Async methods with and without async modifier

Question

What is the difference between methods Add1() and Add2()? Is there a difference at all? For all I know usage (as shown in method UsageTest()) is the same.

private async Task<int> Add1(int a, int b)
{
    return await Task.Run(
        () =>
            {
                Thread.Sleep(1000);
                return a + b;
            });
}

private Task<int> Add2(int a, int b)
{
    return Task.Run(
        () =>
            {
                Thread.Sleep(1000);
                return a + b;
            });
}

private async void UsageTest()
{
    int a = await Add1(1, 2);
    int b = await Add2(1, 3);
}

Stephen Cleary · Accepted Answer · 2013-06-07 15:43:07Z

6

Semantically, they are practically equivalent.

The main difference is that Add1 has more overhead (for the async state machine).

There is also a smaller difference; Add1 will marshal back to its original context while Add2 will not. This can cause a deadlock if the calling code does not use await:

public void Button1_Click(..)
{
  Add1().Wait(); // deadlocks
  Add2().Wait(); // does not deadlock
}

I explain this deadlock situation in more detail on my blog and in a recent MSDN article.

answered Jun 7, 2013 at 15:43

Stephen Cleary

460k78 gold badges721 silver badges858 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Rob Lyndon · Accepted Answer · 2013-06-07 22:25:16Z

1

Add1 will run synchronously until it encounters the await keyword. In this case, that will not have an effect because the await keyword is at the beginning of the method.

To see the effect of this, you can insert a Thread.Sleep() method into the beginning of Add1 and Add2, and notice that the method marked async blocks before it returns.

edited Jun 7, 2013 at 22:25

answered Jun 7, 2013 at 15:37

Rob Lyndon

12.7k5 gold badges56 silver badges79 bronze badges

Collectives™ on Stack Overflow

Async methods with and without async modifier

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related