using System;

public class Program

{

    public static void Main()

    {

        IMessageWriter writer = new ConsoleMessageWriter();

        var salutation = new Salutation(writer);

        salutation.Exclaim();

    }

}

public class Salutation

{

    private readonly IMessageWriter writer;

    public Salutation(IMessageWriter writer)

    {

        if(writer == null)

        {

            throw new ArgumentNullException(writer);

        }

        this.writer = writer;

    }

    public void Exclaim()

    {

        this.writer.Write("Hello DI");

    }

}

​

public interface IMessageWriter

{

    void Write(string nessage);

}

​

public class ConsoleMessageWriter:IMessageWriter

{

    public void Write(string message)

    {

        Console.WriteLine(message);

    }

}