Delegates

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Delegates are the ultimate function pointer. Developers familiar with C and C++ are very familiar with function pointers and their lack of instance-based knowledge. A delegate can be thought of as a call back mechanism, essentially saying, "Please invoke this method for me when the time is right."

Consider the following scenario: Your department has just hired a new employee. Human Resources needs to be notified when a new employee is hired so they can put them to sleep with endless paper work and boring drivel about company policies and practices. This type of interaction is a perfect example of a delegate. Basically, Human Resources is requesting a notification when a new employee is hired and provides a method to be invoked. Listing 2.1.32 shows the basic use of delegates .

Listing 2.1.32 Using Delegates

 1: //File        :part02_31.cs  2: //Author    :Richard L. Weeks  3: //Purpose    :Demonstrate the use of delegates  4:  5: using System;  6:  7: //Define a person struct  8: public struct Person {  9:     public string FName; 10:     public string LName; 11: } 12: 13: //Define a delegate 14: public delegate void OnNewHire( Person person ); 15: 16: //The HR Class 17: public class HR { 18: 19:     //Implement the delegate to be called when a new person is hired 20:     public void OnNewHire( Person person ) { 21:         Console.WriteLine( "HR is in the process of putting {0}  to sleep",  person.FName ); 22:     } 23: } 24: 25: //Create a department 26: public class Department { 27: 28:     //Who to notify 29:     private OnNewHire    m_OnNewHireDelegate = null; 30: 31:     //set the OnNewHire delegate 32:     public void AddOnNewHireDelegate( OnNewHire onh ) { 33:         m_OnNewHireDelegate = onh; 34:     } 35: 36: 37:     public void HirePerson( Person p ) { 38:         //do we need to notify someone? 39:         if( m_OnNewHireDelegate != null ) 40:             m_OnNewHireDelegate( p ); 41:     } 42: } 43: 44: 45: public class DelegateTest { 46: 47:     public static void Main( ) { 48: 49:         HR hr = new HR( ); 50:         Department dept = new Department( ); 51: 52:         //Register the OnNewHire Delegate 53:         dept.AddOnNewHireDelegate( new OnNewHire( hr.OnNewHire ) ); 54: 55:         //Create a person 56:         Person me; me.FName = "Richard"; me.LName = "Weeks"; 57: 58:         //Hire ME!!! 59:         dept.HirePerson( me ); 60:     } 61: } 

Listing 2.1.32 implements the HR scenario and makes use of a delegate to notify HR when a new person has been hired. The delegate OnNewHire is defined on line 14. Notice the use of the delegate keyword to denote what is being declared. Remember that C# does not allow for global methods , so C# would issue an error without the delegate keyword.

The HR class provides a handler for the delegate. The method name does not have to be the same as the name of the delegate; this was done to make it easier to follow. The Department class provides a method AddOnNewHireDelegate to handle the "hooking-up" of the delegate with the intended handler. Notice the call on line 53 that actually adds the HR handler to the Department . A delegate is a type in C# and requires an instance, hence the use of the new keyword to create a new delegate.

I would encourage any developer to explore delegates in detail because their use in .NET is prolific, especially in Windows Forms development.

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