Constant patterns in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of constant pattern matching with int

What is the output of this C# code using constant pattern matching?

C Sharp (C#)

int number = 5;
string result = number switch
{
    1 => "One",
    5 => "Five",
    _ => "Other"
};
Console.WriteLine(result);

A One

B Compilation error

C Other

D Five

💡 Hint

Look at which constant matches the variable 'number'.

The switch expression matches the constant 5, so it returns "Five".

❓ Predict Output

intermediate

Constant pattern with string and null

What will this C# code print?

C Sharp (C#)

string? text = null;
string output = text switch
{
    null => "Null value",
    "hello" => "Greeting",
    _ => "Unknown"
};
Console.WriteLine(output);

A Greeting

B Null value

C Unknown

D Runtime exception

💡 Hint

Check how the null constant pattern works.

The variable 'text' is null, so the null pattern matches and returns "Null value".

❓ Predict Output

advanced

Pattern matching with multiple constant patterns

What is the output of this code snippet?

C Sharp (C#)

char letter = 'a';
string category = letter switch
{
    'a' or 'e' or 'i' or 'o' or 'u' => "Vowel",
    'y' => "Sometimes vowel",
    _ => "Consonant"
};
Console.WriteLine(category);

A Consonant

B Sometimes vowel

C Vowel

D Compilation error

💡 Hint

Check which constant pattern matches the letter 'a'.

The letter 'a' matches the first pattern with multiple constants, so it prints "Vowel".

❓ Predict Output

advanced

Constant pattern with nullable value types

What will this code print?

C Sharp (C#)

int? number = 0;
string result = number switch
{
    0 => "Zero",
    null => "No value",
    _ => "Other"
};
Console.WriteLine(result);

A Zero

B No value

C Other

D Runtime exception

💡 Hint

Remember how nullable types match constant patterns.

The nullable int has value 0, so it matches the constant 0 pattern and prints "Zero".

❓ Predict Output

expert

Constant pattern with enums and default case

Given this enum and code, what is the output?

C Sharp (C#)

enum Status { Ready, Running, Stopped }

Status current = Status.Running;
string message = current switch
{
    Status.Ready => "Ready to start",
    Status.Stopped => "Stopped",
    _ => "In progress"
};
Console.WriteLine(message);

A In progress

B Stopped

C Compilation error

D Ready to start

💡 Hint

Check which enum value matches the constant patterns.

The current status is Running, which matches the default case, so it prints "In progress".

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