Pattern matching in switch in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of pattern matching with type check

What is the output of this C# code using pattern matching in a switch expression?

C Sharp (C#)

object obj = 42;
string result = obj switch
{
    int i when i > 40 => "Greater than 40",
    int i => "Integer",
    string s => "String",
    _ => "Other"
};
System.Console.WriteLine(result);

A String

B Integer

C Other

D Greater than 40

💡 Hint

Look at the order of patterns and the value of obj.

The object is an int with value 42, which matches the first pattern 'int i when i > 40', so the output is 'Greater than 40'.

❓ Predict Output

intermediate

Pattern matching with property check in switch

What will this C# code print when using pattern matching on a record type?

C Sharp (C#)

record Person(string Name, int Age);

Person p = new("Alice", 30);
string category = p switch
{
    { Age: < 18 } => "Minor",
    { Age: >= 18 and < 65 } => "Adult",
    { Age: >= 65 } => "Senior",
    _ => "Unknown"
};
System.Console.WriteLine(category);

A Minor

B Adult

C Senior

D Unknown

💡 Hint

Check the Age property value and which pattern it matches.

The Person has Age 30, which fits the pattern 'Age: >= 18 and < 65', so the output is 'Adult'.

🔧 Debug

advanced

Identify the error in pattern matching switch

What error does this C# code produce when compiled?

C Sharp (C#)

object value = "hello";
switch (value)
{
    case int i:
        System.Console.WriteLine("Integer");
        break;
    case string s when s.Length > 5:
        System.Console.WriteLine("Long string");
        break;
    case string s:
        System.Console.WriteLine("String");
        break;
    case null:
        System.Console.WriteLine("Null");
        break;
}

A No error, code compiles and runs

B CS0161: Not all code paths return a value

C CS0152: The switch cases are unreachable due to order

D CS8120: A pattern is unreachable because a previous pattern matches all strings

💡 Hint

Look at the order of string cases and their conditions.

No compiler error. The guarded string case handles strings with Length > 5, and the general string case handles other strings. Both patterns are reachable.

📝 Syntax

advanced

Which switch pattern syntax is invalid?

Which of these C# switch case patterns will cause a syntax error?

A case int i when i % 2 == 0:

B case { Length: > 5 } s:

C case string s if s.StartsWith("A"):

D case null:

💡 Hint

Check the correct keyword for pattern guards in switch cases.

The keyword 'if' is invalid in switch case patterns; 'when' must be used for guards. Option C uses 'if' causing a syntax error.

🚀 Application

expert

Determine the final value after complex pattern matching

Given the following C# code, what is the value of 'result' after execution?

C Sharp (C#)

object data = new List<int> { 1, 2, 3 };
string result = data switch
{
    List<int> list when list.Count == 0 => "Empty list",
    List<int> list when list.Count > 0 && list[0] == 1 => "Starts with one",
    List<int> list => "Other list",
    _ => "Unknown"
};

A "Starts with one"

B "Empty list"

C "Other list"

D "Unknown"

💡 Hint

Check the list contents and which pattern matches first.

The list has elements and starts with 1, so it matches the second pattern and result is "Starts with one".

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