Wrapping synchronous code into asynchronous call

It's important to make a distinction between two different types of concurrency. Asynchronous concurrency is when you have multiple asynchronous operations in flight (and since each operation is asynchronous, none of them are actually using a thread). Parallel concurrency is when you have multiple threads each doing a separate operation.

The first thing to do is re-evaluate this assumption:

The method itself is synchronous call to the service and there is no possibility to override the implementation.

If your "service" is a web service or anything else that is I/O-bound, then the best solution is to write an asynchronous API for it.

I'll proceed with the assumption that your "service" is a CPU-bound operation that must execute on the same machine as the web server.

If that's the case, then the next thing to evaluate is another assumption:

I need the request to execute faster.

Are you absolutely sure that's what you need to do? Are there any front-end changes you can make instead - e.g., start the request and allow the user to do other work while it's processing?

I'll proceed with the assumption that yes, you really do need to make the individual request execute faster.

In this case, you'll need to execute parallel code on your web server. This is most definitely not recommended in general because the parallel code will be using threads that ASP.NET may need to handle other requests, and by removing/adding threads it will throw the ASP.NET threadpool heuristics off. So, this decision does have an impact on your entire server.

When you use parallel code on ASP.NET, you are making the decision to really limit the scalability of your web app. You also may see a fair amount of thread churn, especially if your requests are bursty at all. I recommend only using parallel code on ASP.NET if you know that the number of simultaneous users will be quite low (i.e., not a public server).

So, if you get this far, and you're sure you want to do parallel processing on ASP.NET, then you have a couple of options.

One of the easier methods is to use Task.Run, very similar to your existing code. However, I do not recommend implementing a CalculateAsync method since that implies the processing is asynchronous (which it is not). Instead, use Task.Run at the point of the call:

private async Task MakeRequest()
{
  // do some stuff
  var task = Task.Run(() => Calculate(myInput));
  // do other stuff
  var myOutput = await task;
  // some more stuff
}

Alternatively, if it works well with your code, you can use the Parallel type, i.e., Parallel.For, Parallel.ForEach, or Parallel.Invoke. The advantage to the Parallel code is that the request thread is used as one of the parallel threads, and then resumes executing in the thread context (there's less context switching than the async example):

private void MakeRequest()
{
  Parallel.Invoke(() => Calculate(myInput1),
      () => Calculate(myInput2),
      () => Calculate(myInput3));
}

I do not recommend using Parallel LINQ (PLINQ) on ASP.NET at all.

I found that the following code can convert a Task to always run asynchronously

private static async Task<T> ForceAsync<T>(Func<Task<T>> func)
{
    await Task.Yield();
    return await func();
}

and I have used it in the following manner

await ForceAsync(() => AsyncTaskWithNoAwaits())

This will execute any Task asynchronously so you can combine them in WhenAll, WhenAny scenarios and other uses.

You could also simply add the Task.Yield() as the first line of your called code.