Why pattern matching matters in C Sharp (C#) - See It in Action

Why pattern matching matters

📖 Scenario: Imagine you work at a store that sells different types of products. Each product has a different way to calculate its discount. You want to write a program that checks the product type and calculates the discount correctly.

🎯 Goal: You will build a simple program that uses pattern matching to check product types and calculate discounts easily and clearly.

📋 What You'll Learn

Create a list of products with different types

Create a variable to hold the total discount

Use pattern matching with switch expressions to calculate discounts based on product type

Print the total discount

💡 Why This Matters

🌍 Real World

Pattern matching helps write clear and simple code when you have different types of data that need different handling, like products with different discount rules.

💼 Career

Many jobs require handling different data types and making decisions based on them. Pattern matching is a modern, readable way to do this in C#.

1

Create product classes and a list

Create three classes called Book, Food, and Clothing. Each class should have a public property Price of type decimal. Then create a list called products containing these exact items: a Book with price 20.0m, a Food with price 10.0m, and a Clothing with price 50.0m.

C Sharp (C#)

// Your code here

Hint

Define each class with a Price property. Then create a list of type object and add one instance of each class with the given prices.

2

Create a variable to hold total discount

Inside the Main method, create a variable called totalDiscount of type decimal and set it to 0.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Book
{
    public decimal Price { get; set; }
}

class Food
{
    public decimal Price { get; set; }
}

class Clothing
{
    public decimal Price { get; set; }
}

class Program
{
    static void Main()
    {
        List products = new List
        {
            new Book { Price = 20.0m },
            new Food { Price = 10.0m },
            new Clothing { Price = 50.0m }
        };
        // Create a decimal variable called totalDiscount and set it to 0
        // Your code here
    }
}

Hint

Use decimal totalDiscount = 0m; to create the variable.

3

Calculate discounts using pattern matching

Use a foreach loop with variable product to go through products. Inside the loop, use a switch expression with pattern matching on product to calculate a discount: if product is Book, discount is 10% of Price; if Food, discount is 5% of Price; if Clothing, discount is 20% of Price; otherwise, discount is 0. Add each discount to totalDiscount.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Book
{
    public decimal Price { get; set; }
}

class Food
{
    public decimal Price { get; set; }
}

class Clothing
{
    public decimal Price { get; set; }
}

class Program
{
    static void Main()
    {
        List products = new List
        {
            new Book { Price = 20.0m },
            new Food { Price = 10.0m },
            new Clothing { Price = 50.0m }
        };
        decimal totalDiscount = 0m;
        // Use foreach with product to loop over products
        // Use switch expression with pattern matching on product to calculate discount
        // Add discount to totalDiscount
        // Your code here
    }
}

Hint

Use foreach (object product in products) and a switch expression with pattern matching to calculate discounts.

4

Print the total discount

Write a Console.WriteLine statement to print the text Total discount: followed by the value of totalDiscount.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Book
{
    public decimal Price { get; set; }
}

class Food
{
    public decimal Price { get; set; }
}

class Clothing
{
    public decimal Price { get; set; }
}

class Program
{
    static void Main()
    {
        List products = new List
        {
            new Book { Price = 20.0m },
            new Food { Price = 10.0m },
            new Clothing { Price = 50.0m }
        };
        decimal totalDiscount = 0m;
        foreach (object product in products)
        {
            decimal discount = product switch
            {
                Book b => b.Price * 0.10m,
                Food f => f.Price * 0.05m,
                Clothing c => c.Price * 0.20m,
                _ => 0m
            };
            totalDiscount += discount;
        }
        // Print the totalDiscount with the text "Total discount: "
        // Your code here
    }
}

Hint

Use Console.WriteLine($"Total discount: {totalDiscount}"); to print the result.

Practice

1. What is the main benefit of using pattern matching in C#?

easy

A. It simplifies checking types and extracting data in one step.

B. It makes the program run faster by optimizing CPU usage.

C. It automatically fixes syntax errors in the code.

D. It replaces all loops with recursive calls.

Solution

1. Step 1: Understand pattern matching purpose

   Pattern matching lets you check a variable's type and extract its data in a single, clear step.

2. Step 2: Compare with other options

   Options A, B, and D describe unrelated or incorrect benefits not provided by pattern matching.

