Switch expression (modern C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a switch expression in modern C#?

A switch expression is a concise way to select a value based on a pattern match, introduced in C# 8.0. It returns a value directly instead of using statements.

beginner

How does a switch expression differ from a traditional switch statement?

A switch expression returns a value and is used inline, while a switch statement executes code blocks and does not return a value directly.

beginner

What symbol is used to separate the input and the result in a switch expression case?

The '=>' symbol is used to separate the input pattern from the result expression in each case of a switch expression.

intermediate

Can switch expressions in C# use pattern matching?

Yes, switch expressions support pattern matching, allowing you to match types, values, and conditions directly in the cases.

intermediate

What happens if no case matches in a switch expression and no default case is provided?

If no case matches and there is no default case (using '_'), the switch expression throws a System.Runtime.CompilerServices.SwitchExpressionException at runtime.

Which symbol is used to separate the pattern and result in a C# switch expression?

A =>

B :

C ->

D =

The '=>' symbol separates the input pattern from the output expression in each case of a switch expression.

What does a switch expression return?

A An exception always

B A value based on the matched case

C A boolean indicating if a case matched

D Nothing, it only executes code blocks

Switch expressions return a value directly based on the matched case.

Which C# version introduced switch expressions?

A C# 7.0

B C# 6.0

C C# 9.0

D C# 8.0

Switch expressions were introduced in C# 8.0.

What keyword is used as a default case in switch expressions?

A _

B default

C else

D case

The underscore '_' is used as a discard pattern to represent the default case in switch expressions.

Can switch expressions use pattern matching with types?

A Only with strings

B No

C Yes

D Only with integers

Switch expressions support pattern matching with types, values, and conditions.

Explain how a switch expression works in modern C# and give a simple example.

Think about how you choose an option and get a result in one line.

Describe the difference between a switch statement and a switch expression in C#.

One is like a list of instructions, the other is like a calculator.

Practice

1. What is the purpose of the _ symbol in a modern C# switch expression?

easy

A. It declares a new variable inside the switch.

B. It marks the end of the switch expression.

C. It acts as a default case to catch all unmatched values.

D. It is used to separate multiple cases.

Solution

1. Step 1: Understand the role of _ in switch expressions

   The underscore _ is used as a discard pattern that matches any value not matched by previous cases.

2. Step 2: Recognize it as the default case

   This means it acts like a default or fallback case to handle all other inputs.

3. Final Answer:

   It acts as a default case to catch all unmatched values. -> Option C

4. Quick Check:

   _ = default case [OK]

Hint: Remember '_' means 'anything else' or default [OK]

Common Mistakes:

• Thinking '_' declares a variable
• Confusing '_' with case separator
• Assuming '_' ends the switch

2. Which of the following is the correct syntax for a simple switch expression in C# that returns "Even" for 2 and 4, and "Odd" for 1 and 3?

easy

A. var result = switch(number) { 2: "Even", 4: "Even", 1 => "Odd", 3 => "Odd" };

B. var result = number switch { 2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd" };

C. var result = number switch (2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd");

D. var result = number switch { case 2: "Even"; case 4: "Even"; case 1: "Odd"; case 3: "Odd"; };

Solution

1. Step 1: Identify correct switch expression syntax

   In modern C#, switch expressions use the syntax: variable switch { pattern => result, ... }.

2. Step 2: Compare options to syntax

   var result = number switch { 2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd" }; matches this syntax exactly. Options B, C, and D use incorrect syntax like parentheses or keywords not used in switch expressions.

3. Final Answer:

   var result = number switch { 2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd" }; -> Option B

4. Quick Check:

   Correct switch expression syntax = var result = number switch { 2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd" }; [OK]

Hint: Switch expressions use curly braces and => for cases [OK]

Common Mistakes:

• Using parentheses instead of braces
• Using 'case' keyword inside switch expression
• Using colons instead of =>

3. What is the output of the following C# code?

int day = 3;
string dayName = day switch
{
  1 => "Monday",
  2 => "Tuesday",
  3 => "Wednesday",
  _ => "Unknown"
};
Console.WriteLine(dayName);

medium

A. Wednesday

B. Monday

C. Unknown

D. Tuesday

Solution

1. Step 1: Match the value of day in the switch expression

   The variable day is 3, so the switch expression looks for the case matching 3.

2. Step 2: Find the corresponding string for case 3

   The case 3 returns "Wednesday". Since it matches, the default case _ is ignored.

3. Final Answer:

   Wednesday -> Option A

4. Quick Check:

   day=3 matches 3 => "Wednesday" [OK]

Hint: Match the variable value to the case pattern [OK]

Common Mistakes:

• Choosing default case instead of matching case
• Confusing case numbers with strings
• Ignoring the switch expression syntax

4. Identify the error in this switch expression code snippet:

string result = value switch
{
  1 => "One",
  2 => "Two",
  3 => "Three",
  default => "Other"
};

medium

A. Using 'default' keyword instead of '_' for default case.

B. Switch expression must use parentheses, not braces.

C. Missing comma after case 3.

D. The variable 'value' is not declared.

Solution

1. Step 1: Check syntax for default case in switch expressions

   Modern C# switch expressions use _ as the default case, not the keyword default.

2. Step 2: Identify the incorrect use of 'default'

   The code uses default => "Other", which is invalid syntax in switch expressions.

3. Final Answer:

   Using 'default' keyword instead of '_' for default case. -> Option A

4. Quick Check:

   Default case = '_' not 'default' [OK]

Hint: Use '_' for default, not 'default' keyword [OK]

Common Mistakes:

• Using 'default' instead of '_'
• Forgetting commas between cases
• Confusing switch statement syntax with switch expression

5. You want to write a switch expression that returns "Small" for numbers less than 10, "Medium" for numbers between 10 and 20 inclusive, and "Large" for numbers greater than 20. Which of the following switch expressions correctly implements this?

hard

A. var size = number switch { < 10 => "Small", 10 <= number <= 20 => "Medium", _ => "Large" };

B. var size = number switch { < 10 => "Small", > 10 => "Medium", _ => "Large" };

C. var size = number switch { < 10 => "Small", > 10 and < 20 => "Medium", _ => "Large" };

D. var size = number switch { < 10 => "Small", >= 10 and <= 20 => "Medium", _ => "Large" };

Solution

1. Step 1: Understand pattern matching syntax in switch expressions

   Modern C# supports relational patterns like < 10 and logical patterns like and to combine conditions.

2. Step 2: Check each option for correct range coverage

   var size = number switch { < 10 => "Small", >= 10 and <= 20 => "Medium", _ => "Large" }; uses < 10 for "Small", and >= 10 and <= 20 for "Medium", which correctly covers 10 to 20 inclusive. The default _ covers "Large".

3. Final Answer:

   var size = number switch { < 10 => "Small", >= 10 and <= 20 => "Medium", _ => "Large" }; -> Option D

4. Quick Check:

   Use relational and logical patterns for ranges [OK]

Hint: Use 'and' to combine conditions in switch patterns [OK]

Common Mistakes:

• Using invalid syntax like '10 <= number <= 20'
• Missing inclusive range boundaries
• Using overlapping or incorrect ranges