using System;

​

/*An object that knows how to call a method (or a group of methods).

A reference to a function.

Why? For designing extensible and flexible applications (eg frameworks).

Delegates refer to System.MulticastDelegate, based on System.Delegate with public properties Method and Target (the class holding that method).

.NET has two generic delegates: Action and Func

    -Action returns void

    -Func<x, y> returns y and takes an x type input.

Interfaces or Delegates?

    -Use a delegate when

        -An eventing design pattern is used.

        -The caller doesn't need to access other properties or methods on the object implementing the method.

*/

​

public class Photo

{

    public static Photo Load(string path)

    {

        return new Photo();

    }

    public void Save()

    {

    }

}

​

public class PhotoProcessor

{

    // public delegate void PhotoFilterHandler(Photo photo); // Define the signature of the method that this delegate will be responsible for calling.

    // public void Process(string path, PhotoFilterHandler filterHandler)

    public void Process(string path, Action<Photo> filterHandler) // Using generic .NET delegate Action<>

    {

        // System.Action<> // Points to a method that returns void.

        // System.Func<> // Points to a method that returns a value.

        var photo = Photo.Load(path);

        filterHandler(photo);

        /* var filters = new PhotoFilters(); // Without a delegate, our code wouldn't be extensible.

        filters.ApplyBrightness(photo);

        filters.ApplyContrast(photo);

        filters.Resize(photo); */

        photo.Save();

    }

}

​

public class PhotoFilters

{

    public void ApplyBrightness(Photo photo)

    {

        Console.WriteLine("Apply brightness");

    }

    public void ApplyContrast(Photo photo)

    {

        Console.WriteLine("Apply contrast");

    }

    public void Resize(Photo photo)

    {

        Console.WriteLine("Resize photo");

    }

}

public class Program

{

    public static void Main()

    {

        var processor = new PhotoProcessor();

        var filters = new PhotoFilters(); // Create an instance of class containing methods.

        // PhotoProcessor.PhotoFilterHandler filterHandler = filters.ApplyBrightness; // Using custom delegate. Here we join classes calling what we need.

        Action<Photo> filterHandler = filters.ApplyBrightness; // Using generic .NET delegate Action<>, which returns void.

        filterHandler += filters.ApplyContrast; // New methods can be added to delegate.

        filterHandler += RemoveRedEyeFilter; // We can even define methods from client side.

        processor.Process("photo.jpg", filterHandler);

    }

    static void RemoveRedEyeFilter(Photo photo)

    {

        Console.WriteLine("Apply RemoveRedEye");

    }

}