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This part of the C# tutorial is dedicated to delegates.

A delegate is a form of type-safe function pointer used by the .NET Framework. Delegates are often used to implement callbacks and event listeners. A delegate does not need to know anything about classes of methods it works with.

A delegate is a reference type. But instead of referring to an object, a delegate refers to a method.

Delegates are used in the following cases:

• Event handlers
• Callbacks
• LINQ
• Implementation of design patterns

There is nothing that is done with delegates that cannot be done with regular methods. Delegates are used because they bring several advantages. They foster flexibility of the application and code reuse. Like interfaces, delegates let us decouple and generalize our code. Delegates also allow methods to be passed as parameters. When we need to decide which method to call at runtime, we use a delegate. Finally, delegates provide a way of specializing behavior of a class without subclassing it. Classes may have complex generic behavior but are still meant to be specialized. Classes are specialized either through inheritance or via delegates.

Using delegates

We will have some simple examples showing how to use delegates.

using System;   

delegate void Mdelegate();

public class SimpleDelegate
{
  static void Main()
  {
    Mdelegate del = new Mdelegate(Callback);
    del();
  }

  static void Callback()
  {
    Console.WriteLine("Calling callback");
  }
} 

We declare a delegate, create an instance of the delegate and invoke it.

delegate void Mdelegate();

This is our delegate declaration. It returns no value and takes no parameters.

Mdelegate del = new Mdelegate(Callback);

We create an instance of the delegate. When called, the delegate will invoke the static Callback() method.

del();

We call the delegate.

$ ./simple.exe 
Calling callback

This is the output.

We can use a different syntax for creating and using a delegate.

using System;   

delegate void Mdelegate();

public class SimpleDelegate2
{
  static void Main()
  {
    Mdelegate del = Callback;
    del();
  }

  static void Callback()
  {
    Console.WriteLine("Calling callback");
  }
} 

We can save some typing when creating an instance of a delegate. This was introduced in C# 2.0.

Mdelegate del = Callback;

This is another way of creating a delegate. We point directly to the method name.

A delegate can point to different methods over time.

using System;

public delegate void NameDelegate(string msg);

public class Person 
{
  public string firstName;
  public string secondName;

  public Person(string firstName, string secondName)
  { 
    this.firstName = firstName;
    this.secondName = secondName;
  }

  public void ShowFirstName(string msg)
  {
    Console.WriteLine(msg + this.firstName);
  }

  public void ShowSecondName(string msg)
  {
    Console.WriteLine(msg + this.secondName);
  }
}

public class CSharpApp
{
  public static void Main()
  {
    Person per = new Person("Fabius", "Maximus");     

    NameDelegate nDelegate = new NameDelegate(per.ShowFirstName);
    nDelegate("Call 1: ");

    nDelegate = new NameDelegate(per.ShowSecondName);
    nDelegate("Call 2: ");   
  }
}

In the example we have one delegate. This delegate is used to point to two methods of the Person class. The methods are called with the delegate.

public delegate void NameDelegate(string msg);

The delegate is created with a delegate keyword. The delegate signature must match the signature of the method being called with the delegate.

NameDelegate nDelegate = new NameDelegate(per.ShowFirstName);
nDelegate("Call 1: ");

We create an instance of a new delegate that points to the ShowFirstName() method. Later we call the method via the delegate.

$ ./moremethods.exe 
Call 1: Fabius
Call 2: Maximus

Both names are printed via the delegate.

Multicast delegate

Multicast delegate is a delegate which holds a reference to more than one method. Multicast delegates must contain only methods that return void, else there is a run-time exception.

using System;   

delegate void Mdelegate(int x, int y);

public class Oper
{
  public static void Add(int x, int y)
  {
    Console.WriteLine("{0} + {1} = {2}", x, y, x + y);
  }

  public static void Sub(int x, int y)
  {
    Console.WriteLine("{0} - {1} = {2}", x, y, x - y);
  }
}

public class CSharpApp
{
  static void Main()
  {
    Mdelegate del = new Mdelegate(Oper.Add);

    del += new Mdelegate(Oper.Sub);
    del(6, 4);      
    del -= new Mdelegate(Oper.Sub);
    del(2, 8);      
  }
}

This is an example of a multicast delegate.

delegate void Mdelegate(int x, int y);

Our delegate will take two parameters. We have an Oper class which has two static methods. One adds two values the other one subtracts two values.

