How do I set up a verifiable expectation on a mocked async method?

Question

I'm trying to use Moq to test integration between a WebAPI controller and a Redis database, using the StackExchange.Redis client, and cannot work out how to set up a verifiable expectation on a mocked async method that includes a callback or some other assertion behaviour.

Ordinarily, I'd use the following syntax:

const string KEY = "some_key";
var db = new Mock<IDatabase>();
db.Setup(d => d.HashSetAsync(KEY, It.IsAny<HashEntry[]>(),It.IsAny<CommandFlags>()))
    .Callback<RedisKey,HashEntry[],CommandFlags>((key, hash, flags) => {
                hash.ShouldContain(entry => entry.Name == "customerid");
                hash.ShouldContain(entry => entry.Name == "quotenumber");
     })
     .Verifiable();

But this is giving me:

'Moq.Language.Flow.IReturnsThrows<StackExchange.Redis.IDatabase,System.Threading.Tasks.Task>' does not contain a definition for 'Verifiable' and no extension method 'Verifiable' accepting a first argument of type 'Moq.Language.Flow.IReturnsThrows' could be found (are you missing a using directive or an assembly reference?)

If I change db.HashSetAsync to db.HashSet in the Setup invocation, it works as expected. It appears that the setting a Callback on a regular method returns an ICallbackResult but setting a callback on an async method invocation returns an IReturnsThrows - and I'm not sure how you mark one of those as verifiable. Any ideas?

You seem to be missing a return from your setup for the call back. what type does `HashSetAsync` return — Nkosi, Apr 05 '16 at 14:59
Why don't you use `ReturnsAsync`? In versions that don't have it you can use `.Returns(Task.FromResult(someValue));` — Panagiotis Kanavos, Apr 05 '16 at 15:11
@PanagiotisKanavos so far I haven't come across `ReturnsAsync`. Where do I look for that? looks interesting. — Nkosi, Apr 05 '16 at 15:19
What Moq version are you using? It's available in Moq 4.2. Even before this, you can return a filled Task with `Task.FromResult` — Panagiotis Kanavos, Apr 05 '16 at 15:24
Check the duplicate question. The *second* answer says that `RetunrsAsync` is available only for methods that return `Task`. You can still use `.Returns(Task.FromResult(...));` — Panagiotis Kanavos, Apr 05 '16 at 15:35
@PanagiotisKanavos yes now I see. learned something new today. thanks. — Nkosi, Apr 05 '16 at 15:39

score 11 · Accepted Answer · edited May 23 '17 at 12:24

For async methods you need to return a completed Task from the setup before using a callback

have a look here:

Using Moq to mock an asynchronous method for a unit test

You're creating a task but never starting it, so it's never completing. However, don't just start the task - instead, change to using Task.FromResult<TResult> which will give you a task which has already completed:

this works

const string KEY = "some_key";
var db = new Mock<IDatabase>();
db.Setup(d => d.HashSetAsync(KEY, It.IsAny<HashEntry[]>(), It.IsAny<CommandFlags>()))
    .Returns(Task.FromResult<object>(null))
    .Callback<RedisKey, HashEntry[], CommandFlags>((key, hash, flags) => {
        hash.ShouldContain(entry => entry.Name == "customerid");
        hash.ShouldContain(entry => entry.Name == "quotenumber");
    })
    .Verifiable();

How do I set up a verifiable expectation on a mocked async method?

1 Answers1