Dev Blog

Summary ​

Conclusions ​

Delegates let you store references to methods and pass behavior as a parameter. Multicast delegates call several methods in the order they were added and return the result of the last one. The built-in delegates Action, Func, Predicate<T>, and Comparison<T> cover most signatures, and lambda expressions let you write a function right where it is used. Lambdas capture variables, forming closures that extend the lifetime of the variables; in a for loop, this can lead to bugs. Higher-order functions build new functions from existing ones. Events implement the “publisher – subscriber” pattern: the event keyword allows outside code only to subscribe and unsubscribe, and the standard EventHandler<TEventArgs> pattern with an On<EventName> method makes events uniform across the .NET platform. Subscriptions that are never removed can cause memory leaks.

Self-check questions ​

  1. What is a delegate? How do you declare a delegate type?
  2. When is a method compatible with a delegate?
  3. How do you safely call a delegate that may be null?
  4. What is a multicast delegate? What result does calling it return?
  5. How do Action, Func, Predicate<T>, and Comparison<T> differ?
  6. What is the syntax of expression lambdas and statement lambdas?
  7. What is the natural type of a lambda?
  8. What are static lambdas used for?
  9. What is a closure? How long does a captured variable live?
  10. What is the trap of capturing a for loop variable?
  11. What is a higher-order function? Give examples.
  12. How does an event differ from a public delegate field?
  13. Describe the standard .NET event pattern.
  14. Why can subscriptions that are never removed cause memory leaks?
  15. Why does unsubscribing with a lambda written again not work?