Task.Wait() vs. await

The await keyword is a new keyword in C# 5.0 which, in tandem with async keyword, allows us to easily author methods which execute asynchronously in regards to the calling code. In a previous post I’ve shown a certain issue you should look out for when using the async keyword. In this post we’ll check a similar issue with the await keyword.

The await Trap

Let’s remember our test code from the previous post:

class AsyncClass

{

    public void Process(string s)

    {

        if (s == null) throw new ArgumentNullException("s");

        ProcessCore(s);

    }

    public Task ProcessAsync1(string s)

    {

        if (s == null) throw new ArgumentNullException("s");

        return Task.Run(() => ProcessCore(s));

    }

    public async Task ProcessAsync2(string s)

    {

        if (s == null) throw new ArgumentNullException("s");

        await Task.Run(() => ProcessCore(s));

    }

    private void ProcessCore(string s)

    {

        for (int len = 1; len <= s.Length; len++)

        {

            Thread.Sleep(1000); // This task is very complicated!!

            Console.WriteLine(s.Substring(0, len));

        }

    }

}

What do you think the following code does?

var asyncTest = new AsyncClass();

await asyncTest.ProcessAsync2(null);

Well, you’re probably saying that this time we are actually awaiting the method to complete so it must throw an ArgumentNullException! And, you’re correct – this is indeed the result of the above code. But how about the following code?

var asyncTest = new AsyncClass();

asyncTest.ProcessAsync2(null).Wait();

Hmmmmm… Isn’t this exactly the same as before? Won’t this throw an ArgumentNullException as well? No! It won’t.

Read more: Stiller on Software

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