setup.ReturnsAsync and friends

Another part of Moq that is obsolete for the most part is the async setup helper verbs setup.ReturnsAsync and setup.ThrowsAsync.

What are they?

These extension methods allow you to set up the result of an async method call without having to manually wrap the desired result in a task:

-mock.Setup(m => m.CountGoldPiecesAsync()).Returns     (Task.FromResult(11873));
+mock.Setup(m => m.CountGoldPiecesAsync()).ReturnsAsync(                11873 );

-mock.Setup(m => m.GoPiratingAsync()).Throws     (Task.FromException(new PirateException("arr!")));
+mock.Setup(m => m.GoPiratingAsync()).ThrowsAsync(                   new PirateException("arr!") );

You could let the compiler do most the work…

Given C# async/await, the same work could be done by the compiler:

-mock.Setup(m => m.CountGoldPiecesAsync()).Returns(Task.FromResult(11873));
+mock.Setup(m => m.CountGoldPiecesAsync()).Returns(    async () => 11873 );

-mock.Setup(m => m.GoPiratingAsync()).Throws(Task.FromException(new PirateException("arr!")));
+mock.Setup(m => m.GoPiratingAsync()).Throws( async () => throw new PirateException("arr!") );

Though the compiler will warn that the lambdas aren’t await-ing anything and will therefore run synchronously, which can be a little annoying.

Alternative API design in Moq 4.16

In version 4.16 (which is actually not published at this time of writing), Moq has gained an alternate API to do the same thing in yet another way—it allows you do set up the .Result of tasks directly:

-mock.Setup(m => m.CountGoldPiecesAsync()       ).Returns(Task.FromResult(11873));
+mock.Setup(m => m.CountGoldPiecesAsync().Result).Returns(                11873 );

-mock.Setup(m => m.GoPiratingAsync()       ).Throws(Task.FromException(new PirateException("arr!")));
+mock.Setup(m => m.GoPiratingAsync().Result).Throws(                   new PirateException("arr!") );

This was added in moq/moq4#1126 and was surprisingly easy to implement… check it out!

It has a few benefits even over a native C# solution, among them the ability to do recursive setups (method chaining) across async calls, and no compiler warnings due to missing awaits.

But it’s entirely possible that not everyone will like this new feature; I do, so I’m going ahead here with removing the async setup verbs!

Removal

This is another feature easily removed, which isn’t surprising at all, since the code parts in question are only extension methods. I’ve done this in a111c23.

The setup.ReturnsAsync and setup.ThrowsAsync extension methods make up much more code (when including tests) than Moq 4.16’s alternative solution: approx. 2,400 lines of code vs. only approx. 600 for the code required to enable setting up task.Result!