c# - Prevent winforms UI block when using async/await

Question

I'm fairly new to async/await programming and sometimes I feel that I understand it, and then all of a sudden something happens and throws me for a loop.

I'm trying this out in a test winforms app and here is one version of a snippet that I have. Doing it this way will block the UI

private async void button1_Click(object sender, EventArgs e)
{

    int d = await DoStuffAsync(c);

    Console.WriteLine(d);

}

private async Task<int> DoStuffAsync(CancellationTokenSource c)
{

        int ret = 0;

        // I wanted to simulator a long running process this way
        // instead of doing Task.Delay

        for (int i = 0; i < 500000000; i++)
        {



            ret += i;
            if (i % 100000 == 0)
                Console.WriteLine(i); 

            if (c.IsCancellationRequested)
            {
                return ret;
            }
        }
        return ret;
}

Now, when I make a slight change by wrapping the body of "DoStuffAsync()" in a Task.Run it works perfectly fine

private async Task<int> DoStuffAsync(CancellationTokenSource c)
    {
        var t = await Task.Run<int>(() =>
        {
            int ret = 0;
            for (int i = 0; i < 500000000; i++)
            {



                ret += i;
                if (i % 100000 == 0)
                    Console.WriteLine(i);

                if (c.IsCancellationRequested)
                {
                    return ret;
                }
            }
            return ret;

        });


        return t;
    }

With all that said, what is the proper way to handle this scenario?

Answer 1

When you write such code:

private async Task<int> DoStuffAsync()
{
    return 0;
}

This way you are doing things synchronously, because you are not using await expression.

Pay attention to the warning:

This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.

Based on the warning suggestion you can correct it this way:

private async Task<int> DoStuffAsync()
{
    return await Task.Run<int>(() =>
    {
        return 0;
    });
}

To learn more about async/await you can take a look at:

• Async and Await by Stephen Cleary
• Asynchronous Programming with Async and Await from msdn