Lambda expression syntax in C Sharp (C#) - Mini Project: Build & Apply

Lambda Expression Syntax in C#

📖 Scenario: You are working on a simple program that processes a list of numbers. You want to use a lambda expression to filter and transform these numbers easily.

🎯 Goal: Build a C# program that uses a lambda expression to select even numbers from a list and then doubles each selected number.

📋 What You'll Learn

Create a list of integers with specific values

Create a lambda expression to filter even numbers

Use the lambda expression to transform the filtered numbers by doubling them

Print the resulting list of doubled even numbers

💡 Why This Matters

🌍 Real World

Filtering and transforming lists of data is common in many programs, such as processing user inputs, handling collections of items, or working with data streams.

💼 Career

Understanding lambda expressions and list operations is essential for C# developers to write concise and readable code, especially when working with collections and LINQ.

1

Create the list of numbers

Create a list of integers called numbers with these exact values: 1, 2, 3, 4, 5, 6.

C Sharp (C#)

# Create the list of numbers
// Your code here

Hint

Use List<int> and initialize it with the values inside curly braces.

2

Create a lambda expression to filter even numbers

Create a variable called evenNumbers that uses numbers.FindAll(n => n % 2 == 0) to select only even numbers from numbers.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List numbers = new List { 1, 2, 3, 4, 5, 6 };
        // Create evenNumbers using a lambda expression to filter even numbers
        // Your code here
    }
}

Hint

Use FindAll method with a lambda expression n => n % 2 == 0 to get even numbers.

3

Double the filtered even numbers using a lambda expression

Create a variable called doubledEvenNumbers that uses evenNumbers.ConvertAll(n => n * 2) to double each number in evenNumbers.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List numbers = new List { 1, 2, 3, 4, 5, 6 };
        List evenNumbers = numbers.FindAll(n => n % 2 == 0);
        // Create doubledEvenNumbers by doubling each number in evenNumbers using a lambda
        // Your code here
    }
}

Hint

Use ConvertAll with a lambda expression n => n * 2 to double each number.

4

Print the doubled even numbers

Use foreach to print each number in doubledEvenNumbers on its own line.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List numbers = new List { 1, 2, 3, 4, 5, 6 };
        List evenNumbers = numbers.FindAll(n => n % 2 == 0);
        List doubledEvenNumbers = evenNumbers.ConvertAll(n => n * 2);
        // Print each number in doubledEvenNumbers
        // Your code here
    }
}

Hint

Use a foreach loop to print each number with Console.WriteLine.

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