3. Final Answer:

   It simplifies checking types and extracting data in one step. -> Option A

4. Quick Check:

   Pattern matching = Simplifies type checks [OK]

Hint: Pattern matching combines type check and data use in one step [OK]

Common Mistakes:

• Thinking pattern matching speeds up CPU performance
• Confusing pattern matching with syntax error fixing
• Believing pattern matching replaces loops

2. Which of the following is the correct syntax for pattern matching with is in C#?

easy

A. if (obj is int number) { /* use number */ }

B. if (obj is int) number { /* use number */ }

C. if (obj is int: number) { /* use number */ }

D. if (obj int is number) { /* use number */ }

Solution

1. Step 1: Recall correct pattern matching syntax

   The correct syntax uses is Type variable inside the if condition, like obj is int number.

2. Step 2: Identify syntax errors in other options

   Options A, B, and C have incorrect order or punctuation that cause syntax errors.

3. Final Answer:

   if (obj is int number) { /* use number */ } -> Option A

4. Quick Check:

   Correct pattern matching syntax = if (obj is int number) { /* use number */ } [OK]

Hint: Remember: 'is Type variable' is the pattern matching syntax [OK]

Common Mistakes:

• Placing variable before type in 'is' expression
• Using colon ':' instead of space
• Mixing order of keywords

3. What will be the output of this code?

object obj = 42;
switch (obj)
{
    case int i:
        Console.WriteLine($"Integer: {i}");
        break;
    case string s:
        Console.WriteLine($"String: {s}");
        break;
    default:
        Console.WriteLine("Unknown type");
        break;
}

medium

A. String: 42

B. Integer: 42

C. Unknown type

D. Compilation error

Solution

1. Step 1: Identify the type of obj

   obj is assigned the integer value 42, so its runtime type is int.

2. Step 2: Match switch cases

   The switch matches case int i because obj is an int, so it prints "Integer: 42".

3. Final Answer:

   Integer: 42 -> Option B

4. Quick Check:

   obj is int 42, so output = "Integer: 42" [OK]

Hint: Match the runtime type to the correct case in switch [OK]

Common Mistakes:

• Assuming obj is string because of quotes missing
• Thinking default case runs first
• Confusing compile-time and runtime types

4. Identify the error in this pattern matching code:

object obj = "hello";
if (obj is int number)
{
    Console.WriteLine(number);
}

medium

A. The variable 'number' is not declared before use.

B. The 'is' keyword cannot be used with variables.

C. Missing braces around the if block.

D. The pattern matching condition will never be true for a string.

Solution

1. Step 1: Analyze the type of obj

   obj holds a string "hello", but the pattern checks if obj is an int.

2. Step 2: Understand pattern matching behavior

   Since obj is a string, the condition obj is int number is false, so the block never runs.

3. Final Answer:

   The pattern matching condition will never be true for a string. -> Option D

4. Quick Check:

   obj is string, pattern checks int, so condition false [OK]

Hint: Check if variable type matches pattern type before using it [OK]

Common Mistakes:

• Assuming variable 'number' must be declared separately
• Thinking 'is' cannot be used with variables
• Ignoring that condition can be false without error

5. Given a list of objects List<object> items = new() { 1, -2, "two", 3.0, null };, which pattern matching expression correctly counts only the positive integer values?

hard

A. items.Count(item => item is string s)

B. items.Count(item => item is int)

C. items.Count(item => item is int number && number > 0)

D. items.Count(item => item != null)

Solution

1. Step 1: Understand the goal

   We want to count only positive integers in the list, ignoring other types and null.

2. Step 2: Analyze each option

   items.Count(item => item is int) counts all ints but does not filter by value; items.Count(item => item is int number && number > 0) counts ints greater than 0, which matches positive integers; items.Count(item => item is string s) counts strings; items.Count(item => item != null) counts all non-null items.

3. Step 3: Choose the best option

   items.Count(item => item is int number && number > 0) uses pattern matching with a condition to count only positive integers, which is a precise use of pattern matching.

4. Final Answer:

   items.Count(item => item is int number && number > 0) -> Option C

5. Quick Check:

   Pattern matching with condition filters integers correctly [OK]

Hint: Use 'is Type var && condition' to filter precisely [OK]

Common Mistakes:

• Counting all ints without condition when needed
• Counting strings or non-null items by mistake
• Not using variable in pattern matching for conditions