c# blocking code in async method

Question

I’m using MvvmCross and the AsyncEx library within a Windows 10 (UWP) App.

In a ViewModel, I have an INotifyTaskCompletion property (1) which is wired-up to an Async method in the ViewModel (2)

In (2), I call an Async library method which:

• Checks a local cache
• Downloads data asynchronously
• Adds the data to the cache

The caching code cannot be made asynchronous and so the library method contains both blocking and asynchronous code.

Q. What’s the best way to prevent blocking the UI thread?

I understand from Stephen Cleary to not to block in asynchronous code and not use Task.Run in library methods. So do I have to….

Move the caching calls into (2) e.g.

• Use Task.Run (to check the cache)
• Call the library method asynchronously
• Use Task.Run again (to cache the data)?

Is there a better way?

Answer 1

If you have completely synchronous code which you can't change to make it return an awaitable, and want to make it asynchronous, then yes, your only choice if you want to use async/await is to use Task.Run().

Something like:

public async Task<T> MyMethod()
{
   T result = await Task.Run(() => CheckCacheOnSyncMethodICantChange());
   if(result != null)
   {
     result = await MyLibraryMethodThatReturnsATask();
     await Task.Run(() => AddToCacheOnSyncMethodICantChange(result));
   }
   return result;
}

Should be ok.