I don't quite understand the difference between Task.Wait
and await
.
I have something similar to the following functions in a ASP.NET WebAPI service:
public class TestController : ApiController
{
public static async Task<string> Foo()
{
await Task.Delay(1).ConfigureAwait(false);
return "";
}
public async static Task<string> Bar()
{
return await Foo();
}
public async static Task<string> Ros()
{
return await Bar();
}
// GET api/test
public IEnumerable<string> Get()
{
Task.WaitAll(Enumerable.Range(0, 10).Select(x => Ros()).ToArray());
return new string[] { "value1", "value2" }; // This will never execute
}
}
Where Get
will deadlock.
What could cause this? Why doesn't this cause a problem when I use a blocking wait rather than await Task.Delay
?
Wait
and await
- while similar conceptually - are actually completely different.
Wait
will synchronously block until the task completes. So the current thread is literally blocked waiting for the task to complete. As a general rule, you should use "async
all the way down"; that is, don't block on async
code. On my blog, I go into the details of how blocking in asynchronous code causes deadlock.
await
will asynchronously wait until the task completes. This means the current method is "paused" (its state is captured) and the method returns an incomplete task to its caller. Later, when the await
expression completes, the remainder of the method is scheduled as a continuation.
You also mentioned a "cooperative block", by which I assume you mean a task that you're Wait
ing on may execute on the waiting thread. There are situations where this can happen, but it's an optimization. There are many situations where it can't happen, like if the task is for another scheduler, or if it's already started or if it's a non-code task (such as in your code example: Wait
cannot execute the Delay
task inline because there's no code for it).
You may find my async
/ await
intro helpful.