Expression-bodied lambdas in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an expression-bodied lambda in C#?

An expression-bodied lambda is a concise way to write a lambda expression using a single expression without braces and return keyword. It returns the value of that expression directly.

beginner

How do you write a simple expression-bodied lambda that adds two integers a and b?

You write it as: (a, b) => a + b;

intermediate

What is the difference between a statement-bodied lambda and an expression-bodied lambda?

A statement-bodied lambda uses braces and can have multiple statements, requiring a return keyword if it returns a value. An expression-bodied lambda has no braces and returns the result of a single expression directly.

intermediate

Can expression-bodied lambdas be used with void-returning methods?

Yes, expression-bodied lambdas can be used with void-returning methods if the expression is a statement like a method call. For example: x => Console.WriteLine(x);

beginner

Why are expression-bodied lambdas useful?

They make code shorter and easier to read when the lambda only needs to return the result of a simple expression.

Which of the following is a valid expression-bodied lambda in C#?

A (x, y) => x * y

B (x, y) => { return x * y; }

C (x, y) => { x * y; }

D (x, y) => return x * y;

Option A is an expression-bodied lambda with a single expression. Option B is a statement-bodied lambda. Option C is invalid because it lacks return. Option D is invalid syntax.

What does this expression-bodied lambda do? x => x + 1

A Returns x minus 1

B Returns x plus 1

C Prints x plus 1

D Does nothing

The lambda takes x and returns x plus 1.

Can expression-bodied lambdas contain multiple statements?

A Yes, always

B Only if inside braces

C Yes, if separated by semicolons

D No, only one expression

Expression-bodied lambdas can only have one expression. Multiple statements require braces and become statement-bodied lambdas.

Which keyword is NOT needed in an expression-bodied lambda?

A return

B =>

C lambda parameters

D expression

Expression-bodied lambdas return the expression value implicitly, so 'return' is not used.

How would you write a void expression-bodied lambda that prints a message?

A x => return Console.WriteLine(x)

B x => { Console.WriteLine(x); }

C x => Console.WriteLine(x)

D x => Console.WriteLine(x); return;

Option C is a valid void expression-bodied lambda. Option B is statement-bodied. Options A and D are invalid syntax.

Explain what an expression-bodied lambda is and how it differs from a statement-bodied lambda.

Think about how many statements each can have and how they return values.

Write an example of an expression-bodied lambda that takes two numbers and returns their product.

Use => with a simple multiplication expression.

Practice

1. What is the main purpose of using an expression-bodied lambda in C#?

easy

A. To write a short function with a single expression without braces or return keyword

B. To declare a variable with a complex type

C. To create a class with multiple methods

D. To handle exceptions in a concise way

Solution

1. Step 1: Understand expression-bodied lambda syntax

   Expression-bodied lambdas use the => operator followed by a single expression, simplifying function syntax.

2. Step 2: Identify the purpose

   This syntax is used to write short functions without braces or explicit return statements.

3. Final Answer:

   To write a short function with a single expression without braces or return keyword -> Option A

4. Quick Check:

   Expression-bodied lambda = short single-expression function [OK]

Hint: Remember: expression-bodied lambdas simplify single-expression functions [OK]

Common Mistakes:

• Confusing with variable declarations
• Thinking it creates classes or handles exceptions
• Assuming it requires braces and return keyword

2. Which of the following is the correct syntax for an expression-bodied lambda that adds two integers a and b?

easy

A. (a, b) => a + b

B. (a, b) => a + b;

C. (a, b) => { return a + b; }

D. (a, b) => { a + b }

Solution

1. Step 1: Identify expression-bodied lambda syntax

   Expression-bodied lambdas use => followed by a single expression without braces or return keyword.

2. Step 2: Check each option

   (a, b) => a + b correctly uses (a, b) => a + b without braces or return. (a, b) => { return a + b; } uses braces and return (not expression-bodied). (a, b) => a + b; has a semicolon after expression (not needed). (a, b) => { a + b } uses braces without return (invalid).

3. Final Answer:

   (a, b) => a + b -> Option A

4. Quick Check:

   Expression-bodied lambda syntax = (params) => expression [OK]

Hint: No braces or return keyword in expression-bodied lambdas [OK]

Common Mistakes:

• Adding braces and return keyword
• Including semicolon after expression
• Using braces without return keyword

3. What is the output of the following C# code?

Func<int, int> square = x => x * x;
Console.WriteLine(square(4));

medium

A. 8

B. 4

C. 16

D. Compilation error

Solution

1. Step 1: Understand the lambda function

   The lambda expression x => x * x defines a function that returns the square of x.

2. Step 2: Evaluate the function call

   Calling square(4) computes 4 * 4 = 16.

3. Final Answer:

   16 -> Option C

4. Quick Check:

   4 squared = 16 [OK]

Hint: Multiply input by itself to get square [OK]

Common Mistakes:

• Confusing output with input
• Expecting compilation error
• Misreading the lambda expression

4. Identify the error in the following expression-bodied lambda:

Func<int, int> increment = x => { x + 1; };

medium

A. Semicolon after expression is invalid

B. Braces are not allowed in expression-bodied lambdas

C. Lambda must have two parameters

D. Missing return keyword inside braces

Solution

1. Step 1: Analyze the lambda syntax

   The lambda uses braces, so it must have an explicit return statement to return a value.

2. Step 2: Identify the missing return

   Inside braces, the expression x + 1 is evaluated but not returned, causing a compile error.

3. Final Answer:

   Missing return keyword inside braces -> Option D

4. Quick Check:

   Braces require return keyword [OK]

Hint: Use return inside braces or remove braces for expression-bodied lambda [OK]

Common Mistakes:

• Assuming braces are disallowed
• Thinking semicolon after expression is invalid
• Expecting multiple parameters

5. Given a list of integers, which expression-bodied lambda correctly filters out even numbers and returns their squares?

var numbers = new List<int> {1, 2, 3, 4, 5};
var result = numbers.Where( /* lambda here */ ).Select(x => x * x).ToList();

hard

A. x => x % 2 == 0

B. x => x % 2 != 0

C. x => { return x % 2 != 0; }

D. x => { x % 2 != 0; }

Solution

1. Step 1: Understand filtering condition

   We want to keep odd numbers, so the condition is x % 2 != 0.

2. Step 2: Choose correct lambda syntax

   x => x % 2 != 0 uses expression-bodied lambda without braces, correctly returning the condition. x => x % 2 == 0 keeps even numbers (wrong). x => { return x % 2 != 0; } uses braces with return (valid but not expression-bodied). x => { x % 2 != 0; } uses braces without return (invalid).

3. Final Answer:

   x => x % 2 != 0 -> Option B

4. Quick Check:

   Filter odds with expression-bodied lambda [OK]

Hint: No braces and return for expression-bodied lambdas [OK]

Common Mistakes:

• Using braces without return
• Choosing wrong condition for filtering
• Mixing expression-bodied with block-bodied syntax