I want to use TaskCompletionSource
to wrap MyService
which is a simple service:
public static Task ProcessAsync(MyService
It appears that MyService
will raise the Completed
event more than once. this causes SetResult
to be called more than once which causes your error.
You have 3 options that I see. Change the Completed event to only be raised once (Seems odd that you can complete more than once), change SetResult
to TrySetResult so it does not throw a exception when you try to set it a 2nd time (this does introduce a small memory leak as the event still gets called and the completion source still tries to be set), or unsubscribe from the event (i3arnon's answer)
An alternative solution to i3arnon's answer would be:
public async static Task<string> ProcessAsync(MyService service, int parameter)
{
var tcs = new TaskCompletionSource<string>();
EventHandler<CustomEventArg> callback =
(s, e) => tcs.SetResult(e.Result);
try
{
contacts.Completed += callback;
contacts.RunAsync(parameter);
return await tcs.Task;
}
finally
{
contacts.Completed -= callback;
}
}
However, this solution will have a compiler generated state machine. It will use more memory and CPU.
The issue here is that the Completed
event is raised on each action but the TaskCompletionSource
can only be completed once.
You can still use a local TaskCompletionSource
(and you should). You just need to unregister the callback before completing the TaskCompletionSource
. That way this specific callback with this specific TaskCompletionSource
will never be called again:
public static Task<string> ProcessAsync(MyService service, int parameter)
{
var tcs = new TaskCompletionSource<string>();
EventHandler<CustomEventArg> callback = null;
callback = (sender, e) =>
{
service.Completed -= callback;
tcs.SetResult(e.Result);
};
service.Completed += callback;
service.RunAsync(parameter);
return tcs.Task;
}
This will also solve the possible memory leak that you have when your service keeps references to all these delegates.
You should keep in mind though that you can't have multiple of these operations running concurrently. At least not unless you have a way to match requests and responses.