleep(100)
End While
cts.Dispose()
End Sub
Shared Sub DoWork(ByVal token As CancellationToken)
While True
'<snippet5>
' Wait on the event if it is not signaled.
Dim waitHandles() As WaitHandle = {mre, token.WaitHandle}
Dim eventThatSignaledIndex =
WaitHandle.WaitAny(waitHandles, _
New TimeSpan(0, 0, 20))
'</snippet5>
' Were we canceled while waiting?
' The first If statement is equivalent to
' token.ThrowIfCancellationRequested()
If eventThatSignaledIndex = 1 Then
Console.WriteLine("The wait operation was canceled.")
Throw New OperationCanceledException(token)
' Were we canceled while running?
ElseIf token.IsCancellationRequested = True Then
Console.WriteLine("Cancelling per user request.")
token.ThrowIfCancellationRequested()
' Did we time out?
ElseIf eventThatSignaledIndex = WaitHandle.WaitTimeout Then
Console.WriteLine("The wait operation timed out.")
Exit While
Else
' Simulating work.
Console.Write("Working... ")
Thread.SpinWait(5000000)
End If
End While
End Sub
End Class
'</snippet9>Example 2
The following example uses a System.Threading.ManualResetEventSlim to demonstrate how to unblock coordination primitives that do support unified cancellation. The same approach can be used with other lightweight coordination primitives, such as System.Threading.SemaphoreSlim and System.Threading.CountdownEvent.
sample · csharpsamples/snippets/csharp/VS_Snippets_Misc/cancellation/cs/cancellationex10.cs10
//<Snippet10>
using System;
using System.Threading;
using System.Threading.Tasks;
class CancelNewStyleEvents
{
// New-style MRESlim that supports unified cancellation
// in its Wait methods.
static ManualResetEventSlim mres = new ManualResetEventSlim(false);
static void Main()
{
var cts = new CancellationTokenSource();
// Pass the same token source to the delegate and to the task instance.
Task.Run(() => DoWork(cts.Token), cts.Token);
Console.WriteLine("Press c to cancel, p to pause, or s to start/restart,");
Console.WriteLine("or any other key to exit.");
// New-style UI thread.
bool goAgain = true;
while (goAgain)
{
char ch = Console.ReadKey(true).KeyChar;
switch (ch)
{
case 'c':
// Token can only be canceled once.
cts.Cancel();
break;
case 'p':
mres.Reset();
break;
case 's':
mres.Set();
break;
default:
goAgain = false;
break;
}
Thread.Sleep(100);
}
cts.Dispose();
}
static void DoWork(CancellationToken token)
{
while (true)
{
if (token.IsCancellationRequested)
{
Console.WriteLine("Canceled while running.");
token.ThrowIfCancellationRequested();
}
// Wait on the event to be signaled
// or the token to be canceled,
// whichever comes first. The token
// will throw an exception if it is canceled
// while the thread is waiting on the event.
//<snippet6>
try
{
// mres is a ManualResetEventSlim
mres.Wait(token);
}
catch (OperationCanceledException)
{
// Throw immediately to be responsive. The
// alternative is to do one more item of work,
// and throw on next iteration, because
// IsCancellationRequested will be true.
Console.WriteLine("The wait operation was canceled.");
throw;
}
Console.Write("Working...");
// Simulating work.
Thread.SpinWait(500000);
//</snippet6>
}
}
}
//</Snippet10>sample · vbsamples/snippets/visualbasic/VS_Snippets_Misc/cancellation/vb/cancellationex10.vb10
' Visual Basic .NET Document
Option Strict On
'<Snippet10>
Imports System.Threading
Imports System.Threading.Tasks
Class CancelNewStyleEvents
' New-style MRESlim that supports unified cancellation
' in its Wait methods.
Shared mres As ManualResetEventSlim = New ManualResetEventSlim(False)
Shared Sub Main10()
Dim cts As New CancellationTokenSource()
' Pass the same token source to the delegate and to the task instance.
Task.Run(Sub() DoWork(cts.Token), cts.Token)
Console.WriteLine("Press c to cancel, p to pause, or s to start/restart,")
Console.WriteLine("or any other key to exit.")
' New-style UI thread.
Dim goAgain As Boolean = True
While goAgain = True
Dim ch As Char = Console.ReadKey(True).KeyChar
Select Case ch
Case "c"c
' Token can only be canceled once.
cts.Cancel()
Case "p"c
mres.Reset()
Case "s"c
mres.Set()
Case Else
goAgain = False
End Select
Thread.Sleep(100)
End While
cts.Dispose()
End Sub
Shared Sub DoWork(ByVal token As CancellationToken)
While True
If token.IsCancellationRequested Then
Console.WriteLine("Canceled while running.")
token.ThrowIfCancellationRequested()
End If
' Wait on the event to be signaled
' or the token to be canceled,
' whichever comes first. The token
' will throw an exception if it is canceled
' while the thread is waiting on the event.
'<snippet6>
Try
' mres is a ManualResetEventSlim
mres.Wait(token)
Catch e As OperationCanceledException
' Throw immediately to be responsive. The
' alternative is to do one more item of work,
' and throw on next iteration, because
' IsCancellationRequested will be true.
Console.WriteLine("Canceled while waiting.")
Throw
End Try
' Simulating work.
Console.Write("Working...")
Thread.SpinWait(500000)
'</snippet6>
End While
End Sub
End Class
'</Snippet10>