Delegates in C#

In C#, a delegate is a type-safe function pointer that allows methods to be referenced and invoked dynamically. It provides a way to treat methods as objects, enabling scenarios such as event handling, callbacks and functional-style programming.

Key Points

• A delegate defines the signature of methods it can point to.
• It can reference both static and instance methods.
• Delegates are type-safe, meaning the method signature must match the delegate declaration.
• They are the foundation of events and anonymous functions in C#.

When to Use Delegates

• For implementing callbacks.
• For handling events.
• For writing flexible, reusable code where behavior can be passed as parameters.
• For functional-style programming with LINQ and lambdas.

Declaration of Delegates

Delegate type can be declared using the delegate keyword. Once a delegate is declared, delegate instance will refer and call those methods whose return type and parameter-list matches with the delegate declaration.

Syntax:

[modifier] delegate [return_type] [delegate_name]([parameter_list]);

• modifier: Defines the accessibility of the delegate (public, private, internal, etc.). It is optional.
• delegate: The keyword used to declare a delegate.
• return_type: The type of value returned by the methods referenced by the delegate (can also be void).
• delegate_name: The identifier you assign to the delegate.
• parameter_list: Defines the parameters the delegate requires. The methods assigned must match this list exactly.

Example:

using System;

public class DelegateExample
{
    // Delegate declaration
    public delegate void MyDelegate(string message);

    // Method matching the delegate signature
    public static void DisplayMessage(string msg)
    {
        Console.WriteLine("Message: " + msg);
    }

    public static void Main()
    {
        // Instantiating delegate
        MyDelegate del = DisplayMessage;

        // Invoking delegate
        del("Hello from delegate!");
    }
}

Message: Hello from delegate!

Explanation:

1. MyDelegate is defined to point to methods that take a string parameter and return void.
2. The DisplayMessage method matches the delegate signature.
3. The delegate instance del references the method and is invoked like a method call.

Multicasting of a Delegate

Delegates can reference multiple methods at once using the + or += operator. Such delegates are called multicast delegates.

Properties: 

• Delegates are combined and when you call a delegate then a complete list of methods is called.
• All methods are called in First in First Out(FIFO) order.
• '+' or '+=' Operator is used to add the methods to delegates.
• '–' or '-=' Operator is used to remove the methods from the delegates list.

using System;

public class MulticastDelegateDemo
{
    public delegate void Notify();

    public static void MethodA() => Console.WriteLine("Method A executed");
    public static void MethodB() => Console.WriteLine("Method B executed");

    public static void Main()
    {
        Notify notify = MethodA;
        notify += MethodB;

        notify(); // Invokes both MethodA and MethodB
    }
}

Method A executed
Method B executed

Note:

If the delegate has a return value, only the result of the last method in the invocation list is returned.

Delegates with Return Types

Delegates can also be used with methods that return values.

using System;

public class ReturnDelegateDemo
{
    public delegate int Operation(int x, int y);

    public static int Add(int a, int b) => a + b;
    public static int Multiply(int a, int b) => a * b;

    public static void Main()
    {
        Operation op = Add;
        Console.WriteLine("Addition: " + op(5, 3));

        op = Multiply;
        Console.WriteLine("Multiplication: " + op(5, 3));
    }
}

Addition: 8
Multiplication: 15

Explanation:

• Delegates can point to methods that return a value and the return type must match the delegate signature.
• If multiple methods are attached, only the last method’s return value is returned.