The most important thing to know about async
and await
is that await
doesn’t wait for the associated call to complete. What await
does is to return the result of the operation immediately and synchronously if the operation has already completed or, if it hasn’t, to schedule a continuation to execute the remainder of the async
method and then to return control to the caller. When the asynchronous operation completes, the scheduled completion will then execute.
The answer to the specific question in your question’s title is to block on an async
method’s return value (which should be of type Task
or Task<T>
) by calling an appropriate Wait
method:
public static async Task<Foo> GetFooAsync()
{
// Start asynchronous operation(s) and return associated task.
...
}
public static Foo CallGetFooAsyncAndWaitOnResult()
{
var task = GetFooAsync();
task.Wait(); // Blocks current thread until GetFooAsync task completes
// For pedagogical use only: in general, don't do this!
var result = task.Result;
return result;
}
In this code snippet, CallGetFooAsyncAndWaitOnResult
is a synchronous wrapper around asynchronous method GetFooAsync
. However, this pattern is to be avoided for the most part since it will block a whole thread pool thread for the duration of the asynchronous operation. This an inefficient use of the various asynchronous mechanisms exposed by APIs that go to great efforts to provide them.
The answer at “await” doesn’t wait for the completion of call has several, more detailed, explanations of these keywords.
Meanwhile, @Stephen Cleary’s guidance about async void
holds. Other nice explanations for why can be found at http://www.tonicodes.net/blog/why-you-should-almost-never-write-void-asynchronous-methods/ and https://jaylee.org/archive/2012/07/08/c-sharp-async-tips-and-tricks-part-2-async-void.html