C# / .NET Mid-level (2–5 yrs) Concepts Very frequent C# delegates Func Action events functional programming

Explain delegates, Func, Action, and events in C# — when do you use each?

cs-mid-005

Your answer

Answer as you would in a real interview — explain your thinking, not just the conclusion.

Model answer

A delegate is a type-safe function pointer. Func<T,TResult> and Action<T> are built-in generic delegate types — Func returns a value, Action does not. You use them to pass behaviour as a parameter (strategy pattern, callbacks). An event is a delegate field protected by add/remove accessor conventions; only the declaring class can invoke it, which prevents external code from firing or clearing the event list. Use Func/Action for one-off callbacks; use events when multiple subscribers need loose-coupled notification (e.g., UI event model, domain events). Avoid returning void in async delegates — prefer Func<Task>.

Code example

// Func (returns a value)
Func<int, int, int> add = (a, b) => a + b;
Console.WriteLine(add(3, 4)); // 7

// Action (void return)
Action<string> log = msg => Console.WriteLine($"[LOG] {msg}");
log("started");

// Predicate (shorthand for Func<T, bool>)
Predicate<int> isEven = n => n % 2 == 0;

// Custom delegate + event pattern
public class OrderProcessor
{
    public event EventHandler<OrderEventArgs> OrderPlaced;

    public void Process(Order order)
    {
        // ... processing ...
        OrderPlaced?.Invoke(this, new OrderEventArgs(order));
    }
}

// Higher-order function using Func
public IEnumerable<T> Filter<T>(IEnumerable<T> source, Func<T, bool> predicate)
    => source.Where(predicate);

Follow-up

What is multicast delegation? What happens if one delegate in a multicast chain throws an exception?