Switch expressions with patterns in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of switch expression with type patterns

What is the output of the following C# code using switch expressions with type patterns?

C Sharp (C#)

object value = 42;
string result = value switch
{
    int i => $"Integer: {i}",
    string s => $"String: {s}",
    _ => "Unknown"
};
System.Console.WriteLine(result);

A String: 42

B Integer: 42

C Unknown

D Compilation error

💡 Hint

Check the type of the variable 'value' and how the switch expression matches patterns.

The variable 'value' is an int with value 42. The switch expression matches the 'int i' pattern and returns "Integer: 42".

❓ Predict Output

intermediate

Switch expression with property pattern output

What will be printed by this C# code using a switch expression with a property pattern?

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 Unknown

B Minor

C Senior

D Adult

💡 Hint

Look at the Age property of the Person record and which pattern it matches.

The Person has Age 30, which matches the pattern { Age: >= 18 and < 65 }, so the result is "Adult".

🔧 Debug

advanced

Identify the error in this switch expression with tuple patterns

This C# code uses a switch expression with tuple patterns. Which option correctly describes the error it produces?

C Sharp (C#)

var point = (x: 3, y: 4);
string quadrant = point switch
{
    ( > 0, > 0 ) => "First",
    ( < 0, > 0 ) => "Second",
    ( < 0, < 0 ) => "Third",
    ( > 0, < 0 ) => "Fourth",
    _ => "Origin or axis"
};
System.Console.WriteLine(quadrant);

A Compilation error due to missing parentheses around relational patterns

B Runtime NullReferenceException

C Output: First

D Output: Origin or axis

💡 Hint

Check if the tuple patterns and relational patterns syntax are correct and what the values are.

The tuple (3,4) matches the pattern ( > 0, > 0 ) so the output is "First". The syntax is valid in C# 9+.

📝 Syntax

advanced

Which switch expression syntax is correct for matching a string prefix?

Which option shows the correct C# switch expression syntax to match a string starting with "Hello" using a pattern?

A string greeting = input switch { string s when s.StartsWith("Hello") => "Hi", _ => "Bye" };

B string greeting = input switch { string s if s.StartsWith("Hello") => "Hi", _ => "Bye" };

C string greeting = input switch { string s => s.StartsWith("Hello") ? "Hi" : "Bye" };

D string greeting = input switch { string s where s.StartsWith("Hello") => "Hi", _ => "Bye" };

💡 Hint

Recall the correct keyword for adding a condition in a switch expression pattern is 'when'.

Option A uses 'when' correctly to add a condition in the pattern. Options B and D use invalid keywords 'if' and 'where'. Option A is a valid expression but not a pattern with condition.

🚀 Application

expert

Determine the number of items in the dictionary created by this switch expression

Consider this C# code that uses a switch expression inside a dictionary initializer. How many key-value pairs will the resulting dictionary contain?

C Sharp (C#)

var inputs = new object[] { 1, "two", 3.0, null };
var dict = new Dictionary<object, string>();
foreach (var item in inputs)
{
    dict[item] = item switch
    {
        int i => $"Int {i}",
        string s => $"Str {s}",
        null => "Null",
        _ => "Other"
    };
}
System.Console.WriteLine(dict.Count);

A 4

B 3

C 2

D 1

💡 Hint

Check how many unique keys are added to the dictionary and if any keys are overwritten.

The dictionary keys are the original objects: 1 (int), "two" (string), 3.0 (double), and null. All are unique keys, so the count is 4.

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