/// <summary>
/// DoAsyncOperationOverLegacyBad shows an async operation that does not properly clean up references to the canceallation token.
/// </summary>
public Task <string> DoAsyncOperationOverLegacyBad(CancellationToken cancellationToken)
{
// The following TaskCompletionSource hasn't been creating with the TaskCreationOptions.RunContinuationsAsynchronously
// option. This means that the calling code will resume in the OnCompleted callback. This has a couple of consequences
// 1. It will extend the lifetime of these objects since they will be on the stack when user code is resumed.
// 2. If the calling code blocks, it could *steal* the thread from the LegacyAsyncOperation.
var tcs = new TaskCompletionSource <string>();
var operation = new LegacyAsyncOperation();
if (cancellationToken.CanBeCanceled)
{
// CancellationToken.Register returns a CancellationTokenRegistration that needs to be disposed.
// If this isn't disposed, it will stay around in the CancellationTokenSource until the
// backing CancellationTokenSource is disposed.
cancellationToken.Register(state =>
{
((LegacyAsyncOperation)state).Cancel();
},
operation);
}
// Not removing the event handler can result in a memory leak
// this object is referenced by the callback which itself isn't cleaned up until
// the token is disposed or the registration is disposed.
operation.Completed += OnCompleted;
operation.Start();
return(tcs.Task);
void OnCompleted(string result, bool cancelled)
{
if (cancelled)
{
tcs.TrySetCanceled(cancellationToken);
}
else
{
tcs.TrySetResult(result);
}
}
}
public Task <string> DoAsyncOperationOverLegacy(CancellationToken cancellationToken)
{
var tcs = new TaskCompletionSource <string>(TaskCreationOptions.RunContinuationsAsynchronously);
var operation = new LegacyAsyncOperation();
var registration = default(CancellationTokenRegistration);
if (cancellationToken.CanBeCanceled)
{
registration = cancellationToken.Register(state =>
{
((LegacyAsyncOperation)state).Cancel();
},
operation);
}
operation.Completed += OnCompleted;
operation.Start();
return(tcs.Task);
void OnCompleted(string result, bool cancelled)
{
registration.Dispose();
operation.Completed -= OnCompleted;
if (cancelled)
{
tcs.TrySetCanceled(cancellationToken);
}
else
{
tcs.TrySetResult(result);
}
}
}