Mdelegate del = new Mdelegate(Oper.Add);

We create an instance of our delegate. The delegate points to the static Add() method of the Oper class.

del += new Mdelegate(Oper.Sub);
del(6, 4);  

We plug another method to the existing delegate instance. The first call of the delegate invokes two methods.

del -= new Mdelegate(Oper.Sub);
del(2, 8);  

We remove one method from the delegate. The second call of the delegate invokes only one method.

$ ./multicast.exe 
6 + 4 = 10
6 - 4 = 2
2 + 8 = 10

This is the output of the multicast.exe program.

Anonymous methods

It is possible to use anonymous methods with delegates.

using System;   

delegate void Mdelegate();

public class Anonymous
{
  static void Main()
  {
    Mdelegate del = delegate 
    { 
      Console.WriteLine("Anonymous method"); 
    };

    del();
  }
} 

We can omit a method declaration when using an anonymous method with a delegate. The method has no name and can be invoked only via the delegate.

Mdelegate del = delegate { 
  Console.WriteLine("Anonymous method"); 
};

Here we create a delegate that points to an anonymous method. The anonymous method has a body enclosed by { } characters, but it has no name.

Delegates as method parameters

Delegates can be used as method parameters.

using System;   

delegate int Arithm(int x, int y);

public class CSharpApp
{
  static void Main()
  {
    DoOperation(10, 2, Multiply);
    DoOperation(10, 2, Divide);
  }

  static void DoOperation(int x, int y, Arithm del)
  {
    int z = del(x, y);
    Console.WriteLine(z);
  }

  static int Multiply(int x, int y)
  {
    return x * y;
  }

  static int Divide(int x, int y)
  {
    return x / y;
  }
} 

We have a DoOperation() method which takes a delegate as a parameter.

delegate int Arithm(int x, int y);

This is a delegate declaration.

static void DoOperation(int x, int y, Arithm del)
{
  int z = del(x, y);
  Console.WriteLine(z);
}

This is DoOperation() method implementation. The third parameter is a delegate. The DoOperation() method calls a method which is passed to it as a third parameter.

DoOperation(10, 2, Multiply);
DoOperation(10, 2, Divide);

We call the DoOperation() method. We pass two values and a method to it. What we do with the two values depends on the method that we pass. This is the flexibility that come with using delegates.

Events

Events are messages triggered by some action. A click on a button or a tick of a clock are such actions. The object that triggers an event is called a sender and the object that receives the event is called a receiver.

By convention, event delegates in the .NET Framework have two parameters: the source that raised the event and the data for the event.

using System;   

public delegate void OnFiveHandler(object sender, EventArgs e);

class FEvent 
{ 
  public event OnFiveHandler FiveEvent;

  public void OnFiveEvent() 
  { 
    if(FiveEvent != null) 
      FiveEvent(this, EventArgs.Empty); 
  } 
} 

public class SimpleEvent
{
  static void Main()
  {
    FEvent fe = new FEvent();
    fe.FiveEvent += new OnFiveHandler(Callback);

    Random random = new Random();

    for (int i = 0; i<10; i++) 
    {
      int rn = random.Next(6);
       
      Console.WriteLine(rn);
      
      if (rn == 5)
      {
        fe.OnFiveEvent();
      }
    }
  }

  public static void Callback(object sender, EventArgs e)
  {
    Console.WriteLine("Five Event occured");
  }
} 

We have a simple example in which we create and launch an event. An random number is generated. If the number equals to 5 a FiveEvent event is generated.

public event OnFiveHandler FiveEvent;

An event is declared with a event keyword.

fe.FiveEvent += new OnFiveHandler(Callback);

Here we plug the event called FiveEvent to the Callback() method. In other words, if the ValueFive event is triggered, the Callback() method is executed.

public void OnFiveEvent() 
{ 
  if(FiveEvent != null) 
    FiveEvent(this, EventArgs.Empty); 
} 

When the random number equals to 5, we invoke the OnFiveEvent() method. In this method, we raise the FiveEvent event. This event carries no arguments.

$ ./simpleevent.exe 
3
0
5
Five Event occured
0
5
Five Event occured
2
3
4
4
0

The output of the program might look like this.

