How does running several tasks asynchronously on UI thread using async/await work?

Question

I\'ve read (and used) async/await quite a lot for some time now but I still have one question I can\'t get an answer to. Say I have this code.

private async          


        

Answer 1

The TaskParallelLibrary (TPL) uses a TaskScheduler which can be configured with TaskScheduler.FromCurrentSynchronizationContext to return to the SynchronizationContext like this :

textBox1.Text = "Start";
// The SynchronizationContext is captured here
Factory.StartNew( () => DoSomeAsyncWork() )
.ContinueWith( 
    () => 
    {
       // Back on the SynchronizationContext it came from            
        textBox1.Text = "End";
    },TaskScheduler.FromCurrentSynchronizationContext());

When an async method suspends at an await, by default it will capture the current SynchronizationContext and marshall the code after the await back on the SynchronizationContext it came from.

        textBox1.Text = "Start";

        // The SynchronizationContext is captured here

       /* The implementation of DoSomeAsyncWork depends how it runs, this could run on the threadpool pool 
          or it could be an 'I/O operation' or an 'Network operation' 
          which doesnt use the threadpool */
        await DoSomeAsyncWork(); 

        // Back on the SynchronizationContext it came from
        textBox1.Text = "End";

async and await example:

async Task MyMethodAsync()
{
  textBox1.Text = "Start";

  // The SynchronizationContext is captured here
  await Task.Run(() => { DoSomeAsyncWork(); }); // run on the threadPool

  // Back on the SynchronizationContext it came from
  textBox1.Text = "End";
}