Switch expressions with patterns in C Sharp (C#) - Mini Project: Build & Apply

Switch Expressions with Patterns

📖 Scenario: You are building a simple program that describes animals based on their type and age. This helps a zoo staff quickly understand animal details.

🎯 Goal: Create a C# program that uses a switch expression with patterns to return descriptions for different animals.

📋 What You'll Learn

Create a record called Animal with properties Type (string) and Age (int).

Create an Animal variable with specific values.

Create a switch expression using pattern matching on the Animal variable.

Print the description returned by the switch expression.

💡 Why This Matters

🌍 Real World

Pattern matching with switch expressions helps write clear and concise code when you want to handle different cases based on object properties, like categorizing animals or processing user input.

💼 Career

Many C# jobs require understanding modern language features like switch expressions and pattern matching to write clean, maintainable code.

1

Create the Animal record and an instance

Create a record called Animal with properties Type (string) and Age (int). Then create a variable called animal and set it to new Animal("Dog", 5).

C Sharp (C#)

// Define the Animal record and create the animal variable
// Your code here

Hint

Use the record keyword to create Animal. Then create animal with new Animal("Dog", 5).

2

Create a variable for the description using switch expression

Create a variable called description and assign it a switch expression on animal. Use pattern matching with Animal to match these cases: Type == "Dog" and Age < 3 returns "Puppy Dog", Type == "Dog" and Age >= 3 returns "Adult Dog", Type == "Cat" returns "Cat", and a default case returns "Unknown Animal".

C Sharp (C#)

public record Animal(string Type, int Age);

Animal animal = new Animal("Dog", 5);

// Create description variable using switch expression with patterns
// Your code here

Hint

Use a switch expression with pattern matching on animal. Use property patterns like Animal { Type: "Dog", Age: < 3 }.

3

Change the animal to a Cat aged 2

Change the animal variable to new Animal("Cat", 2).

C Sharp (C#)

public record Animal(string Type, int Age);

// Change animal to a Cat aged 2
// Your code here

string description = animal switch
{
    Animal { Type: "Dog", Age:  "Puppy Dog",
    Animal { Type: "Dog", Age: >= 3 } => "Adult Dog",
    Animal { Type: "Cat" } => "Cat",
    _ => "Unknown Animal"
};

Hint

Just replace the animal variable assignment with new Animal("Cat", 2).

4

Print the description

Write a Console.WriteLine statement to print the description variable.

C Sharp (C#)

public record Animal(string Type, int Age);

Animal animal = new Animal("Cat", 2);

string description = animal switch
{
    Animal { Type: "Dog", Age:  "Puppy Dog",
    Animal { Type: "Dog", Age: >= 3 } => "Adult Dog",
    Animal { Type: "Cat" } => "Cat",
    _ => "Unknown Animal"
};

// Print the description
// Your code here

Hint

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

Practice

1. What does a switch expression with patterns in C# primarily help you do?

easy

A. Create loops that run a fixed number of times

B. Declare variables with dynamic types

C. Match values and types cleanly without long if-else chains

D. Handle exceptions in asynchronous code

Solution

1. Step 1: Understand switch expressions

   Switch expressions allow matching on values or types in a concise way.

2. Step 2: Compare with if-else chains

   They replace long if-else chains with clearer, more readable code.

3. Final Answer:

   Match values and types cleanly without long if-else chains -> Option C

4. Quick Check:

   Switch expressions = clean matching [OK]

Hint: Switch expressions simplify matching values and types [OK]

Common Mistakes:

• Confusing switch expressions with loops
• Thinking switch expressions handle exceptions
• Believing switch expressions declare variables

2. Which of the following is the correct syntax for a switch expression with a type pattern in C#?

easy

A. var result = obj switch { int i => i * 2, string s => s.Length, _ => 0 };

B. var result = switch obj { int i => i * 2, string s => s.Length, _ => 0 };

C. var result = obj { int i => i * 2, string s => s.Length, _ => 0 } switch;

D. var result = obj switch (int i => i * 2, string s => s.Length, _ => 0);

Solution

1. Step 1: Identify correct switch expression syntax

   The switch expression syntax is: variable switch { pattern => result, ... }.

