FullSUTMock.cs

namespace Examples;

using System;
using FluentAssertions;
using NSubstitute;
using Xunit;

public interface IFullMockService
{
    string GetValue(Guid id);
}

public class FullSUTMock
{
    private readonly IFullMockService _service;

    public FullSUTMock(IFullMockService service)
    {
        _service = service;
    }

    public string GetValue(Guid id)
    {
        // Here we would like to make sure that we're able to stub other virtual methods of the same class.
        // We wanna be sure that we get a partial mock which is useful for checking a scenario
        // in which a method knows an algorithm. It doesn't know any lower level details
        // it just does something according to some algorithm.
        // In that algorithm it may call other services which were injected and we can stub(we're not concerned about those here)
        // it also may call some other members of this class which could be virtual or quite often they're
        // "protected virtual internal" those methods just may do some low level things so that our algorithm
        // looks nice and clean.
        // 
        // In this particular example the algorithm is pretty dumb it just delegates call to GetValueEx
        // We would like be able to stub completely the call if we wish.
        return GetValueEx(id);
    }

    protected internal virtual string GetValueEx(Guid id)
    {
        throw new NotImplementedException();
    }
}

public class FullMockTests : TestsSubstituteOf<FullSUTMock>
{
    [Fact]
    public void MethodIsNotSubbed_OriginalImplementationWasNotCalled()
    {
        var id = Guid.NewGuid();
        var wrapper = new Wrapper();

        Use(wrapper);

        var actual = SUT.GetValue(id);

        actual.Should().BeNullOrWhiteSpace();
    }

    [Fact]
    public void MethodIsSubbed_TheValueWhichWasSetOnStubbedIsReturned()
    {
        var id = Guid.NewGuid();
        var wrapper = new Wrapper();

        Use(wrapper);

        SUT.GetValueEx(id).Returns("42");

        var actual = SUT.GetValue(id);

        actual.Should().Be("42");
    }

    private class Wrapper : IFullMockService
    {
        public string GetValue(Guid id)
        {
            return id.ToString();
        }
    }
}