An async version of xUnit's Async.Throws. Use it like so: await ThrowsAsync<AuthenticationException>(async () => await obj.GetStuffAsync());

ThrowsAsync.cs

public async static Task<T> ThrowsAsync<T>(Func<Task> testCode) where T : Exception
{
    try
    {
        await testCode();
        Assert.Throws<T>(() => { }); // Use xUnit's default behavior.
    }
    catch (T exception)
    {
        return exception;
    }
    return null;
}