How to handle exceptions thrown by Tasks in xUnit .net's Assert.Throws?

Question

The following asynchronous xUnit.net test with a lambda marked with the async modifier fails by reporting that no exception was thrown

Answer 1

Update

This has been solved in xUnit 2, with the addition of Assert.ThrowsAsync.

---

I am suspecting that Assert.Throws is not async-aware. I recommend raising this issue with the xUnit team, suggesting a ThrowsAsync be added.

An async delegate in this case is returning Task or Task, and the ArgumentNullException is not thrown out of the delegate directly; instead, it is placed on the Task (Task.Exception.InnerException). Assert.Throws is expecting the exception to be thrown out of the delegate directly, not placed on a property of the return value.

You can create your own AssertEx.ThrowsAsync as such:

public static async Task ThrowsAsync(Func func)
{
  var expected = typeof(TException);
  Type actual = null;
  try
  {
    await func();
  }
  catch (Exception e)
  {
    actual = e.GetType();
  }
  Assert.Equal(expected, actual);
}

which can be used as such:

[Theory, AutoWebData]
public async Task SearchWithNullQueryThrows(
    SearchService sut,
    CancellationToken dummyToken)
{
    // Fixture setup
    // Exercise system and verify outcome
    await AssertEx.ThrowsAsync(async () =>
        await sut.SearchAsync(null, dummyToken));
    // Teardown
}

I use a similar approach in MSTest.