How do Delegate, Action, and Func differ, and how does the event keyword protect encapsulation?

Short answer

A Delegate is a type-safe function pointer; Func returns a value and Action returns nothing. The event keyword prevents external subscribers from invoking the event or clearing its listener list; they can only use += and -=.

Explanation

A Delegate is a type that represents a function signature. Instead of defining a custom delegate for every case, Microsoft provides Func<T, TResult> for functions with return values and Action<T> for void functions. Adding the event keyword to a delegate protects Encapsulation: code outside the class cannot invoke the event or clear other listeners with = null; it can only subscribe with += and unsubscribe with -=.

Code example

public class OrderProcessor {
    public event Action<Order>? OrderCompleted;
    public void Process(Order order) {
        OrderCompleted?.Invoke(order); // Only invoked from within the class
    }
}

Common mistakes

• Not unsubscribing from events (-=) on long-lived objects, preventing the Garbage Collector from reclaiming listener objects and causing a memory leak.

Follow-up questions

• How do Nullable Reference Types work in C#, and what is the role of Static Flow Analysis?

Sources

• Microsoft Learn — Events overview