using System;

public class Program

{

    public static void Main()

    {

        //the 'real' version

        var usingClass = new UsingClass(null);//because we did not choose to pass in the object to use, it uses the default

        Console.WriteLine( usingClass.HelloWorld("yoYO"));

        //now we mock it.

        var usingClassMocked = new UsingClass(implementationCLass: new MockDoStuff());

        Console.WriteLine(usingClassMocked.HelloWorld("my mock input"));

    }

}

​

public interface IImplementStuff

{

    string HelloWorld(string inputString);  

}

​

public class UsingClass

{

    private readonly IImplementStuff localImplementStuff;

    public UsingClass(IImplementStuff implementationCLass = null)

    {

        this.localImplementStuff = implementationCLass ?? new DoStuff();

    }

    public string HelloWorld(string input)

    {

        return this.localImplementStuff.HelloWorld(input);

    }

}

​

public class DoStuff : IImplementStuff

{

    public string HelloWorld(string inputString)

    {

        return String.Format("Hello World input:{0}",inputString);

    }

}

​

public class MockDoStuff : IImplementStuff

{

​

    public string HelloWorld(string inputString)

    {

        return "This was totally Mocked";   

    }

}