Constant patterns in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constant pattern in C#?

A constant pattern checks if a value matches a specific constant value. It is used in pattern matching to compare values directly.

beginner

How do you write a constant pattern in a C# switch expression?

You write the constant value directly before the =>, like 5 => "five", or "hello" => "hello",.

beginner

What happens if a constant pattern does not match the value?

The pattern matching moves on to the next pattern or the default case if no match is found.

beginner

Can constant patterns be used with is expressions in C#?

Yes, you can use constant patterns with is to check if a variable equals a constant, like if (x is 10).

beginner

Example: What will this code print?

int x = 3;
string result = x switch {
  1 => "One",
  2 => "Two",
  3 => "Three",
  _ => "Other"
};
Console.WriteLine(result);

It will print Three because the value 3 matches the constant pattern 3 in the switch expression.

What does a constant pattern do in C#?

A Checks if a value equals a specific constant

B Declares a constant variable

C Creates a new object

D Loops through a collection

A constant pattern compares a value to a fixed constant to check for equality.

Which keyword is used with constant patterns in C# to test a value?

A new

B is

C var

D for

The 'is' keyword is used with constant patterns to check if a value matches a constant.

In a switch statement, how do you write a constant pattern for the number 10?

A case x == 10:

B case const 10:

C case 10:

D case (10):

You write 'case 10:' to match the constant value 10.

What will happen if no constant pattern matches in a switch expression?

A The default case runs if provided

B The program crashes

C The first case runs anyway

D The switch is ignored

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

Can constant patterns be used with strings in C#?

A Only in loops

B No, only numbers

C Only with variables

D Yes, strings can be constant patterns

Constant patterns can match string values directly.

Explain what a constant pattern is and how it is used in C# pattern matching.

Think about how you check if a value equals a fixed number or text.

Describe how a switch expression uses constant patterns to decide which code to run.

Imagine choosing an option based on exact matches.

Practice

1. What does a constant pattern in C# do when used in a switch statement?

easy

A. It checks if the value is greater than a constant.

B. It checks if the value exactly matches a fixed constant.

C. It checks if the value is less than a constant.

D. It checks if the value is of a certain type.

Solution

1. Step 1: Understand constant pattern purpose

   Constant patterns compare a value to a fixed constant to see if they are equal.

2. Step 2: Apply to switch statement

   In a switch, constant patterns match cases where the value equals the constant exactly.

3. Final Answer:

   It checks if the value exactly matches a fixed constant. -> Option B

4. Quick Check:

   Constant pattern = exact value check [OK]

Hint: Constant pattern means exact value match check [OK]

Common Mistakes:

• Thinking it checks ranges instead of exact values
• Confusing with type patterns
• Assuming it checks inequality

2. Which of the following is the correct syntax for a constant pattern in an if statement in C#?

easy

A. if (is x 10) { }

B. if (x == is 10) { }

C. if (x is 10) { }

D. if (x = 10) { }

Solution

1. Step 1: Recall constant pattern syntax

   The syntax uses is followed by the constant value to check equality.

2. Step 2: Check each option

   if (x is 10) { } uses if (x is 10), which is correct. Others have syntax errors.

3. Final Answer:

   if (x is 10) { } -> Option C

4. Quick Check:

   Correct constant pattern syntax = if (x is 10) [OK]

Hint: Use 'is' keyword followed by constant value [OK]

Common Mistakes:

• Using assignment '=' instead of 'is'
• Placing 'is' incorrectly
• Adding extra operators

3. What will be the output of the following C# code?

int number = 5;
switch (number)
{
    case 3:
        Console.WriteLine("Three");
        break;
    case 5:
        Console.WriteLine("Five");
        break;
    default:
        Console.WriteLine("Other");
        break;
}

medium

A. Five

B. Three

C. Other

D. No output

Solution

1. Step 1: Identify the value of 'number'

   The variable 'number' is assigned the value 5.

2. Step 2: Match the switch cases

   The switch checks cases 3 and 5. Since number is 5, it matches case 5 and prints "Five".

3. Final Answer:

   Five -> Option A

4. Quick Check:

   number = 5 matches case 5 [OK]

Hint: Match value to case constant exactly [OK]

Common Mistakes:

• Choosing default instead of matching case
• Confusing case values
• Ignoring break statements

4. Identify the error in this C# code using constant patterns:

object obj = 10;
if (obj is 10)
{
    Console.WriteLine("Ten");
}

medium

A. No error, code is correct

B. Missing cast to int before pattern matching

C. Cannot use constant pattern with object type

D. Constant pattern requires a variable after 'is'

Solution

1. Step 1: Understand constant pattern with object

   Constant patterns can be used with object type to check if it equals a constant value.

2. Step 2: Check code correctness

   The code uses obj is 10 correctly to check if obj equals 10. This is valid syntax and logic.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   Constant pattern works with object type [OK]

Hint: Constant pattern works directly on object values [OK]

Common Mistakes:

• Assuming cast needed before pattern
• Thinking constant pattern needs variable
• Confusing with type patterns

5. Given a list of integers, which C# code correctly uses constant patterns to count how many times the number 7 appears?

var numbers = new List<int> {1, 7, 3, 7, 5};
int count = 0;
foreach (var n in numbers)
{
    // Fill in the blank
}
Console.WriteLine(count);

hard

A. if (n == 7) count = 7;

B. if (n is not 7) count++;

C. if (n is int) count++;

D. if (n is 7) count++;

Solution

1. Step 1: Understand the goal

   We want to count how many times 7 appears in the list.

2. Step 2: Use constant pattern to check for 7

   The correct constant pattern is if (n is 7) which checks if n equals 7, then increments count.

3. Final Answer:

   if (n is 7) count++; -> Option D

4. Quick Check:

   Constant pattern for 7 increments count [OK]

Hint: Use 'is' with constant to check exact value [OK]

Common Mistakes:

• Assigning count instead of incrementing
• Checking type instead of value
• Counting non-7 values