C# Tutorial 12.1: Delegates

Introduction to Delegates:

Delegates in C# are a type-safe, object-oriented way of representing and referencing methods. They allow you to treat methods as first-class citizens, meaning you can pass methods as parameters to other methods, store them in variables, and invoke them dynamically. Delegates are particularly useful for implementing callback mechanisms and event handling.

1. Declaration and Instantiation of Delegates:

In C#, you declare a delegate using the delegate keyword. Here's a simple example:

// Declaration of a delegate
public delegate void MyDelegate(string message);
// Instantiation of the delegate
MyDelegate myDelegate = new MyDelegate(MyMethod);

In this example, MyDelegate is a delegate that can reference any method that takes a string parameter and returns void. The MyMethod method has the same signature, so it can be assigned to the delegate.

2. Using Delegates:

2.1 Invoking Delegates:

You can invoke a delegate using the Invoke method or simply by calling the delegate like a regular method:

myDelegate("Hello, delegates!"); // Invoking the delegate

2.2 Multicast Delegates:

Delegates can be combined to create multicast delegates, allowing you to call multiple methods through a single delegate:

MyDelegate multiDelegate = myDelegate + AnotherMethod;
multiDelegate("Combined delegates");

Here, both MyMethod and AnotherMethod will be called when multiDelegate is invoked.

2.3 AsyncCallback using Delegates:

Delegates are commonly used in asynchronous programming, especially when dealing with BeginInvoke and EndInvoke methods. Here's a simple example:

public delegate void MyAsyncCallback(string result);
public void PerformAsyncOperation(MyAsyncCallback callback)
{
    // Simulating an asynchronous operation
    string result = "Async operation completed";
    
    // Invoking the callback
    callback(result);
}
// Example of using AsyncCallback
MyAsyncCallback asyncCallback = new MyAsyncCallback(OnAsyncOperationCompleted);
PerformAsyncOperation(asyncCallback);
void OnAsyncOperationCompleted(string result)
{
    Console.WriteLine(result);
}

3. Built-in Delegates:

C# provides several built-in delegates in the System namespace, such as Action, Func, and Predicate. These are generic delegates that can be used without explicitly declaring a custom delegate:

Action<string> actionDelegate = MyMethod;
actionDelegate("Using Action delegate");

Conclusion:

Delegates in C# provide a powerful way to work with methods in a flexible and type-safe manner. Whether you're implementing event handling or asynchronous programming, understanding how to use delegates is essential for any C# developer.