Delegates, Events & Lambdas

Q: What is a delegate in C#?

A delegate is a type-safe reference to a method, letting you pass behaviour as a parameter and invoke it later. Func<>, Action<> and Predicate<> are built-in delegates.

Q: What is the difference between Func, Action and Predicate?

Func returns a value; Action returns void; Predicate is a Func used for tests.

Q: What is a closure and what is a common pitfall?

A closure is a lambda that captures variables from its enclosing scope. A classic pitfall is capturing a loop variable by reference so all lambdas see its final value.

Q: How do events differ from plain delegates?

An event is a delegate wrapped so external code can only subscribe (+=) or unsubscribe (-=), not invoke or overwrite it — enforcing the publisher/subscriber boundary.

Q: What causes event-handler memory leaks and how do you prevent them?

A long-lived publisher holding a delegate to a subscriber keeps the subscriber alive. Prevent it by unsubscribing (-=), using weak event patterns, or scoping lifetimes.

Q: What is the difference between multicast delegate invocation and exception handling?

A multicast delegate invokes subscribers in order; if one throws, later subscribers are skipped and the exception propagates. Invoke manually via GetInvocationList to isolate failures.

6 question(s)

What is a delegate in C#?

Beginner

A delegate is a type-safe reference to a method, letting you pass behaviour as a parameter and invoke it later. Func<>, Action<> and Predicate<> are built-in delegates.

Func<int,int> square = x => x*x;
Action<string> log = Console.WriteLine;
Console.WriteLine(square(4)); // 16

Real-world example Passing a comparison or projection function into a sorting or LINQ method.

What is the difference between Func, Action and Predicate?

Beginner

Func<...,TResult> returns a value; Action<...> returns void; Predicate<T> is a Func<T,bool> used for tests.

Func<int,bool> isEven = n => n%2==0;
Action greet = () => Console.Write("hi");

Real-world example Passing isEven to List.FindAll to filter numbers.

What is a closure and what is a common pitfall?

Intermediate

A closure is a lambda that captures variables from its enclosing scope. A classic pitfall is capturing a loop variable by reference so all lambdas see its final value.

var fns = new List<Func<int>>();
foreach (var i in Enumerable.Range(0,3)) fns.Add(() => i); // each captures its own i (C# 5+)

Real-world example Building event handlers in a loop that must remember the item they were created for.

How do events differ from plain delegates?

Intermediate

An event is a delegate wrapped so external code can only subscribe (+=) or unsubscribe (-=), not invoke or overwrite it — enforcing the publisher/subscriber boundary.

public event EventHandler? Saved;
protected void OnSaved() => Saved?.Invoke(this, EventArgs.Empty);

Real-world example A ViewModel raises a Saved event that views subscribe to, without views being able to fire it.

What causes event-handler memory leaks and how do you prevent them?

Advanced

A long-lived publisher holding a delegate to a subscriber keeps the subscriber alive. Prevent it by unsubscribing (-=), using weak event patterns, or scoping lifetimes.

source.Changed += Handler;
// later, to allow GC:
source.Changed -= Handler;

Real-world example A screen that subscribes to a singleton service must unsubscribe on close or it never gets collected.

What is the difference between multicast delegate invocation and exception handling?

Advanced

A multicast delegate invokes subscribers in order; if one throws, later subscribers are skipped and the exception propagates. Invoke manually via GetInvocationList to isolate failures.

foreach (var d in handler.GetInvocationList().Cast<Action>())
    try { d(); } catch (Exception ex) { Log(ex); }

Real-world example Notifying many listeners where one failing listener must not block the rest.