Switch expressions with patterns in C Sharp (C#)

Introduction

Switch expressions with patterns let you choose a result based on the shape or value of data in a clear and simple way.

You want to return different results based on the type of an object.

You need to check if a value fits certain conditions and respond accordingly.

You want to replace long if-else chains with cleaner code.

You want to match values and extract parts of data easily.

Syntax

C Sharp (C#)

var result = value switch
{
    pattern1 => expression1,
    pattern2 => expression2,
    _ => defaultExpression
};

The switch expression evaluates value against each pattern in order.

The underscore _ is a catch-all pattern for anything not matched before.

Examples

This matches exact numbers and returns a string for each case.

C Sharp (C#)

int number = 5;
string result = number switch
{
    1 => "One",
    2 => "Two",
    _ => "Other"
};

This matches the type of obj and extracts the value.

C Sharp (C#)

object obj = "hello";
string result = obj switch
{
    int i => $"Number {i}",
    string s => $"Text '{s}'",
    _ => "Unknown"
};

This uses relational patterns to check ranges.

C Sharp (C#)

int age = 20;
string category = age switch
{
    < 13 => "Child",
    >= 13 and < 20 => "Teen",
    _ => "Adult"
};

Sample Program

This program checks each item in an array and prints a description based on its type or value.

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        object[] items = { 42, "apple", 3.14, null };

        foreach (var item in items)
        {
            string description = item switch
            {
                int i => $"Integer: {i}",
                string s => $"String: '{s}'",
                null => "Null value",
                _ => "Other type"
            };
            Console.WriteLine(description);
        }
    }
}

Important Notes

Switch expressions are expressions, so they return a value directly.

Patterns can be combined with and, or, and not for more complex checks.

Always include a catch-all _ pattern to handle unexpected cases.

Summary

Switch expressions with patterns let you match values and types cleanly.

They help replace long if-else chains with clearer code.

Patterns can check types, values, and conditions easily.

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