Testing for exceptions in async methods

Question

I'm a bit stuck with this code (this is a sample):

public async Task Fail()
{
    await Task.Run(() => { throw new Exception(); });
}

[Test]
public async Task TestFail()
{
    Action a = async () => { await Fail(); };
    a.ShouldThrow<Exception>();
}

The code doesn't catch the exception, and fails with

Expected a System.Exception to be thrown, but no exception was thrown.

I'm sure I'm missing something, but docs seem to suggest this is the way to go. Some help would be appreciated.

Answer 1

You should use Func<Task> instead of Action:

[Test]
public void TestFail()
{
    Func<Task> f = async () => { await Fail(); };
    f.ShouldThrow<Exception>();            
}

That will call the following extension which is used to verify asynchronous methods

public static ExceptionAssertions<TException> ShouldThrow<TException>(
    this Func<Task> asyncAction, string because = "", params object[] becauseArgs)
        where TException : Exception        

Internally this method will run task returned by Func and wait for it. Something like

try
{
    Task.Run(asyncAction).Wait();
}
catch (Exception exception)
{
    // get actual exception if it wrapped in AggregateException
}

Note that test itself is synchronous.