Task vs async Task

How often do you write code and think about what will it look like the compiler is done with it? If you're like me, not often. But is it a good thing that over the time we've learned to trust unquestionably the compiler and blindly rely on it to do the job for us?

I was lucky to get some guidance from Daniel Marbach on async/await and the importance of understanding code optimizations that compiler is performing. Without any further due, let's dive into an example.

Consider the following method:

Task MainAsync()
{
   return Task.Delay(1000);
}

Now the same method with a slight variation, marking the method as async and awaiting the delay.

async Task MainAsync()
{
   await Task.Delay(1000);
}

Looks almost identical. But is it? Let's look at what compiler generates.

For the first method, it's identical to the original code:

Task MainAsync()
{
   return Task.Delay(1000);
}

But for the second method, the compiler does... magic and voodoo.

private Task MainAsync2()
{
    UserQuery.\u003CMainAsync2\u003Ed__2 mainAsync2D2 = new UserQuery.\u003CMainAsync2\u003Ed__2();
    mainAsync2D2.\u003C\u003E4__this = this;
    mainAsync2D2.\u003C\u003Et__builder = AsyncTaskMethodBuilder.Create();
    mainAsync2D2.\u003C\u003E1__state = -1;
    AsyncTaskMethodBuilder taskMethodBuilder = mainAsync2D2.\u003C\u003Et__builder;
    ((AsyncTaskMethodBuilder) @taskMethodBuilder).Start<UserQuery.\u003CMainAsync2\u003Ed__2>((M0&) @mainAsync2D2);
    return ((AsyncTaskMethodBuilder) @mainAsync2D2.\u003C\u003Et__builder).get_Task();
}

No magic. The compiler just creates a state machine due to async/await keywords.

 [/*Attribute with token 0C000007*/CompilerGenerated]
  private sealed class \u003CMainAsync2\u003Ed__2 : IAsyncStateMachine
  {
    public int \u003C\u003E1__state;
    public AsyncTaskMethodBuilder \u003C\u003Et__builder;
    public UserQuery \u003C\u003E4__this;
    private TaskAwaiter \u003C\u003Eu__1;

    public \u003CMainAsync2\u003Ed__2()
    {
      base.\u002Ector();
    }

    void IAsyncStateMachine.MoveNext()
    {
      int num1 = this.\u003C\u003E1__state;
      try
      {
        TaskAwaiter taskAwaiter;
        int num2;
        if (num1 != 0)
        {
          taskAwaiter = Task.Delay(1000).GetAwaiter();
          // ISSUE: explicit reference operation
          if (!((TaskAwaiter) @taskAwaiter).get_IsCompleted())
          {
            this.\u003C\u003E1__state = num2 = 0;
            this.\u003C\u003Eu__1 = taskAwaiter;
            UserQuery.\u003CMainAsync2\u003Ed__2 mainAsync2D2 = this;
                ((AsyncTaskMethodBuilder) @this.\u003C\u003Et__builder).AwaitUnsafeOnCompleted<TaskAwaiter, UserQuery.\u003CMainAsync2\u003Ed__2>((M0&) @taskAwaiter, (M1&) @mainAsync2D2);
            return;
          }
        }
        else
        {
          taskAwaiter = this.\u003C\u003Eu__1;
          this.\u003C\u003Eu__1 = (TaskAwaiter) null;
          this.\u003C\u003E1__state = num2 = -1;
        }
        ((TaskAwaiter) @taskAwaiter).GetResult();
        taskAwaiter = (TaskAwaiter) null;
      }
      catch (Exception ex)
      {
        this.\u003C\u003E1__state = -2;
        ((AsyncTaskMethodBuilder) @this.\u003C\u003Et__builder).SetException(ex);
        return;
      }
      this.\u003C\u003E1__state = -2;
      ((AsyncTaskMethodBuilder) @this.\u003C\u003Et__builder).SetResult();
    }

    [/*Attribute with token 0C000008*/DebuggerHidden]
    void IAsyncStateMachine.SetStateMachine(/*Parameter with token 08000001*/IAsyncStateMachine stateMachine)
    {
    }
  }

The moral of this is simple: if you don't need to await, just return the Task. It will do the same, and you'll save a lot of unnecessary state machine construction, with its wasteful memory and execution where it's not needed.

2 Comments

  • Nice article (and articles! -- I enjoy your blog). Async is definitely an improvement, but it does have rough edges and could use some further polish. I've added this article as a reference to a vote to improve the asyncrhonous model here:
    https://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/9126493-improve-asynchronous-programming-model

  • Thank you Mike-EEE.
    Something like AsyncFixer helps to handle code like the one I've outlined in the post. Definitely worth installation.
    https://visualstudiogallery.msdn.microsoft.com/03448836-db42-46b3-a5c7-5fc5d36a8308

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