Testing for exceptions in async methods

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.

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.

With Fluent Assertions v5+ the code will be like :

ISubject sut = BuildSut();
//Act and Assert
Func<Task> sutMethod = async () => { await sut.SutMethod("whatEverArgument"); };
await sutMethod.Should().ThrowAsync<Exception>();

This should work.

Other variation of usage ThrowAsync method:

await Should.ThrowAsync<Exception>(async () => await Fail());