Task.Run Etiquette Examples: Don't Use Task.Run for the Wrong Thing

I had quite a few comments on my last post asking for more explicit examples of Correct vs. Incorrect Task.Run usage.

First, let’s consider the “beginner’s error”. This is where the user misuses Task.Run because they want to make their code “asynchronous” but aren’t sure how to do it.

This kind of user starts off with existing code, which usually does some kind of synchronous work (often database access or a web request).

class MyService
{
  public int RetrieveValue(int id)
  {
    // Do some blocking work.
    // DB access, web request, etc.
    Thread.Sleep(500);
    return 42;
  }
}

They’ve read a bit about how async helps in those areas, and decide to give it a spin. “Let’s see if I can figure out this async thing. I’ll just add an async and see what happens. Oh, I have to change the return type to Task, too.”

// Warning: bad code!
class MyService
{
  public async Task<int> RetrieveValueAsync(int id)
  {
    // Do some blocking work.
    // DB access, web request, etc.
    Thread.Sleep(500);
    return 42;
  }
}

“Now the compiler is complaining that I’m not using await. OK, so what can I await? [Google-Fu] Ah, Task.Run looks promising!”

// Warning: bad code!
class MyService
{
  public async Task<int> RetrieveValueAsync(int id)
  {
    return await Task.Run(() =>
    {
      // Do some blocking work.
      // DB access, web request, etc.
      Thread.Sleep(500);
      return 42;
    });
  }
}

“Hey, it worked! My UI thread is no longer blocked! Yay for async!”

Unfortunately, this is a misuse of Task.Run. The problem is that it’s not truly asynchronous. It’s still executing blocking work, blocking a thread pool thread the whole time the operation is in progress.

The proper approach is to change the blocking call to an asynchronous call first and then allow async to grow from there. So, starting from the same point, we first change the blocking call to an asynchronous call. In the real world, this would be like replacing WebClient with HttpClient or converting your Entity Framework queries to be asynchronous. In this example, I’m replacing Thread.Sleep with Task.Delay.

// Warning: bad code!
class MyService
{
  public int RetrieveValue(int id)
  {
    // Converted to nonblocking work.
    // DB access, web request, etc.
    await Task.Delay(500);
    return 42;
  }
}

Now we’re getting a compiler error, and we need to make the method async.

class MyService
{
  public async Task<int> RetrieveValueAsync(int id)
  {
    // Converted to nonblocking work.
    // DB access, web request, etc.
    await Task.Delay(500);
    return 42;
  }
}

And now we end up with a more correct implementation.

Note that this was an example of using Task.Run for the wrong thing. To reiterate a sentence from my last post, use Task.Run to call CPU-bound methods (from GUI threads). Do not use it just to “provide something awaitable for my async method to use”.