Lambda expression syntax in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple lambda expression

What is the output of this C# code snippet?

C Sharp (C#)

Func<int, int> square = x => x * x;
Console.WriteLine(square(5));

A Compilation error

B 25

C 10

D 5

💡 Hint

The lambda takes an integer and returns its square.

The lambda expression x => x * x takes an integer x and returns x multiplied by itself. So square(5) returns 25.

❓ Predict Output

intermediate

Lambda with multiple parameters and statement body

What will this code print?

C Sharp (C#)

Func<int, int, int> addAndMultiply = (a, b) => {
    int sum = a + b;
    return sum * 2;
};
Console.WriteLine(addAndMultiply(3, 4));

A 14

B 7

C 12

D Compilation error

💡 Hint

The lambda sums two numbers and then doubles the result.

The lambda adds 3 and 4 to get 7, then multiplies by 2 to get 14.

❓ Predict Output

advanced

Lambda capturing outer variable

What is the output of this code?

C Sharp (C#)

int factor = 3;
Func<int, int> multiply = x => x * factor;
factor = 5;
Console.WriteLine(multiply(4));

A Compilation error

B 15

C 12

D 20

💡 Hint

Lambdas capture variables by reference, not by value.

The lambda captures 'factor' by reference. Since 'factor' is changed to 5 before calling multiply(4), the output is 4 * 5 = 20.

❓ Predict Output

advanced

Lambda with incorrect syntax

Which option will cause a compilation error?

C Sharp (C#)

Func<int, int> f = x => x * 2;

A Func<int, int> f = (x) => x * 2;

B Func<int, int> f = x => { return x * 2; };

C Func<int, int> f = x => { x * 2; };

D Func<int, int> f = (int x) => x * 2;

💡 Hint

Check if the lambda body returns a value properly.

Option C has a block body but no return statement, so it does not return a value, causing a compilation error.

🧠 Conceptual

expert

Understanding lambda expression type inference

Given the code below, what is the inferred delegate type of the lambda expression?

C Sharp (C#)

var list = new List<int> {1, 2, 3};
var result = list.Select(x => x.ToString());

A Func<int, string>

B Action<int>

C Predicate<int>

D Func<string, int>

💡 Hint

Select expects a function that takes an int and returns a string.

The Select method expects a Func. Here TSource is int and TResult is string, so the lambda is Func.

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