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.

• 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:

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 invoked in the order they were added to the delegate (invocation order).
• '+' or '+=' Operator is used to add the methods to delegates.
• '–' or '-=' Operator is used to remove the methods from the delegates list.

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.

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.