27/07/2015

A hint how to use TaskCompletionSource<T>

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Some time ago I wrote about using TaskCompletionSource<T> class in order to take advantage of async/await keywords. In that post I included the following code:
public async Task<Stream> ProcessFileAsync(string key, string secret, string path)
{
   var client = new DropNetClient(key, secret);
   //...
   var tcs = new TaskCompletionSource<Stream>();
   client.GetFileAsync(path, response => tcs.SetResult(new MemoryStream(response.RawBytes)), tcs.SetException);
   return tcs.Task;
}
Now, Let's assume that we want to provide a possibility to cancel a task returned from ProcessFileAsync method. We can do something like that:
public async Task<Stream> ProcessFileAsync(string key, string secret, string path, CancellationToken ct)
{
   var client = new DropNetClient(key, secret);
   //...
   var tcs = new TaskCompletionSource<Stream>();

   ct.Value.Register(tcs.SetCanceled);

   client.GetFileAsync(path, response => tcs.SetResult(new MemoryStream(response.RawBytes)), tcs.SetException);
   return tcs.Task;
}
I used CancellationToken.Register method in order to register a callback that will be executed when a token is canceled. This callback is responsible for notifying TaskCompletionSource<T> that underlying task should be cancelled.

You may say that it is not enough because this code doesn't inform DropNetClient that an action should be cancelled. You are right. However, according to my knowledge DropNet API doesn't provide such a possibility.

It leads to the situation when a task is cancelled but DropNetClient continues processing and finnaly TaskCompletionSource.SetResult method will be executed. This will cause ObjectDisposedException because this method cannot be executed for a disposed task. What can we do in this case?

The first solution is to check if a task is cancelled before calling SetResult method. However, it can still happen that a task will be cancelled after this check but before calling SetResult method.

My proposition is to use methods from TaskCompletionSource.Try* family. They don't throw exceptions for disposed tasks.
public async Task<Stream> ProcessFileAsync(string key, string secret, string path, CancellationToken ct)
{
   var client = new DropNetClient(key, secret);
   //...
   var tcs = new TaskCompletionSource<Stream>();

   ct.Value.Register(tcs.SetCanceled);

   client.GetFileAsync(path, response => tcs.TrySetResult(new MemoryStream(response.RawBytes)), tcs.TrySetException);
   return tcs.Task;
}
I'm aware that it is not a perfect solution because it actually does not cancel processing. However, without modifying DropNet code it is not possible. It the case of my application it is an acceptable solution but it is not a rule.

4 comments:

for ict 99 said...

great

Radley Co Tad said...
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Unknown said...

Thanks for info!

Family Matters in Islam said...
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