Pattern matching in switch in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is pattern matching in a switch statement?

Pattern matching in a switch statement lets you check an expression against different patterns, not just fixed values. It helps you test types, properties, or conditions directly in each case.

beginner

How do you match a type in a switch case using pattern matching?

You write the type name followed by a variable name, like case int number:. This matches if the value is an int and assigns it to number.

intermediate

What does the when keyword do in a switch case?

The when keyword adds an extra condition to a case. The case matches only if the pattern matches and the when condition is true.

intermediate

Explain the difference between case int n: and case int n when n > 0:.

case int n: matches any integer and assigns it to n. case int n when n > 0: matches only positive integers because of the when condition.

beginner

Can you use pattern matching in switch expressions as well as statements?

Yes! Pattern matching works in both switch statements and switch expressions, allowing concise and readable code for selecting values based on patterns.

What does case string s: do in a switch statement?

A Matches any string and assigns it to s

B Matches only empty strings

C Matches any integer and assigns it to s

D Matches null values only

The case matches any value of type string and assigns it to the variable s.

Which keyword adds an extra condition to a switch case pattern?

A if

B where

C when

D case

The 'when' keyword adds a condition that must be true for the case to match.

What happens if no pattern matches in a switch statement with pattern matching?

A The switch statement is skipped

B The program crashes

C The first case runs anyway

D The default case runs if provided

If no pattern matches, the default case runs if it exists; otherwise, no case runs.

Which of these is a valid pattern matching case in C#?

A case n int:

B case int n when n < 10:

C case (int) n:

D case int:

Option B correctly uses type matching with a variable and a when condition.

Can pattern matching in switch check properties of an object?

A Yes, using property patterns

B Only with loops

C Only with if statements

D No, only types

Pattern matching supports property patterns to check object properties inside switch cases.

Describe how pattern matching improves switch statements in C#.

Think about how you can check more than just fixed values.

Explain how to use the 'when' keyword in a switch case with an example.

It adds extra conditions to cases.

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