Delegates And Events
Lesson 6.2: Delegates and Events: Event-Driven Programming
In this lesson, you'll dive into two important C# concepts: delegates and events. These are the building blocks for creating responsive and interactive applications using an event-driven programming model.
What are Delegates?
Think of a delegate as a "type-safe function pointer" or a "named method signature."
In simpler terms, a delegate is a variable that can hold a reference to a method.
This means you can pass methods around like data and call them indirectly through the delegate.
Delegates are useful for:
Callbacks: You can pass a method as an argument to another method, and the receiving method can call your method at a later time.
Customizing Behavior: You can change the behavior of a program by dynamically assigning different methods to a delegate.
Creating and Using a Delegate
Declare a delegate type:
C#
delegate void MyDelegate(string message); // Defines a delegate type that takes a string parameter
Use code with caution.
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Create delegate instances and assign methods:
C#
static void PrintMessage(string message)
{
Console.WriteLine(message);
}
MyDelegate myDelegate = PrintMessage; // Assign the PrintMessage method to the delegate
// Call the method through the delegate
myDelegate("Hello from the delegate!");
What are Events?
Events are a way for objects to notify other objects that something interesting has happened (e.g., a button was clicked, data was received, etc.).
The object that raises the event is called the publisher, and the objects that respond to the event are called subscribers.
Events use delegates under the hood to manage the communication between publishers and subscribers.
Creating and Using Events
Declare an event:
C#
public event MyDelegate MyEvent; // Declare an event of the MyDelegate type
Subscribe to the event (add a handler):
C#
MyEvent += PrintMessage; // Subscribe the PrintMessage method to the event
Raise the event (trigger it):
C#
MyEvent?.Invoke("This is a message from the event!"); // ?. is for null check
Unsubscribe from the event (remove a handler):
C#
MyEvent -= PrintMessage;
Event-Driven Programming
In event-driven programming, the flow of your program is determined by events.
Your code reacts to events as they occur, rather than following a predetermined sequence.
This makes your applications more responsive and easier to maintain.
Example: A Simple Button Click Event
C#
// In a Windows Forms or WPF application:
button.Click += (sender, e) => // Subscribe to the button's Click event
{
MessageBox.Show("Button clicked!"); // Show a message when clicked
};
Key Points
Delegates: Hold references to methods and allow you to call methods indirectly.
Events: A way for objects to communicate with each other about things that have happened.
Event-Driven Programming: A model where the flow of your program is determined by events.
By understanding delegates and events, you'll be able to write more interactive and responsive C# applications!