Example from https://groups.google.com/d/msg/nsubstitute/D_wDeBTgckU/vqmpftNpDgAJ

a.cs

using Xunit;
using NSubstitute;

namespace NSubWorkshop
{
    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"));
        }
        
        [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"));
        }
    }
}