Functional C#: Delegates & Actions

A Delegate is a type-safe function pointer. It allows you to pass a "Method" as a parameter to another method. This is the foundation of LINQ, Events, and modern asynchronous callbacks.

1. The Trio: Func, Action, and Predicate

You rarely need to define custom delegates anymore. Microsoft provided three generic wrappers that cover 99% of use cases.

• Action: For methods that return void.
• Func<T, TResult>: For methods that return a value.
• Predicate<T>: A Func that specifically returns a bool.

// 1. An action that takes a string
Action<string> log = msg => Console.WriteLine(msg);

// 2. A func that takes two ints and returns an int
Func<int, int, int> add = (a, b) => a + b;

// 3. Using them as parameters
public void ProcessData(int val, Action<int> callback) 
{
    callback(val * 2);
}

2. Multicast Delegates

A single delegate can hold references to multiple methods. When you call the delegate, all methods fire in sequence. This is how the Observer Pattern is implemented in C# via Events.

4. Interview Mastery

Q: "What is a Closure in C# Lambda expressions?"

Architect Answer: "A Closure occurs when a lambda expression 'captures' a variable from its outer scope. Even if that outer variable is a local variable on the stack that should have been destroyed, the C# compiler cleverly generates a hidden class to store that variable on the heap, ensuring it stays alive as long as the lambda exists. Be careful: capturing large objects in closures can prevent the Garbage Collector from cleaning them up, leading to memory leaks."