// custom Setup Common Services
public class CommonServicesSetuper : ICommonServicesSetuper
{
    public void SetUp(ServiceCollection services)
    {
        services.AddSingleton(i =>
        {
            var mock = new Mock<IConstantInt>();
            mock.SetupGet(j => j.ConstantInt).Returns(20);
            return mock.Object;
        });
    }
}

public interface IAdder
{
    int Add(int x, int y);
}

public interface IConstantInt
{
    int ConstantInt { get; }
}

public class AddTester
{
    private IAdder _Adder;
    private IConstantInt _ConstantInt;

    public int GivenInt = 0;

    public int GiWhenInt = 0;

    public AddTester(IAdder adder, IConstantInt constantInt)
    {
        _Adder = adder;
        _ConstantInt = constantInt;
    }

    public int Add(int x, int y) => _Adder.Add(x, y) + GivenInt + GiWhenInt + _ConstantInt.ConstantInt;
}

public class AddTest : SpecsFor<AddTester>
{
    public override void SetUp(IServiceCollection services)
    {
        services.AddSingleton(i =>
        {
            var mock = new Mock<IAdder>();
            mock.Setup(j => j.Add(It.IsAny<int>(), It.IsAny<int>()))
            .Returns<int, int>((x, y) => x + y);
            return mock.Object;
        });
    }

    public override void Given()
    {
        SUT.GivenInt = 6;
    }

    public override void When()
    {
        SUT.GiWhenInt = 4;
    }

    [Fact(DisplayName = "4 + 5 + 6 + 4 + 20 = 35")]
    public void Add4And5()
    {
        Assert.Equal(39, SUT.Add(4, 5));
    }

    [Fact(DisplayName = "6 + 6 + 6 + 4 + 20 = 42")]
    public void Add6And6()
    {
        Assert.Equal(42, SUT.Add(6, 6));
    }
}