Delegates

  • A delegate is a type that represents reference to methods with a particular parameter list and return type
  • A delegate is the glue/pipeline between
    an event and event handler
  • Specialised class based on the MulticastDelegate base class.
  • Often called as function pointer.
  • Delegates allows you to reference a method and later invoke it.
  • A object that knows how to call a method or group of methods

When to use Delegate

  • When you need a callback.
    • Commonly used in past performing background work
  • When a publisher notifies that an event
    has occurred, every subscriber is executed and can act on it.
  • Allow a method to run additional functionality or replace functionality during runtime.

Extensibility 

				
					House house = new House();

var buildHouse = new HouseBuild();
buildHouse.PaintBlue(house);
buildHouse.SemiFurnished(house);
buildHouse.DecorateBasicInterior(house);


public class House
{
}
				
			
				
					
public class HouseBuild
{
    public void PaintBlue(House house)
    {
        Console.WriteLine("Painting Blue Color");
    }

    public void DecorateBasicInterior(House house)
    {
        Console.WriteLine("Decorate basic interior");
    }

    public void SemiFurnished(House house)
    {
        Console.WriteLine("Semi Furnished !!");
    }
}

output :- 

Painting Blue Color
Semi Furnished !!
Decorate basic interior

  • Now If I want to modify the HouseBuild, I dont want to touch the existing class, rather I want to modify in my own way.
  • Lets use Delegate to extend it.
  • The same thing can be done using interface as well.
				
					
House house = new House();

var buildHouse = new HouseBuild();
//buildHouse.PaintBlue(house);
//buildHouse.SemiFurnished(house);
//buildHouse.DecorateBasicInterior(house);


HouseHandler obj = buildHouse.DecorateBasicInterior;
//obj += buildHouse.SemiFurnished;
obj += PaintWhite;
obj += FullFurnishedHouse;
obj.Invoke(house);


static void FullFurnishedHouse(House house)
{
    Console.WriteLine("This is full furnished house with all the amenities");
}

static void PaintWhite(House house)
{
    Console.WriteLine("The house is painted white");
}

public delegate void HouseHandler(House house);

Output:- 

Decorate basic interior
The house is painted white
This is full furnished house with all the amenities

				
					Action<House> obj = buildHouse.DecorateBasicInterior;
//obj += buildHouse.SemiFurnished;
obj += PaintWhite;
obj += FullFurnishedHouse;
obj.Invoke(house);
  • Alternatively we can use interface
  • How to decide whether to use interface or delegate ??
    • The caller doesnt need to access other properties or methods on the object implementing the method
    • A class may need more than one implementation of the method.

Delegate vs Interfaces Example 

				
					delegate void MyHouseHandler(string name);

interface IHouseHandler
{
    void BuildHouse(string house);
}

class HouseUsingDelegate
{
    // All four methods below can be used to implement the MyHouseHandler delegate
    void BuildHouse1(string house) { }
    void BuildHous2(string house) { }
    void BuildHous3(string house) { }
    void BuildHous4(string house) { }
}

class HouseUsingInterface : IHouseHandler
{
    public void BuildHouse(string house)
    {
        // Only this method could be used to call an implementation through an interface
    }
}

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