How to return passed parameters in an async method with Moq in unit tests?

Question

I am struggling with the problem of how to use the Moq-Setup-Return construct.

First, my setting:

Some repository of type IRepository-Interface has to implement the StoreAsync-Method that returns a StoreResult object with the stored entity as property included.

using System.Threading.Tasks;
using Moq;
using Xunit;

namespace Tests
{
    public class Entity { }

    public class StoreResult
    {
        public Entity Entity { get; set; }
    }

    public interface IRepository
    {
        Task<StoreResult> StoreAsync(Entity entity);
    }

    public class Tests
    {
        [Fact]
        public void Test()
        {
            var moq = new Mock<IRepository>();
            moq.Setup(m => m.StoreAsync(It.IsAny<Entity>())).Returns(e => Task.FromResult<Task<StoreResult>>(new StoreResult {Entity = e}));
        }
    }
}

Now I try to write a Mock-Objekt for the IRepository-Interface, but I do not know how to code the Return-Statement so that the StoreResult-Object includes the entity given as a parameter to the StoreAsync-Function.

I read about this topic in Moq ReturnsAsync() with parameters and Returning value that was passed into a method.

I have tried

moq.Setup(m => m.StoreAsync(It.IsAny<Entity>()))
     .ReturnsAsync(entity => new StoreResult {Entity = entity});

with the error statement "Cannot convert lambda expression to type "StoreResult", because it is not a delegate type.

And with the same error message I tried

moq.Setup(m => m.StoreAsync(It.IsAny<Entity>()))
     .Returns(e => Task.FromResult<Task<StoreResult>>(new StoreResult {Entity = e}));

I am using a .NET Core xUnit environment with Moq 4.6.36-alpha

Thank you for your help.

Shouldn't it just be: `Task.FromResult(new StoreResult {Entity = e})` — Callum Linington, Aug 01 '16 at 10:07
@CallumLinington - With `.Returns(Task.FromResult(new StoreResult {Entity = e}));` e is not defined. — Ralf Bönning, Aug 01 '16 at 10:11
@CallumLinington Aaah, I see. `moq.Setup(m => m.StoreAsync(It.IsAny())).Returns((Entity e) => Task.FromResult(new StoreResult {Entity = e}));` did the trick. — Ralf Bönning, Aug 01 '16 at 10:55
Yeah, you need to specify the object always, because of the amount of overloads, there are ambiguous calls — Callum Linington, Aug 01 '16 at 12:07
@NateBarbettini - Oh yes, this solved the issue. For the records I added the information as answer. — Ralf Bönning, Aug 02 '16 at 07:06

score 14 · Accepted Answer · answered Aug 02 '16 at 07:04

Thanks to the tip from Callum Linigton I came to the following solution:

moq
 .Setup(m => m.StoreAsync(It.IsAny<Entity>()))
 .Returns((Entity e) => Task.FromResult(new StoreResult {Entity = e}));

The key difference is to specify the type for the input parameter of the lambda expression in order to avoid ambiguous calls.

How to return passed parameters in an async method with Moq in unit tests?

1 Answers1