In looking at various C# Async CTP samples I see some async functions that return
SLaks and Killercam's answers are good; I thought I'd just add a bit more context.
Your first question is essentially about what methods can be marked
A method marked as
Task<T>. What are the differences between them?
Task<T> returning async method can be awaited, and when the task completes it will proffer up a T.
Task returning async method can be awaited, and when the task completes, the continuation of the task is scheduled to run.
void returning async method cannot be awaited; it is a "fire and forget" method. It does work asynchronously, and you have no way of telling when it is done. This is more than a little bit weird; as SLaks says, normally you would only do that when making an asynchronous event handler. The event fires, the handler executes; no one is going to "await" the task returned by the event handler because event handlers do not return tasks, and even if they did, what code would use the Task for something? It's usually not user code that transfers control to the handler in the first place.
Your second question, in a comment, is essentially about what can be
What kinds of methods can be
awaited? Can a void-returning method be
No, a void-returning method cannot be awaited. The compiler translates
await M() into a call to
GetAwaiter might be an instance method or an extension method. The value awaited has to be one for which you can get an awaiter; clearly a void-returning method does not produce a value from which you can get an awaiter.
Task-returning methods can produce awaitable values. We anticipate that third parties will want to create their own implementations of
Task-like objects that can be awaited, and you will be able to await them. However, you will not be allowed to declare
async methods that return anything but
(UPDATE: My last sentence there may be falsified by a future version of C#; there is a proposal to allow return types other than task types for async methods.)