Handling specimen creation inconsistencies between AutoFixture and Moq

autofixture, moq

Solution

As there's no extensibility point in Moq that enables AutoFixture to hook in and supply a value of `ThingOne`, there's not a whole lot you can do.

However, you can use the `SetReturnsDefault<T>` method of Moq. Modifying the above test would then be like this:

[Theory, BasicConventions]
public void WhyCannotInstantiateProxyOfClass(
    ThingOne one, ThingTwo two, IThingBuilders builder, SomeClass sut)
{
    Assert.IsAssignableFrom<IThings>(one);
    Assert.IsAssignableFrom<IThings>(two);
    Mock.Get(builder).SetReturnsDefault(one); // Add this to make the test pass

    var actual = sut.GetCommands(builder);

    Assert.Equal(1, actual.OfType<ThingOne>().Count());
    Assert.Equal(1, actual.OfType<ThingTwo>().Count());
}

This is a bit easier than having to write a specific `Setup`/`Returns` pair, but not much. You could move that code to an AutoFixture Customization, but again, since this is a generic method on a a Mock instance, you'll explicitly need to call this for e.g. ThingOne in order to set the default for that return type. Not particularly flexible.

Problem

I am using AutoMoqCustomization in my test conventions. Consider the code below. Everything works great until I add a constructor to one of the concrete classes. When I do, I get "could not find a parameterless constructor". We know AutoFixture doesn't have an issue with the constructor because it delivered me the test object `one` which proved to be assignable from IThings... no failure there. So it must be moq. This makes some sense because I assume `builder` was generated by moq and passed into the `GetCommands` method. So I think I can see that control has been passed from AutoFixture to moq at that point. That takes care of the why, but what should I do about it? Is there a way to instruct moq on how to deal with the `ThingOne` or is there a way to instruct AutoFixture to ignore moq for `IThingBuilders` and instead do something Fixtury? ``` public class TestClass { public interface IThingBuilders { T1 Build<T1>() where T1 : IThings; } public interface IThings { } public class ThingOne : IThings { public ThingOne(string someparam) { } } public class ThingTwo : IThings { } public class SomeClass { public List<IThings> GetCommands(IThingBuilders builder) { var newlist = new List<IThings>(); newlist.Add(builder.Build<ThingOne>()); newlist.Add(builder.Build<ThingTwo>()); return newlist; } } [Theory, BasicConventions] public void WhyCannotInstantiateProxyOfClass(ThingOne one, ThingTwo two, IThingBuilders builder, SomeClass sut) { Assert.IsAssignableFrom<IThings>(one); Assert.IsAssignableFrom<IThings>(two); var actual = sut.GetCommands(builder); Assert.Equal(1, actual.OfType<ThingOne>().Count()); Assert.Equal(1, actual.OfType<ThingTwo>().Count()); } } ```

Original source