Question

I have the following extension method:

internal static string ReadLine(this DataReader stream)
{
   Task<string> line = ReadLineAsync(stream);
   // line.Wait(); - Not Required, as next call blocks
   return line.Result;
}

It’s basically a Synchronous method call wrapper for an Asynchronous method call that returns a string. The code works fine if I step through it line by line, however it seems that if I let it execute freely, I encounter an indefinite block. Any ideas?

Related to a previous question I posted: How can I update my API to use the async keyword without using await for all callers

Answer 1

As some commented on the answer to your other question, if you use Task.Result in a GUI application, you may cause a deadlock (as I explain in detail on my blog).

In short:

• You start the asynchronous operation on the UI thread. Note that the task line represents the ReadLineAsync method, and will complete when that method completes.
• ReadLineAsync calls await on some operation that is incomplete. This causes ReadLineAsync to return an incomplete task (line).
• You block the UI thread waiting for line to complete.
• When the awaited operation completes, it schedules the remainder of ReadLineAsync to the UI thread.
• The UI thread cannot complete ReadLineAsync because it is synchronously blocked waiting for ReadLineAsync to complete. Deadlock.

See my answer to your other question for a way around this deadlock. In short:

• Use ConfigureAwait(false) everywhere.
• Change your error handling to account for Result wrapping its errors in AggregateException.