Expression-bodied lambdas in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Expression-bodied lambdas

Define lambda with => expression

↓

Invoke lambda

↓

Evaluate expression

↓

Return result

An expression-bodied lambda uses => to define a function with a single expression that returns a value immediately.

Execution Sample

C Sharp (C#)

Func<int, int> square = x => x * x;
int result = square(3);
Console.WriteLine(result);

Defines a lambda that squares a number and prints the result of squaring 3.

Execution Table

Step	Action	Expression Evaluated	Result
1	Define lambda 'square' as x => x * x	x * x	Lambda stored, no output
2	Call square(3)	3 * 3	9
3	Print result	Console.WriteLine(9)	Outputs 9

💡 Program ends after printing the result 9

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
square	null	lambda function (x => x * x)	lambda function	lambda function
result	undefined	undefined	9	9

Key Moments - 2 Insights

Why does the lambda not need curly braces or a return statement?

Because expression-bodied lambdas use a single expression after => which automatically returns its value, as shown in step 1 and 2 of the execution_table.

What happens when we call square(3)?

The expression x * x is evaluated with x=3, resulting in 9, as shown in step 2 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'result' after step 2?

A undefined

B 9

C 3

D null

💡 Hint

Check the 'Result' column in row for step 2 in execution_table.

At which step is the lambda function assigned to 'square'?

A Step 3

B Step 2

C Step 1

D Before Step 1

💡 Hint

Look at the 'Action' column in execution_table for when 'square' is defined.

If we changed the lambda to x => x + 1, what would be the output at step 3?

A 4

B 9

C 3

D 1

💡 Hint

Consider the expression evaluated in step 2 and how it affects the printed result in step 3.

Concept Snapshot

Expression-bodied lambdas use => followed by a single expression.
They automatically return the expression's value.
No curly braces or return keyword needed.
Example: Func<int,int> square = x => x * x;
Call like: square(3) returns 9.

Full Transcript

This example shows how expression-bodied lambdas work in C#. First, we define a lambda named 'square' that takes an integer x and returns x multiplied by itself. This is done using the => syntax with a single expression 'x * x'. When we call square(3), the expression evaluates to 9. Finally, we print the result 9 to the console. The lambda does not need curly braces or a return statement because the expression after => is automatically returned.

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