2. Step 2: Match options to syntax

   var result = obj switch { int i => i * 2, string s => s.Length, _ => 0 }; matches this syntax exactly with type patterns and a discard pattern.

3. Final Answer:

   var result = obj switch { int i => i * 2, string s => s.Length, _ => 0 }; -> Option A

4. Quick Check:

   Correct switch expression syntax = var result = obj switch { int i => i * 2, string s => s.Length, _ => 0 }; [OK]

Hint: Remember: variable switch { patterns } is correct syntax [OK]

Common Mistakes:

• Placing switch before the variable
• Using parentheses instead of braces
• Misplacing the switch keyword

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

object value = 5;
string result = value switch
{
  int i when i > 0 => "Positive integer",
  int i when i < 0 => "Negative integer",
  string s => "String",
  _ => "Unknown"
};
Console.WriteLine(result);

medium

A. Negative integer

B. Positive integer

C. String

D. Unknown

Solution

1. Step 1: Analyze the switch expression input

   The variable 'value' is an object holding the integer 5.

2. Step 2: Match patterns in order

   First pattern matches int i when i > 0, which is true for 5, so result is "Positive integer".

3. Final Answer:

   Positive integer -> Option B

4. Quick Check:

   5 > 0 matches first pattern [OK]

Hint: Check pattern order and conditions carefully [OK]

Common Mistakes:

• Ignoring the 'when' condition
• Assuming string pattern matches first
• Choosing default case incorrectly

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

object obj = "hello";
var output = obj switch
{
  int i => i.ToString(),
  string s => s.ToUpper(),
  default => "none"
};

medium

A. Patterns must be in parentheses

B. Missing semicolon after switch expression

C. Cannot switch on object type

D. Using 'default' instead of '_' for discard pattern

Solution

1. Step 1: Check switch expression patterns

   In C# switch expressions, the discard pattern uses '_' not 'default'.

2. Step 2: Identify the incorrect pattern usage

   The code uses 'default' which causes a syntax error; it should be '_'.

3. Final Answer:

   Using 'default' instead of '_' for discard pattern -> Option D

4. Quick Check:

   Discard pattern = '_' not 'default' [OK]

Hint: Use '_' for default case in switch expressions [OK]

Common Mistakes:

• Using 'default' keyword in switch expressions
• Confusing switch statements with switch expressions
• Forgetting to end statements with semicolon

5. Given a list of objects, write a switch expression that returns:
- "Number" if the object is an int or double
- "Text" if the object is a non-empty string
- "Empty" if the object is an empty string
- "Other" for anything else

Which of the following switch expressions correctly implements this logic?

hard

A. obj switch { int _ or double _ => "Number", string s when s.Length == 0 => "Empty", string s when s.Length > 0 => "Text", _ => "Other" }

B. obj switch { int or double => "Number", string s when s == "" => "Empty", string s when s != "" => "Text", _ => "Other" }

C. obj switch { int or double => "Number", string s when s.Length > 0 => "Text", string s when s.Length == 0 => "Empty", _ => "Other" }

D. obj switch { int or double => "Number", string s when s != "" => "Text", string s when s == "" => "Empty", _ => "Other" }

Solution

1. Step 1: Understand type patterns and conditions

   We must match int or double first, then string with length checks for empty/non-empty.

2. Step 2: Check order and conditions

   obj switch { int _ or double _ => "Number", string s when s.Length == 0 => "Empty", string s when s.Length > 0 => "Text", _ => "Other" } correctly checks int or double, then string with Length == 0 for "Empty", then Length > 0 for "Text", then default.

3. Final Answer:

   obj switch { int _ or double _ => "Number", string s when s.Length == 0 => "Empty", string s when s.Length > 0 => "Text", _ => "Other" } -> Option A

4. Quick Check:

   Order and conditions matter in switch patterns [OK]

Hint: Order patterns from specific to general, check string length for empty [OK]

Common Mistakes:

• Using 'or' incorrectly without parentheses
• Checking string emptiness with == "" instead of Length
• Placing empty string check after non-empty