using System;

using NSubstitute;

​

public class BarService {

    public virtual string GetTheFoo() {

        return "9000";

    }

}

public class FooService

{

    private readonly BarService barService;

    protected FooService() { }

    public FooService(BarService barService) {

        this.barService = barService;   

    }

    public string GetServiceId() {

        return barService.GetTheFoo() + " foo";

    }

}

​

public class FooServiceTest {

    [Fact]

    public void GetServiceId_ReturnsFooAppended() {

        var barServiceMock = Substitute.For<BarService>();

        var fooService = new FooService(barServiceMock);        

        barServiceMock

            .GetTheFoo()

            .Returns("bar");

        var actual = fooService.GetServiceId();

        actual.Should().BeEquivalentTo("bar foo");

        barServiceMock

            .Received()

            .GetTheFoo();

    }

}