Mocking generic methods in Moq without specifying T

I have found an alternative that I think gets closer to what you want. Anyway it was useful for me so here goes. The idea is to create an intermediate class which is almost purely abstract, and implements your interface. The part which is not abstract is the part Moq can't handle. E.g.

public abstract class RepoFake : IRepo
{
    public IA<T> Reserve<T>()
    {
        return (IA<T>)ReserveProxy(typeof(T));
    }

    // This will be mocked, you can call Setup with it
    public abstract object ReserveProxy(Type t);

    // TODO: add abstract implementations of any other interface members so they can be mocked
}

Now you can mock RepoFake instead of IRepo. Everything works the same except for you write your setups on ReserveProxy instead of Reserve. You can handle the Callback if you want to perform assertions based on type, though the Type parameter to ReserveProxy is totally optional.

Unless I'm misunderstand what you need, you could build a method like this:

private Mock<IRepo> MockObject<T>()
{
    var mock = new Mock<IRepo>();
    return mock.Setup(pa => pa.Reserve<T>())
        .Returns(new Mock<IA<T>>().Object).Object;
}

Simply do this:

[TestMethod]
public void ExampleTest()
{
  var mock = new Mock<IRepo> { DefaultValue = DefaultValue.Mock, };
  // no setups needed!

  ...
}

Since your mock does not have behavior Strict, it will be happy with calls that you haven't even set up. In that case a "default" is simply returned. Then

DefaultValue.Mock

ensures that this "default" is a new Mock<> of appropriate type, instead of just a null reference.

The limitation here is that you cannot control (e.g. make special setups on) the individual "sub-mocks" that are returned.

In Moq 4.13 they introduced the It.IsAnyType type which you can using to mock generic methods. E.g.

public interface IFoo
{
    bool M1<T>();
    bool M2<T>(T arg);
}

var mock = new Mock<IFoo>();
// matches any type argument:
mock.Setup(m => m.M1<It.IsAnyType>()).Returns(true);

// matches only type arguments that are subtypes of / implement T:
mock.Setup(m => m.M1<It.IsSubtype<T>>()).Returns(true);

// use of type matchers is allowed in the argument list:
mock.Setup(m => m.M2(It.IsAny<It.IsAnyType>())).Returns(true);
mock.Setup(m => m.M2(It.IsAny<It.IsSubtype<T>>())).Returns(true);