Delegates

• Delegate is an object that knows how to call a method or a  group of methods which have the same signature. 
• It is a reference to a function.
• Of course, we can call methods directly, but we need delegates for designing extensible and flexible applications. (Ex: Frameworks).
• Imagine that you are designing a framework which is use for processing the photos. In that framework, you have defined few photo processing ways such as "Apply Brightness" , "Apply Contrast" and etc. In future, may be a client who is using your framework need to add their own photo filter which we haven't defined. So in that case, we have to change our application , recompile and redeploy. It is not a good idea. If we use delegates in our frameworks, client can add any no of filters without relying on us. See below example. 

    public class Photo
    {
        public Photo Load(string path)
        {
            return new Photo();
        }
    }
	
    public class Filter
    {
        public void Resize(Photo photo)
        {
            Console.WriteLine("Changed the Size");
        }

        public void ChangeBrightness(Photo photo)
        {
            Console.WriteLine("Changed the brightness");
        }

        public void ChangeContrast(Photo photo)
        {
            Console.WriteLine("Changed contrast of the photo");
        }

    }
	
    public class PhotoMaker
    {
        // Define the delegate
        public delegate void PhotoFilterHandler(Photo photo);
        public void Process(string path, PhotoFilterHandler photofilterhandler )
        {
            var photoObject = new Photo();
            var photo = photoObject.Load(path);
            photofilterhandler(photo);
        }
    }
	
    class Program
    {
        static void Main(string[] args)
        {
            var photomaker = new PhotoMaker();
            var filter = new Filter();
			
	    // Adding ChangeBrightness filter
            PhotoMaker.PhotoFilterHandler filterhandler = filter.ChangeBrightness;
			
	    //Adding ChangeContrast filter
	    filterhandler += filter.ChangeContrast;
			
	    // Remove ChangeBrightness filter
            filterhandler -= filter.ChangeBrightness;
			
	    //Adding custom method  without changing the PhotoProcess class
            filterhandler+= RemoveRedEye;
           
	    photomaker.Process("", filterhandler);	

            Console.Read();
        }

        static void RemoveRedEye(Photo photo)
        {
            Console.WriteLine("Remove Red Eye");
        }
    }
	

• In .Net, there are two delegates which are generic. They are Action and Func. Difference between Func and Action is Func point to a method that has a return value and Action point to a method that returns void (no return type). 

• Below code uses in built Action delegate instead of creating a custom delegate. 

 	
public class Photo
    {
        public Photo Load(string path)
        {
            return new Photo();
        }
    }
	
    public class Filter
    {
        public void Resize(Photo photo)
        {
            Console.WriteLine("Changed the Size");
        }

        public void ChangeBrightness(Photo photo)
        {
            Console.WriteLine("Changed the brightness");
        }

        public void ChangeContrast(Photo photo)
        {
            Console.WriteLine("Changed contrast of the photo");
        }
    }
	
    public class PhotoMaker
    {
        public void Process(string path, Action<Photo> photofilterhandler )
        {
            var photoObject = new Photo();
            var photo = photoObject.Load(path);
            photofilterhandler(photo);
        }
    }
	
    class Program
    {
        static void Main(string[] args)
        {
            var photomaker = new PhotoMaker();
            var filter = new Filter();
			
            // Adding ChangeBrightness filter
            Action<Photo> filterhandler = filter.ChangeBrightness;
			
	    //Adding ChangeContrast filter
	    filterhandler += filter.ChangeContrast;
			
	    // Remove ChangeBrightness filter
            filterhandler -= filter.ChangeBrightness;
			
	    photomaker.Process("", filterhandler);	

            Console.Read();
        }
    }

Use a delegate in the following circumstances:

• An eventing design pattern is used.
• The caller has no need to access other properties, methods, or interfaces on the object implementing the method.