Promise equivalent in C#

Question

In Scala there is a Promise class that could be used to complete a Future manually. I am looking for an alternative in C#.

I am writing a test and I want it to look it similar to this:

// var MyResult has a field `Header`
var promise = new Promise<MyResult>;

handlerMyEventsWithHandler( msg =>
    promise.Complete(msg);
);

// Wait for 2 seconds
var myResult = promise.Future.Await(2000);

Assert.Equals("my header", myResult.Header);

I understand that this is probably not the right pattern for C#, but I couldn't figure out a reasonable way to achieve the same thing even with somewhat different pattern.

EDIT: please note, that async/await doesn't help here, as I don't have a Task to await! I just have an access to a handler that will be run on another thread.

Answer 1

In C#:

• Task<T> is a future (or Task for a unit-returning future).
• TaskCompletionSource<T> is a promise.

So your code would translate as such:

// var promise = new Promise<MyResult>;
var promise = new TaskCompletionSource<MyResult>();

// handlerMyEventsWithHandler(msg => promise.Complete(msg););
handlerMyEventsWithHandler(msg => promise.TrySetResult(msg));

// var myResult = promise.Future.Await(2000);
var completed = await Task.WhenAny(promise.Task, Task.Delay(2000));
if (completed == promise.Task)
  ; // Do something on timeout
var myResult = await completed;

Assert.Equals("my header", myResult.Header);

The "timed asynchronous wait" is a bit awkward, but it's also relatively uncommon in real-world code. For unit tests, I would just do a regular asynchronous wait:

var promise = new TaskCompletionSource<MyResult>();

handlerMyEventsWithHandler(msg => promise.TrySetResult(msg));

var myResult = await promise.Task;

Assert.Equals("my header", myResult.Header);