Switch expression (modern C#) - Mini Project: Build & Apply

Using Switch Expression in C#

📖 Scenario: You are building a simple program that assigns a description to a day of the week. This helps users understand what kind of day it is.

🎯 Goal: Create a program that uses a modern C# switch expression to convert a day name into a short description.

📋 What You'll Learn

Create a string variable called day with a day name.

Create a switch expression called description that matches day to a description.

Use the switch expression to assign descriptions for Monday, Friday, and Sunday.

Print the description to the console.

💡 Why This Matters

🌍 Real World

Switch expressions help write clear and concise code for decision making, such as mapping inputs to outputs in apps.

💼 Career

Understanding switch expressions is useful for C# developers to write modern, readable, and maintainable code.

1

Create the day variable

Create a string variable called day and set it to the value "Monday".

C Sharp (C#)

// Create a string variable called day and set it to "Monday"
// Your code here

Hint

Use string day = "Monday"; to create the variable.

2

Create the description switch expression

Create a string variable called description and use a switch expression on day to assign:
- "Start of work week" for "Monday"
- "Almost weekend" for "Friday"
- "Rest day" for "Sunday"
Use _ as the default case with "Regular day".

C Sharp (C#)

string day = "Monday";
// Create a switch expression called description based on day
// Your code here

Hint

Use the syntax: string description = day switch { "Monday" => "Start of work week", ... };

3

Change day to "Friday"

Change the value of the day variable to "Friday".

C Sharp (C#)

string day = "Monday";
string description = day switch
{
    "Monday" => "Start of work week",
    "Friday" => "Almost weekend",
    "Sunday" => "Rest day",
    _ => "Regular day"
};
// Change day to "Friday"
// Your code here

Hint

Just replace the value inside the quotes: string day = "Friday";

4

Print the description

Write a Console.WriteLine statement to print the value of description.

C Sharp (C#)

string day = "Friday";
string description = day switch
{
    "Monday" => "Start of work week",
    "Friday" => "Almost weekend",
    "Sunday" => "Rest day",
    _ => "Regular day"
};
// Print the description
// Your code here

Hint

Use Console.WriteLine(description); to show the result.

Practice

1. What is the purpose of the _ symbol in a modern C# switch expression?

easy

A. It declares a new variable inside the switch.

B. It marks the end of the switch expression.

C. It acts as a default case to catch all unmatched values.

D. It is used to separate multiple cases.

Solution

1. Step 1: Understand the role of _ in switch expressions

   The underscore _ is used as a discard pattern that matches any value not matched by previous cases.

2. Step 2: Recognize it as the default case

   This means it acts like a default or fallback case to handle all other inputs.

3. Final Answer:

   It acts as a default case to catch all unmatched values. -> Option C

4. Quick Check:

   _ = default case [OK]

Hint: Remember '_' means 'anything else' or default [OK]

Common Mistakes:

• Thinking '_' declares a variable
• Confusing '_' with case separator
• Assuming '_' ends the switch

2. Which of the following is the correct syntax for a simple switch expression in C# that returns "Even" for 2 and 4, and "Odd" for 1 and 3?

easy

A. var result = switch(number) { 2: "Even", 4: "Even", 1 => "Odd", 3 => "Odd" };

B. var result = number switch { 2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd" };

C. var result = number switch (2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd");

D. var result = number switch { case 2: "Even"; case 4: "Even"; case 1: "Odd"; case 3: "Odd"; };

Solution

1. Step 1: Identify correct switch expression syntax

   In modern C#, switch expressions use the syntax: variable switch { pattern => result, ... }.

2. Step 2: Compare options to syntax

   var result = number switch { 2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd" }; matches this syntax exactly. Options B, C, and D use incorrect syntax like parentheses or keywords not used in switch expressions.

3. Final Answer:

   var result = number switch { 2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd" }; -> Option B

4. Quick Check:

   Correct switch expression syntax = var result = number switch { 2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd" }; [OK]

Hint: Switch expressions use curly braces and => for cases [OK]

Common Mistakes:

• Using parentheses instead of braces
• Using 'case' keyword inside switch expression
• Using colons instead of =>

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

int day = 3;
string dayName = day switch
{
  1 => "Monday",
  2 => "Tuesday",
  3 => "Wednesday",
  _ => "Unknown"
};
Console.WriteLine(dayName);

medium

A. Wednesday

B. Monday

C. Unknown

D. Tuesday

Solution

1. Step 1: Match the value of day in the switch expression

   The variable day is 3, so the switch expression looks for the case matching 3.

2. Step 2: Find the corresponding string for case 3

   The case 3 returns "Wednesday". Since it matches, the default case _ is ignored.

3. Final Answer:

   Wednesday -> Option A

4. Quick Check:

   day=3 matches 3 => "Wednesday" [OK]

Hint: Match the variable value to the case pattern [OK]

Common Mistakes:

• Choosing default case instead of matching case
• Confusing case numbers with strings
• Ignoring the switch expression syntax

4. Identify the error in this switch expression code snippet:

string result = value switch
{
  1 => "One",
  2 => "Two",
  3 => "Three",
  default => "Other"
};

medium

A. Using 'default' keyword instead of '_' for default case.

B. Switch expression must use parentheses, not braces.

C. Missing comma after case 3.

D. The variable 'value' is not declared.

Solution

1. Step 1: Check syntax for default case in switch expressions

   Modern C# switch expressions use _ as the default case, not the keyword default.

2. Step 2: Identify the incorrect use of 'default'

   The code uses default => "Other", which is invalid syntax in switch expressions.

3. Final Answer:

   Using 'default' keyword instead of '_' for default case. -> Option A

4. Quick Check:

   Default case = '_' not 'default' [OK]

Hint: Use '_' for default, not 'default' keyword [OK]

Common Mistakes:

• Using 'default' instead of '_'
• Forgetting commas between cases
• Confusing switch statement syntax with switch expression

5. You want to write a switch expression that returns "Small" for numbers less than 10, "Medium" for numbers between 10 and 20 inclusive, and "Large" for numbers greater than 20. Which of the following switch expressions correctly implements this?

hard

A. var size = number switch { < 10 => "Small", 10 <= number <= 20 => "Medium", _ => "Large" };

B. var size = number switch { < 10 => "Small", > 10 => "Medium", _ => "Large" };

C. var size = number switch { < 10 => "Small", > 10 and < 20 => "Medium", _ => "Large" };

D. var size = number switch { < 10 => "Small", >= 10 and <= 20 => "Medium", _ => "Large" };

Solution

1. Step 1: Understand pattern matching syntax in switch expressions

   Modern C# supports relational patterns like < 10 and logical patterns like and to combine conditions.

2. Step 2: Check each option for correct range coverage

   var size = number switch { < 10 => "Small", >= 10 and <= 20 => "Medium", _ => "Large" }; uses < 10 for "Small", and >= 10 and <= 20 for "Medium", which correctly covers 10 to 20 inclusive. The default _ covers "Large".

3. Final Answer:

   var size = number switch { < 10 => "Small", >= 10 and <= 20 => "Medium", _ => "Large" }; -> Option D

4. Quick Check:

   Use relational and logical patterns for ranges [OK]

Hint: Use 'and' to combine conditions in switch patterns [OK]

Common Mistakes:

• Using invalid syntax like '10 <= number <= 20'
• Missing inclusive range boundaries
• Using overlapping or incorrect ranges