Complex event example

Next we have a more complex example. This time we will send some data with the generated event.

using System;   

public delegate void OnFiveHandler(object sender, FiveEventArgs e);

public class FiveEventArgs : EventArgs
{
  public int count;
  public DateTime time;

  public FiveEventArgs(int count, DateTime time)
  {
    this.count = count;
    this.time = time;
  }
}

public class FEvent 
{ 
  public event OnFiveHandler FiveEvent;

  public void OnFiveEvent(FiveEventArgs e) 
  { 
    FiveEvent(this, e); 
  } 
} 

public class RandomEventGenerator
{
  public void Generate()
  {
    int count = 0;
    FiveEventArgs args;

    FEvent fe = new FEvent();
    fe.FiveEvent += new OnFiveHandler(Callback);

    Random random = new Random();

    for (int i = 0; i<10; i++) 
    {
      int rn = random.Next(6);
       
      Console.WriteLine(rn);
      
      if (rn == 5)
      {  
        count++;
        args = new FiveEventArgs(count, DateTime.Now);
        fe.OnFiveEvent(args);
      }
    }
  }

  public void Callback(object sender, FiveEventArgs e)
  {
    Console.WriteLine("Five event {0} occured at {1}", 
      e.count, e.time);
  }
}

public class ComplexEvent
{
  static void Main()
  {
    RandomEventGenerator reg = new RandomEventGenerator();
    reg.Generate();
  }
} 

We have four classes. The FiveEventArgs carries some data with the event object. The FEvent class encapsulates the event object. The RandomEventGenerator class is responsible for random number generation. It is the event sender. Finally, the ComplexEvent is the main application class.

public class FiveEventArgs : EventArgs
{
  public int count;
  public DateTime time;
...

The FiveEventArgs carries data inside the event object. It inherits from the EventArgs base class. The count and time members are data that will be initialized and carried with the event.

if (rn == 5)
{  
  count++;
  args = new FiveEventArgs(count, DateTime.Now);
  fe.OnFiveEvent(args);
}

If the generated random number equals to 5, we instantiate the FiveEventArgs class with the current count and DateTime values. The count variable counts the number of times this event was generated. The DateTime value holds the time when the event was generated.

$ ./complexevent.exe 
2
2
3
5
Five event 1 occured at 11/7/2010 12:13:59 AM
5
Five event 2 occured at 11/7/2010 12:13:59 AM
1
1
0
5
Five event 3 occured at 11/7/2010 12:13:59 AM
5
Five event 4 occured at 11/7/2010 12:13:59 AM

This is a sample output of the complexevent.exe program.

Predefined delegates

The .NET framework has several built-in delegates that a reduce the typing needed and make the programming easier for developers.

An action delegate encapsulates a method that has no parameters and does not return a value.

using System;   

public class ActionDelegate
{
  static void Main()
  {
    Action act = ShowMessage;
    act();
  }

  static void ShowMessage()
  {
    Console.WriteLine("C# language");
  }
} 

Using predefined delegates further simplifies programming. We do not need to declare a delegate type.

Action act = ShowMessage;
act();

We instantiate an action delegate. The delegate points to the ShowMessage() method. When the delegate is invoked, the ShowMessage() method is executed.

There are multiple types of action delegates. For example, the Action<T> delegate encapsulates a method that takes a single parameter and does not return a value.

using System;   

public class CSharpApp
{
  static void Main()
  {
    Action<string> act = ShowMessage;
    act("C# language");
  }

  static void ShowMessage(string message)
  {
    Console.WriteLine(message);
  }
} 

We modify the previous example to use the action delegate that takes one parameter.

Action<string> act = ShowMessage;
act("C# language");

We create an instance of the Action<T> delegate and call it with one parameter.

Predicate delegate

A predicate is a method that returns true or false. A predicate delegate is a reference to a predicate. Predicates are very useful for filtering lists of values.

using System;
using System.Collections.Generic;

public class PredicateDelegate
{
  static void Main()
  {
    List<int> list = new List<int> { 4, 2, 3, 0, 6, 7, 1, 9 };

    Predicate<int> predicate = greaterThanThree;

    List<int> list2 = list.FindAll(predicate);

    foreach ( int i in list2)
    {
      Console.WriteLine(i);
    }
  }

  static bool greaterThanThree(int x)
  {
    return x > 3;
  }
}

We have a list of integer values. We want to filter all numbers that are bigger than three. For this, we use the predicate delegate.

List<int> list = new List<int> { 4, 2, 3, 0, 6, 7, 1, 9 };

This is a generic list of integer values.

Predicate<int> predicate = greaterThanThree;

We create an instance of a predicate delegate. The delegate points to a predicate, a special method that returns true or false.

List<int> list2 = list.FindAll(predicate);

The FindAll() method retrieves all the elements that match the conditions defined by the specified predicate.

static bool greaterThanThree(int x)
{
  return x > 3;
}

The predicate returns true for all values that are greater than three.

This part of the C# tutorial was dedicated to the delegates.