Accessing UI controls in Task.Run with async/await on WinForms

Question

I have the following code in a WinForms application with one button and one label:

using System;
using System.IO;
using System.Threading.Tasks;
using System.         


        

Answer 1

Try this

private async Task Run()
{
    await Task.Run(async () => {
       await File.AppendText("temp.dat").WriteAsync("a");
       });
    label1.Text = "test";
}

Or

private async Task Run()
{
    await File.AppendText("temp.dat").WriteAsync("a");        
    label1.Text = "test";
}

Or

private async Task Run()
{
    var task = Task.Run(async () => {
       await File.AppendText("temp.dat").WriteAsync("a");
       });
    var continuation = task.ContinueWith(antecedent=> label1.Text = "test",TaskScheduler.FromCurrentSynchronizationContext());
    await task;//I think await here is redundant        
}

async/await doesn't guarantee that it will run in UI thread. await will capture the current SynchronizationContext and continues execution with the captured context once the task completed.

So in your case you have a nested await which is inside Task.Run hence second await will capture the context which is not going to be UiSynchronizationContext because it is being executed by WorkerThread from ThreadPool.

Does this answers your question?