Lambda with captures (closures) in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a lambda expression in C#?

A lambda expression is a short way to write an anonymous function using the syntax (parameters) => expression or block. It can be used to create inline functions.

intermediate

What does it mean when a lambda "captures" a variable?

Capturing means the lambda uses a variable from its surrounding scope. The lambda keeps a reference to that variable, so it can use or change it even after the original scope ends.

intermediate

Explain what a closure is in C#.

A closure is a lambda or anonymous function that captures variables from its surrounding scope. It 'closes over' those variables, keeping them alive and accessible inside the lambda.

intermediate

Consider this code snippet:<br>

int x = 5;<br>Func<int> f = () => x + 1;<br>x = 10;<br>Console.WriteLine(f());

<br>What will be printed and why?

It will print 11. The lambda captures the variable x by reference, not by value. So when x changes to 10, the lambda uses the updated value.

beginner

Why are closures useful in programming?

Closures let you keep state inside functions without using classes or global variables. They help write cleaner, more flexible code by bundling data with behavior.

What does a lambda capture in C#?

A Nothing, lambdas cannot capture variables

B Only constants

C Only parameters passed to it

D Variables from its surrounding scope

Lambdas can capture variables from the surrounding scope, allowing them to access or modify those variables.

If a lambda captures a variable, what happens when the variable changes after the lambda is created?

A The lambda sees the updated value

B The lambda keeps the original value forever

C The lambda throws an error

D The lambda copies the value once and ignores changes

The lambda captures the variable by reference, so it sees any changes made to the variable after creation.

What is a closure in C#?

A A static method

B A lambda that captures variables from its environment

C A method with no parameters

D A class with private fields

A closure is a lambda or anonymous function that captures variables from its surrounding scope.

Which of these is a correct lambda syntax in C#?

A () => 42

B lambda() { return 42; }

C function() => 42

D def lambda(): return 42

The correct C# lambda syntax uses =>, for example: () => 42.

Why might you use a closure instead of a class?

A Because closures are faster than classes

B Because classes cannot hold state

C To keep state in a small, simple function without extra code

D Because closures do not use memory

Closures let you keep state inside a function without needing to create a full class, making code simpler and cleaner.

Describe how a lambda captures variables and what effect this has on variable values inside the lambda.

Think about what happens if the variable changes after the lambda is created.

Explain what a closure is and why it is useful in C# programming.

Closures bundle data and behavior together.