6 important uses of Delegates and Events

private int Add(int i,int y)
{
return i + y;
}

// Declare delegate
public delegate int PointetoAddFunction(int i,int y);

// Create delegate reference
PointetoAddFunction myptr = null;

// Point the reference to the add method
myptr = this.Add;

// Invoke the delegate
myptr.Invoke(20, 10)

public class clsMaths
{
public delegate int PointerMaths(int i, int y);
}

public class clsMaths
{
public delegate int PointerMaths(int i, int y);
public PointerMaths getPointer(int intoperation)
{
PointerMaths objpointer = null;
if (intoperation == 1)
{
objpointer = Add;
}
else if (intoperation == 2)
{
objpointer = Sub;
}
else if (intoperation == 3)
{
objpointer = Multi;
}
else if (intoperation == 4)
{
objpointer = Div;
}
return objpointer;
}
}

public class clsMaths
{
public delegate int PointerMaths(int i, int y);
public PointerMaths getPointer(int intoperation)
{
PointerMaths objpointer = null;
if (intoperation == 1)
{
objpointer = Add;
}
else if (intoperation == 2)
{
objpointer = Sub;
}
else if (intoperation == 3)
{
objpointer = Multi;
}
else if (intoperation == 4)
{
objpointer = Div;
}
return objpointer;
}
private int Add(int i, int y)
{
return i + y;
}
private int Sub(int i, int y)
{
return i - y;
}
private int Multi(int i, int y)
{
return i * y;
}
private int Div(int i, int y)
{
return i / y;
}
}

So at the client side the calls becomes generic without any coupling with the actual method names like 'Add' , 'Sub' etc.

int intResult = objMath.getPointer(intOPeration).Invoke(intNumber1,intNumber2);

Multicast delegates

In our previous example we have see how we can create a delegate pointer to a function or method. We can also create a delegate which can point to multiple functions and methods. If we invoke such delegate it will invoke all the methods and functions associated with the same one after another sequentially.
Below is the code snippet which attaches 2 methods i.e. method1 and method2 with delegate 'delegateptr'. In order to add multiple methods and function we need to use '+=' symbols. Now if we invoke the delegate it will invoke 'Method1' first and then 'Method2'. Invocation happen in the same sequence as the attachment is done.

// Associate method1
delegateptr += Method1;
// Associate Method2
delegateptr += Method2;
// Invoke the Method1 and Method2 sequentially
delegateptr.Invoke();

So how we can use multicast delegate in actual projects. Many times we want to create publisher / subscriber kind of model. For instance in an application we can have various error logging routine and as soon as error happens in a application you would like to broadcast the errors to the respective components.

Simple demonstration of multicast delegates

In order to understand multicast delegates better let's do the below demo. In this demo we have 'Form1', 'Form2' and 'Form3'. 'Form1' has a multicast delegate which will propagate event to 'Form2' and 'Form3'.


At the form level of 'Form1' (this form will be propagating events to form2 and form3) we will first define a simple delegate and reference of the delegate as shown in the code snippet below. This delegate will be responsible for broadcasting events to the other forms.

// Create a simple delegate
public delegate void CallEveryOne();
// Create a reference to the delegate
public CallEveryOne ptrcall=null;
// Create objects of both forms
public Form2 obj= new Form2();
public Form3 obj1= new Form3();

In the form load we invoke the forms and attach 'CallMe' method which is present in both the forms in a multicast fashion ( += ).

private void Form1_Load(object sender, EventArgs e)
{
// Show both the forms
obj.Show();
obj1.Show();
// Attach the form methods where you will make call back
ptrcall += obj.CallMe;
ptrcall += obj1.CallMe;
}

Finally we can invoke and broadcast method calls to both the forms.

private void button1_Click(object sender, EventArgs e)
{
// Invoke the delegate
ptrcall.Invoke();
}

Problem with multicast delegates – naked exposure

The first problem with above code is that the subscribers (form2 and form3) do not have the rights to say that they are interested or not interested in the events. It's all decided by 'form1'.
We can go other route i.e. pass the delegate to the subscribers and let them attach their methods if they wish to subscribe to the broadcast sent by 'form1'. But that leads to a different set of problems i.e. encapsulation violation.
If we expose the delegate to the subscriber he can invoke delegate, add his own functions etc. In other words the delegate is completely naked to the subscriber.

