Making async Task interface backwards compatible with monodroid

Question

I have a bunch of interfaces for a repository in a core class library that defines return types as Task. These interfaces are used in other projects to define async methods.

I need to implement these same interfaces in a Xamarain monodroid project and was wondering how I should use the same return types without using async await since that's not supported yet in MonoDroid. I don't want to modify the existing core library interfaces at all if possible.

Answer 1

Technically, you don't have to use async/await; it's just a lot easier than doing it "by hand". Your implementations just have to return Tasks that are never null and already started (if applicable).

The TAP document has some useful techniques and guidelines. One key type is TaskCompletionSource<TResult> which you can use to wrap any asynchronous implementation (e.g., APM). Synchronous implementations can use Task.FromResult (which is just a small wrapper around TaskCompletionSource. If you have CPU work to do, you can use Task.Factory.StartNew, which is closely related but not quite equivalent to Task.Run. For more complex implementations, you would need continuations a la ContinueWith.

In other words, all the .NET 4.0 TPL techniques can be used, as long as your returned Tasks are always started. (The only way to get an unstarted Task is to construct it with the Task constructor and not call Start. So as long as you don't do this, you should be fine.)