Why Pattern matching in switch in C Sharp (C#)? - Purpose & Use Cases

The Big Idea

What if you could replace many confusing if-else checks with one simple, clear switch statement?

The Scenario

Imagine you have a box with different types of fruits, and you want to do something special depending on the fruit type and its properties. Without pattern matching, you have to write many if-else checks, one for each fruit and condition.

The Problem

This manual way is slow and confusing. You might forget a condition or write repeated code. It's like sorting fruits by hand every time, which wastes time and causes mistakes.

The Solution

Pattern matching in switch lets you check the type and properties of an object in one clear place. It's like having a smart sorter that knows exactly what to do with each fruit type and condition, making your code neat and easy to read.

Before vs After

✗ Before

if (obj is int) { /* handle int */ } else if (obj is string s && s.Length > 5) { /* handle long string */ } else { /* default */ }

✓ After

switch (obj) { case int i: /* handle int */ break; case string s when s.Length > 5: /* handle long string */ break; default: /* default */ break; }

What It Enables

You can write clearer, shorter code that handles many cases with less chance of errors.

Real Life Example

In a game, you can easily decide what to do with different objects like enemies, items, or players by matching their types and states in one switch statement.

Key Takeaways

Manual type and condition checks are slow and error-prone.

Pattern matching in switch combines type and condition checks neatly.

This makes your code easier to write, read, and maintain.

Practice

1. What does pattern matching in a switch statement primarily help you do in C#?

easy

A. Check an object's type and properties in a clear way

B. Loop through collections more efficiently

C. Declare variables with less code

D. Handle exceptions automatically

Solution

1. Step 1: Understand pattern matching purpose

   Pattern matching in switch lets you test an object's type and properties clearly.

2. Step 2: Compare with other options

   Looping, variable declaration, and exception handling are unrelated to pattern matching in switch.

3. Final Answer:

   Check an object's type and properties in a clear way -> Option A

4. Quick Check:

   Pattern matching = type and property checks [OK]

Hint: Pattern matching checks types and conditions clearly [OK]

Common Mistakes:

• Confusing pattern matching with loops
• Thinking it handles exceptions
• Believing it declares variables automatically

2. Which of the following is the correct syntax to match an integer variable num equal to 10 using pattern matching in a switch expression?

easy

A. case int n when n == 10:

B. case int n == 10:

C. case int n => 10:

D. case int n is 10:

Solution

1. Step 1: Recall correct pattern matching syntax

   To match a value with a condition, use case Type variable when condition:.

2. Step 2: Check each option

   case int n when n == 10: uses when n == 10, which is correct. Others use invalid syntax.

3. Final Answer:

   case int n when n == 10: -> Option A

4. Quick Check:

   Use 'when' for conditions in pattern matching [OK]

Hint: Use 'when' keyword for conditions in switch patterns [OK]

Common Mistakes:

• Using '==' directly after case without 'when'
• Using '=>' or 'is' incorrectly in case
• Omitting the variable name

3. What will be the output of this C# code?

object obj = 42;
switch (obj)
{
    case int n when n > 40:
        Console.WriteLine("Greater than 40");
        break;
    case int n:
        Console.WriteLine("Integer");
        break;
    default:
        Console.WriteLine("Other");
        break;
}

medium

A. Integer

B. Other

C. Greater than 40

D. No output (compile error)

Solution

1. Step 1: Identify the object type and value

   obj is an integer with value 42.

2. Step 2: Evaluate switch cases in order

   First case matches int n when n > 40, which is true for 42, so prints "Greater than 40" and breaks.

3. Final Answer:

   Greater than 40 -> Option C

4. Quick Check:

   Pattern with 'when' matched first [OK]

Hint: Check cases top to bottom; first match runs [OK]

Common Mistakes:

• Ignoring the 'when' condition
• Assuming second case runs first
• Thinking default runs for int

4. Identify the error in this switch statement using pattern matching:

object val = "hello";
switch (val)
{
    case string s when s.Length > 3
        Console.WriteLine("Long string");
        break;
    default:
        Console.WriteLine("Other");
        break;
}

medium

A. Cannot use 'when' with string type

B. Missing colon ':' after case pattern

C. Missing parentheses around condition

D. Incorrect variable name in case

Solution

1. Step 1: Check switch case syntax

   Each case pattern must end with a colon ':' before the statements.

2. Step 2: Locate missing colon

   The case line lacks ':' after when s.Length > 3, causing syntax error.

3. Final Answer:

   Missing colon ':' after case pattern -> Option B

4. Quick Check:

   Case patterns need ':' after condition [OK]

Hint: Always put ':' after case pattern line [OK]

Common Mistakes:

• Forgetting ':' after case pattern
• Adding parentheses around 'when' condition
• Thinking 'when' can't be used with strings

5. Given a list of objects items containing integers and strings, how would you use pattern matching in a switch expression to return a new list of strings where:
- Integers greater than 10 become "Big number"
- Strings longer than 5 characters become "Long text"
- All others become "Other"?

Which code snippet correctly implements this?

hard

A. items.Select(item => item switch {
int n when n > 10: "Big number",
string s when s.Length > 5: "Long text",
_ => "Other"
}).ToList();

B. items.Select(item => switch (item) {
case int n if n > 10: "Big number";
case string s if s.Length > 5: "Long text";
default: "Other";
}).ToList();

C. items.Select(item => item switch {
int n if n > 10: "Big number",
string s if s.Length > 5: "Long text",
_ => "Other"
}).ToList();

D. items.Select(item => item switch {
int n when n > 10 => "Big number",
string s when s.Length > 5 => "Long text",
_ => "Other"
}).ToList();

Solution

1. Step 1: Understand switch expression syntax

   Switch expressions use '=>' for cases, and 'when' for conditions.

2. Step 2: Check each option's syntax

   items.Select(item => item switch { int n when n > 10 => "Big number", string s when s.Length > 5 => "Long text", _ => "Other" }).ToList(); uses correct 'item switch { case => result }' with 'when' and '=>'. Other options use invalid syntax ('switch' inside lambda or 'if' instead of 'when'). items.Select(item => item switch { int n when n > 10: "Big number", string s when s.Length > 5: "Long text", _ => "Other" }).ToList(); uses ':' instead of '=>', which is invalid in switch expressions.

3. Final Answer:

   items.Select(item => item switch { int n when n > 10 => "Big number", string s when s.Length > 5 => "Long text", _ => "Other" }).ToList(); -> Option D

4. Quick Check:

   Switch expressions use '=>' and 'when' [OK]

Hint: Use '=>' and 'when' in switch expressions for conditions [OK]

Common Mistakes:

• Using ':' instead of '=>' in switch expressions
• Using 'if' instead of 'when' for conditions
• Trying to put 'switch' keyword inside lambda