Alternative way to implement Service, but will not compile

pattern-alternativeTightlyBound.cs

class AlternateService
{
    public void Serve()
    {
        Console.WriteLine("Service Called");
        //To Do: Some Stuff
    }
}

class TestClient
{
    public static Main()
    {
        // will not compile. Cannot mock out the alternate service
        Client client = new Client(new AlternateService());
        client.Start();

        Console.ReadKey();
    }
}