Partial substitution on interface with concrete implementation (like composition)

[Dheerendra Rathor]

I've an interface which implements two interfaces, and a class implementing one of those interfaces

interface IA : IB, IC {
}

class BImpl : IB {
    ...
}

Is it possible to mock IA in a way that all calls which are defined in IB lands to object of BImpl while calls to methods in IC are mocked.

Something like:

var b = new BImpl();

var iaMock = Substitute.For<IA>(b); // or Substitute.For<IB, IC>(b)

iaMock.MethodDefinedInIb(); // This should call b.MethodDefinedInIb();

iaMock.MethodDefinedInIc().Returns("blah"); // This is calling generated mock implementation

Thanks

[David Tchepak]

Hello,

NSubstitute does not support saying "forward every call from `IB` to this instance of `BImpl`", but you can do this on a per method basis.

The snippet below uses NSubstitute 4.0:

    public interface IC {

        int GetInt(string s);

    }

    public interface IB {

        string GetString(string s);

    }

    public interface IA : IB, IC { }

    public class BImpl : IB {

        public virtual string GetString(string s) {

            return $"{s}!";

        }

    }

    public class UnitTest1 {

        [Fact]

        public void Partial() {

            var sub = Substitute.For<IA, BImpl>();

            // Method on IB / BImpl should call base implementation:

            sub.When(x => x.GetString(Arg.Any<string>())).CallBase();

            // Stub method on IC:

            sub.GetInt("hi").Returns(42);

            Assert.Equal("hi!", sub.GetString("hi"));

            Assert.Equal(42, sub.GetInt("hi"));

        }

    }

A bit more clumsy, but probably more obvious, is using `Returns` to forward to a real instance:

        [Fact]

        public void ForwardSpecificCalls() {

            var b = new BImpl();

            var sub = Substitute.For<IA>();

            sub.GetString("").ReturnsForAnyArgs(x => b.GetString(x.Arg<string>()));

            sub.GetInt("hi").Returns(42);

            Assert.Equal("hi!", sub.GetString("hi"));

            Assert.Equal(42, sub.GetInt("hi"));

        }

You can view the full code here:  https://gist.github.com/dtchepak/3e561944b1cccd9238a1da8b34ffc9be

Hope this helps.

Regards,

David

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