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?

Reza Aghaei · Accepted Answer · 2019-10-03 05:29:32Z

18

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

edited Oct 3, 2019 at 5:29

answered Nov 7, 2015 at 21:27

Reza Aghaei

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5 Comments

Benji Over a year ago

The second option you put is what I have in the working code. So then that is the proper and correct way to handle this ?

Reza Aghaei Over a year ago

Yes it is. Read the compiler warning.

NoChance Over a year ago

Just wondering how to code the Cancel button in this case? Thanks.

Reza Aghaei Over a year ago

What does the cancel button suppose to do?

J. Ouwehand Over a year ago

You might want to remove the async directive and use return Task.FromResult(...) if the implementation is synchronous, instead of using Task.Run.

Hiran · Accepted Answer · 2017-08-08 10:31:13Z

3

You just need to change the DoStuffAsync task little bit, as below.

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

answered Aug 8, 2017 at 10:31

Hiran

1,20011 silver badges19 bronze badges

1 Comment

CMS Over a year ago

👍🏻 from the CancellationTokenSource

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