Events – Encapsulation on delegates

Events help to solve the delegate encapsulation problem. Events sit on top of delegates and provide encapsulation so that the destination source can only listen and not have full control of the delegate object.
Below figure shows how the things look like:-
• Method and functions are abstracted /encapsulated using delegates
• Delegates are further extended to provide broadcasting model by using multicast delegate.
• Multicast delegate are further encapsulated using events.

Implementing events

So let's take the same example which we did using multicast delegates and try to implement the same using events. Event uses delegate internally as event provides higher level of encapsulation over delegates.
So the first step in the publisher ('Form1') we need to define the delegate and the event for the delegate. Below is the code snippet for the same and please do notice the 'event' keyword.
We have defined a delegate 'CallEveryOne' and we have specified an event object for the delegate called as 'EventCallEveryOne'.

public delegate void CallEveryone();
public event CallEveryone EventCallEveryOne;

From the publisher i.e. 'Form1' create 'Form2' and 'Form3' objects and attach the current 'Form1' object so that 'Form2' and 'Form3' will listen to the events. Once the object is attached raise the events.

Form2 obj = new Form2();
obj.obj = this;
Form3 obj1 = new Form3();
obj1.obj = this;
obj.Show();
obj1.Show();
EventCallEveryOne();

At the subscriber side i.e. (Form2 and Form3) attach the method to the event listener.

obj.EventCallEveryOne += Callme;

This code will show the same results as we have got from multicast delegate example.

Difference between delegates and events

So what's really the difference between delegates and events other than the sugar coated syntax of events. As already demonstrated previously the main difference is that event provides one more level of encapsulation over delegates.
So when we pass delegates it's naked and the destination / subscriber can modify the delegate. When we use events the destination can only listen to it.

Delegates for Asynchronous method calls

One of the other uses of delegates is asynchronous method calls. You can call methods and functions pointed by delegate asynchronously.
Asynchronous calling means the client calls the delegate and the control is returned back immediately for further execution. The delegate runs in parallel to the main caller. When the delegate has finished doing his work he makes a call back to the caller intimating that the function / subroutine has completed executing.


To invoke a delegate asynchronously we need call the 'begininvoke' method. In the 'begininvoke' method we need to specify the call back method which is 'CallbackMethod' currently.

delegateptr.BeginInvoke(new AsyncCallback(CallbackMethod), delegateptr);

Below is the code snippet for 'CallbackMethod'. This method will be called once the delegate finishes his task.

static void CallbackMethod(IAsyncResult result)
{
int returnValue = flusher.EndInvoke(result);
}

Summarizing Use of delegates

There are 6 important uses of delegates:-
1. Abstract and encapsulate a method (Anonymous invocation)

This is the most important use of delegates; it helps us to define an abstract pointer which can point to methods and functions. The same abstract delegate can be later used to point to that type of functions and methods. In the previous section we have shown a simple example of a maths class. Later addition of new algorithm functions does not affect the UI code.

2. Callback mechanism
Many times we would like to provide a call back mechanism. Delegates can be passed to the destination and destination can use the same delegate pointer to make callbacks.

3. Asynchronous processing
By using 'BeginInvoke' and 'EndInvoke' we can call delegates asynchronously. In our previous section we have explained the same in detail.

4. Multicasting - Sequential processing
Some time we would like to call some methods in a sequential manner which can be done by using multicast delegate. This is already explained in the multicast example shown above.

5. Events - Publisher subscriber model
We can use events to create a pure publisher / subscriber model.

Difference between delegates and C++ pointer

C++ pointers are not type safe, in other words it can point to any type of method. On the other hand delegates are type safe. A delegate which is point to a return type of int cannot point to a return type of string.

Source code

You can download source code for this tutorial from top of this article.