C# Delegates

Delegates

Delegates in C# are a powerful feature that allows you to encapsulate a reference to a method inside a delegate object. This enables methods to be passed as parameters, returned as values from other methods, and stored in collections.

Let's delve into a tutorial on delegates in C#:

1. What is a Delegate?

A delegate is a type-safe function pointer. It represents references to methods with a specific function signature. Being type-safe ensures that the method's signature you're pointing to matches the delegate's definition.

2. Declaring Delegates:

You declare a delegate using the delegate keyword:

delegate void MyDelegate(string message);

In the above example, MyDelegate is a delegate type that can point to methods taking a single string parameter and returning void.

3. Using Delegates:

3.1. Simple Usage:

Given the delegate declaration from above, you can have:

public static void ShowMessage(string message) { Console.WriteLine(message); } static void Main(string[] args) { MyDelegate del = ShowMessage; del("Hello, World!"); // Outputs: Hello, World! }

3.2. Multicast Delegates:

A delegate can refer to more than one method:

public static void ShowMessage(string message) { Console.WriteLine("Message: " + message); } public static void ShowLength(string message) { Console.WriteLine("Length: " + message.Length); } static void Main(string[] args) { MyDelegate del = ShowMessage; del += ShowLength; del("Hello, World!"); // Outputs: // Message: Hello, World! // Length: 13 }

4. Delegates with Return Values:

If a delegate has a return type other than void, and it's pointing to multiple methods (multicast delegate), only the value of the last invoked method will be returned.

delegate int MathOperation(int a, int b); public static int Add(int a, int b) { return a + b; } public static int Subtract(int a, int b) { return a - b; } static void Main(string[] args) { MathOperation operation = Add; operation += Subtract; int result = operation(5, 3); // This will return -2, not 8. Console.WriteLine(result); // Outputs: -2 }

5. Anonymous Methods:

You can use delegates with anonymous methods:

MyDelegate del = delegate(string message) { Console.WriteLine(message); }; del("Hello from an anonymous method!"); // Outputs: Hello from an anonymous method!

6. Lambda Expressions:

Lambda expressions provide a concise way to create delegate instances:

MyDelegate del = msg => Console.WriteLine(msg); del("Hello from a lambda!"); // Outputs: Hello from a lambda!

7. Predicates, Action, and Func:

C# provides some built-in generic delegate types:

  • Predicate<T>: Represents a method containing a set of criteria and checks them against a value. Returns a bool.

  • Action<T>: Represents a method that takes a single parameter and returns void.

  • Func<T, TResult>: Represents a method that takes a parameter of type T and returns a value of type TResult.

8. Use Cases:

  • Event Handling: Delegates form the foundation of events in C#. They're used extensively in GUI applications to handle user events like button clicks, mouse movements, etc.

  • Callback Mechanisms: When a task is running asynchronously, you might want to be notified or execute some code when it completes. This can be achieved using delegates.

  • LINQ and Lambda Expressions: Delegates are fundamental to LINQ and provide the mechanism to use lambda expressions for querying data.

Conclusion:

Delegates are a core concept in C# and a fundamental building block for various advanced features in .NET, such as events, lambda expressions, and LINQ. Understanding how to create and use delegates will significantly enhance your C# programming skills.

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