Lambda expression syntax 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 anonymous functions using the syntax (input) => expression or (input) => { statements }. It is often used for inline code like in LINQ queries or event handlers.

beginner

How do you write a lambda expression that takes one integer and returns its square?

You write it as: x => x * x. Here, x is the input parameter, and x * x is the expression returned.

intermediate

What is the difference between a lambda expression with an expression body and one with a statement body?

An expression body lambda returns the result of a single expression directly, like x => x + 1. A statement body lambda uses braces and can have multiple statements, like x => { int y = x + 1; return y; }.

beginner

Can a lambda expression have multiple parameters? Show an example.

Yes, a lambda can have multiple parameters separated by commas. Example: (x, y) => x + y adds two numbers.

intermediate

How do you specify the type of parameters in a lambda expression?

You can specify types explicitly like (int x, int y) => x + y, but usually the compiler infers the types from context.

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

A x => x * 2

B (int x) => x * 2

C x => { return x * 2; }

D => x * 2

Option A is correct. Option B is valid syntax but not the best choice here since the question implies invalidity; Option C is missing a semicolon after return, which is corrected here. Option D is missing the parameter.

What does the lambda expression (x, y) => x - y do?

A Divides x by y

B Adds x and y

C Multiplies x and y

D Subtracts y from x

The expression subtracts y from x.

How do you write a lambda expression with no parameters that returns 42?

A => 42

B x => 42

C () => 42

D (x) => 42

Option C correctly shows a lambda with no parameters returning 42.

Which syntax is used for a statement body lambda?

A (x) => { return x + 1; }

B x => { x + 1 }

C x => x + 1

D x => return x + 1

Option A uses braces and a return statement, which is the statement body syntax.

Can the compiler infer parameter types in lambda expressions?

A Yes, always

B Yes, if context provides type information

C Only for single parameter lambdas

D No, never

The compiler infers types when it can, based on the context where the lambda is used.

Explain the basic syntax of a lambda expression in C# and give an example.

Think about how you write a small function without a name.

Describe the difference between expression body and statement body lambdas with examples.

One is short and simple, the other can do more steps.

Practice

1. What does the lambda expression x => x * 2 represent in C#?

easy

A. A variable declaration

B. A function that doubles the input value

C. A loop that runs twice

D. A conditional statement

Solution

1. Step 1: Understand lambda syntax

   The expression x => x * 2 means a function taking input x and returning x * 2.

2. Step 2: Interpret the operation

   It doubles the input value by multiplying it by 2.

3. Final Answer:

   A function that doubles the input value -> Option B

4. Quick Check:

   Lambda x => x * 2 doubles input [OK]

Hint: Lambda uses => to define short functions quickly [OK]

Common Mistakes:

• Thinking lambda is a variable
• Confusing lambda with loops
• Assuming lambda is a conditional

2. Which of the following is the correct syntax for a lambda expression that takes two integers and returns their sum?

easy

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

B. a, b => a + b

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

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

Solution

1. Step 1: Check parameter syntax

   Parameters must be enclosed in parentheses when there is more than one, so (a, b) is correct.

2. Step 2: Check expression body

   The expression a + b without braces is valid for single expressions. (a, b) => a + b matches this.

3. Final Answer:

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

4. Quick Check:

   Two parameters need parentheses and expression body [OK]

Hint: Use parentheses for multiple parameters in lambda [OK]

Common Mistakes:

• Omitting parentheses for multiple parameters
• Adding semicolon inside lambda expression
• Using braces without return keyword incorrectly

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

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

medium

A. Compilation error

B. 8

C. 4

D. 16

Solution

1. Step 1: Understand the lambda function

   The lambda x => x * x returns the square of the input number.

2. Step 2: Calculate the output for input 4

   4 * 4 = 16, so square(4) returns 16.

3. Final Answer:

   16 -> Option D

4. Quick Check:

   Square of 4 is 16 [OK]

Hint: Lambda returns expression result directly if no braces [OK]

Common Mistakes:

• Confusing multiplication with addition
• Expecting lambda to print instead of return
• Thinking lambda causes syntax error

4. Identify the error in this lambda expression:

Func<int, int> add = (x, y) => x + y;

medium

A. Parentheses around parameters are not allowed

B. Missing return keyword inside lambda

C. Lambda has too many parameters for Func<int, int>

D. Lambda expression syntax is correct

Solution

1. Step 1: Check Func delegate signature

   Func<int, int> expects one input parameter and one output (int).

2. Step 2: Compare lambda parameters

   The lambda has two parameters (x, y), which does not match the delegate expecting one.

3. Final Answer:

   Lambda has too many parameters for Func<int, int> -> Option C

4. Quick Check:

   Func<int, int> means one input, one output [OK]

Hint: Match lambda parameters count with delegate type [OK]

Common Mistakes:

• Assuming multiple parameters allowed for Func
• Forgetting delegate parameter count rules
• Thinking return keyword is always needed

5. Given a list of integers, which lambda expression correctly filters out all even numbers using List<int>.Where()?

var numbers = new List<int> {1, 2, 3, 4, 5, 6};
var odds = numbers.Where( /* which lambda? */ );

hard

A. x => x % 2 == 1

B. x => x % 2 = 1

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

D. x => x / 2 == 1

Solution

1. Step 1: Understand filtering condition

   We want to keep odd numbers, so condition is x % 2 == 1.

2. Step 2: Check lambda syntax and logic

   x => x % 2 == 1 uses correct syntax and logic to filter odds. x => x % 2 = 1 uses assignment (=) which is invalid. x => { return x % 2 == 0; } filters evens, not odds. x => x / 2 == 1 checks division, not oddness.

3. Final Answer:

   x => x % 2 == 1 -> Option A

4. Quick Check:

   Filter odds with modulo == 1 [OK]

Hint: Use == for comparison, % 2 == 1 filters odd numbers [OK]

Common Mistakes:

• Using = instead of == in lambda
• Filtering evens instead of odds
• Confusing division with modulo