Async/Await

https://msdn.microsoft.com/en-us/magazine/jj991977.aspx

Figure 1 Summary of Asynchronous Programming Guidelines

Name	Description	Exceptions
Avoid async void	Prefer async Task methods over async void methods	Event handlers
Async all the way	Don’t mix blocking and async code	Console main method
Configure context	Use ConfigureAwait(false) when you can	Methods that require con­text

Avoid Async Void

You should prefer "async Task" to "async void". Async Task methods enable easier error-handling(propagate up or not), composability (Task.waitAll ...) and testability. The exception to this guideline is asynchronous event handlers, which must return void. This exception includes methods that are logically event handlers even if they’re not literally event handlers (for example, ICommand.Execute implementations).

Async All the Way

"Async all the way” means that you shouldn’t mix synchronous and asynchronous code without carefully considering the consequences. In particular, it’s usually a bad idea to block on async code by calling Task.Wait or Task.Result.  This is an common problem for programmers who try to convert just a small part of their application and wrapping it in a synchronous API so the rest of the application is isolated from the changes. Unfortunately, this can cause deadlocks, in the case of GUI or ASP.NET (not if in a console application). The exception semantic for await and Task.Wait is also different, Exception versus AggregateException. So do not do this except in the Main method for console applications.

Figure 5 The “Async Way” of Doing Things

To Do This …	Instead of This …	Use This
Retrieve the result of a background task	Task.Wait or Task.Result	await
Wait for any task to complete	Task.WaitAny	await Task.WhenAny
Retrieve the results of multiple tasks	Task.WaitAll	await Task.WhenAll
Wait a period of time	Thread.Sleep	await Task.Delay

Configure Context

Await require context, see the following code, if you swap the commented-out lines in DelayAsync, it will not deadlock,

public static class DeadlockDemo
{
  private static async Task DelayAsync()
  {
    await Task.Delay(1000);
　　//await Task.Delay(1000).ConfigureAwait(continueOnCapturedContext: false);

  }   // This method causes a deadlock when called in a GUI or ASP.NET context.
　　public static void Test()   
　　{     // Start the delay.     
　　　　var delayTask = DelayAsync();     // Wait for the delay to complete.     
　　　　delayTask.Wait();   
　　} 
}

 This technique is particularly useful if you need to gradually convert an application from synchronous to asynchronous.

You should not use ConfigureAwait when you have code after the await in the method that needs the context.