Async Lambdas

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Today I learned something new and I love that!

I was looking at some code that looked like this:

    await obj.GetSomeAsync();
    Assert.True(false, "SomeException was not thrown");
catch (SomeException)

That’s odd. We’re using xUnit. Why not use the Assert.Throws method? So I tried with the following naïve code.

Assert.Throws<SomeException>(() => await obj.GetSomeAsync());

Well that didn’t work. I got the following helpful compiler error:

error CS4034: The 'await' operator can only be used within an async lambda expression. Consider marking this lambda expression with the 'async' modifier.

Oh, I never really thought about applying the async keyword to a lambda expression, but it makes total sense. So I tried this:

Assert.Throws<SomeException>(async () => await obj.GetSomeAsync());

Hey, that worked! I rushed off to tell the internets on Twitter.

But I made a big mistake. That only made the compiler happy. It doesn’t actually work. It turns out that Assert.Throws takes in an Action and thus that expression doesn’t return a Task to be awaited upon. Stephen Toub explains the issue in this helpful blog post, Potential pitfalls to avoid when passing around async lambdas.

Ah, I’m gonna need to write my own method that takes in a Func<Task>. Let’s do this!

I wrote the following:

public async static Task<T> ThrowsAsync<T>(Func<Task> testCode)
      where T : Exception
    await testCode();
    Assert.Throws<T>(() => { }); // Use xUnit's default behavior.
  catch (T exception)
    return exception;
  // Never reached. Compiler doesn't know Assert.Throws above always throws.
  return null;

Here’s an example of a unit test (using xUnit) that makes use of this method.

public async Task RequiresBasicAuthentication()
  await ThrowsAsync<SomeException>(async () => await obj.GetSomeAsync());

And that works. I mean it actually works. Let me know if you see any bugs with it.

Note that you have to change the return type of the test method (fact) from void to return Task and mark it with the async keyword as well.

So as I was posting all this to Twitter, I learned that Brendan Forster (aka @ShiftKey) already built a library that has this type of assertion. But it wasn’t on NuGet so he’s dead to me.

But he remedied that five minutes later.

Install-Package AssertEx.

So we’re all good again.

If I were you, I’d probably just go use that. I just thought this was an enlightening look at how await works with lambdas.