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

Recall & Review

beginner

What is a relational pattern in C#?

A relational pattern in C# is a way to compare a value against a condition using relational operators like <, <=, >, >= inside a pattern matching expression.

beginner

Which relational operators can be used in C# relational patterns?

You can use <, <=, >, and >= operators in relational patterns to check if a value is less than, less than or equal to, greater than, or greater than or equal to another value.

intermediate

How do relational patterns improve code readability?

Relational patterns let you write conditions directly inside switch or if statements in a clear and concise way, avoiding extra variables or complex if-else chains.

beginner

Example: What does this C# code do?

case >= 90: Console.WriteLine("Excellent"); break;

This case matches when the value is greater than or equal to 90 and prints "Excellent". It uses a relational pattern to check the value directly.

intermediate

Can relational patterns be combined with other patterns in C#?

Yes, relational patterns can be combined with logical patterns like 'and', 'or', and 'not' to create complex matching conditions.

Which of these is a valid relational pattern in C#?

A case > 10:

B case == 10:

C case != 10:

D case === 10:

Relational patterns use operators like >, <, >=, <=. Equality (==) is a constant pattern, not a relational pattern. '===' is not valid in C#.

What will this code print if value = 75?

switch(value) { case >= 90: Console.WriteLine("A"); break; case >= 70: Console.WriteLine("B"); break; default: Console.WriteLine("C"); }

A A

B B

C C

D No output

75 is not >= 90, so first case fails. 75 is >= 70, so it prints "B".

Can you use relational patterns inside an if statement in C#?

A Yes, but only with variables.

B No, only in switch statements.

C Yes, using the 'is' keyword.

D No, relational patterns are not supported.

Relational patterns can be used with 'is' in if statements, like 'if (x is > 10)'.

Which C# version introduced relational patterns?

A C# 7.0

B C# 8.0

C C# 10.0

D C# 9.0

Relational patterns were introduced in C# 9.0.

What does this pattern mean? 'case < 0:'

A Matches values less than zero.

B Matches values equal to zero.

C Matches values greater than zero.

D Matches all values.

The pattern matches any value less than zero.

Explain what relational patterns are and how they are used in C#.

Think about how you compare numbers in conditions.

Describe how relational patterns can be combined with logical patterns in C#.

Consider how you join multiple conditions in if statements.

Practice

1. What does the relational pattern is > 10 check in C#?

easy

A. If a value is greater than 10

B. If a value is equal to 10

C. If a value is less than 10

D. If a value is not equal to 10

Solution

1. Step 1: Understand the relational pattern syntax

   The pattern is > 10 checks if a value is greater than 10.

2. Step 2: Compare with other options

   Options A, B, and D check less than, equality, or inequality, which are different from > 10.

3. Final Answer:

   If a value is greater than 10 -> Option A

4. Quick Check:

   Relational pattern is > 10 means greater than 10 [OK]

Hint: Relational patterns use normal math signs like >, <, == [OK]

Common Mistakes:

• Confusing > with >= or ==.
• Thinking it checks equality instead of comparison.
• Mixing up less than and greater than signs.

2. Which of the following is the correct syntax to check if an integer n is less than or equal to 5 using a relational pattern?

easy

A. if (n is <= 5) { }

B. if (n <= 5) { }

C. if (n is < = 5) { }

D. if (n is <=5) { }

Solution

1. Step 1: Identify correct relational pattern syntax

   The relational pattern syntax uses is followed by the operator and value without spaces inside the operator, like is <= 5.

2. Step 2: Check each option

   if (n is <= 5) { } uses correct syntax: if (n is <= 5). if (n <= 5) { } is normal comparison, not a relational pattern. if (n is < = 5) { } has a space between < and = which is invalid. if (n is <=5) { } misses a space after is, which is required.

3. Final Answer:

   if (n is <= 5) { } -> Option A

4. Quick Check:

   Relational pattern syntax: is <= value [OK]

Hint: Relational patterns need 'is' then operator with no spaces inside operator [OK]

Common Mistakes:

• Using normal comparison without 'is'.
• Adding spaces inside the operator like '< ='.
• Omitting space after 'is'.

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

int x = 7;
if (x is >= 5 and < 10)
    Console.WriteLine("In range");
else
    Console.WriteLine("Out of range");

medium

A. No output

B. Out of range

C. In range

D. Compilation error

Solution

1. Step 1: Evaluate the relational pattern condition

   The condition x is >= 5 and < 10 checks if x is between 5 and 9 inclusive. Since x = 7, it satisfies both parts.

2. Step 2: Determine which branch runs

   Because the condition is true, the Console.WriteLine("In range") runs, printing "In range".

3. Final Answer:

   In range -> Option C

4. Quick Check:

   7 is between 5 and 10, so output is "In range" [OK]

Hint: Check both parts of 'and' relational pattern carefully [OK]

Common Mistakes:

• Thinking 'and' means 'or'.
• Ignoring one part of the condition.
• Assuming syntax error due to 'and'.

4. Identify the error in this code snippet using relational patterns:

int score = 85;
switch (score)
{
    case is > 90:
        Console.WriteLine("Excellent");
        break;
    case is >= 80 and < 90:
        Console.WriteLine("Good");
        break;
    default:
        Console.WriteLine("Average");
        break;
}

medium

A. The 'and' keyword is not allowed in switch case patterns

B. Missing braces around case blocks

C. Relational patterns cannot be used in switch cases

D. The pattern 'is > 90' should be 'case > 90:' without 'is'

Solution

1. Step 1: Identify the syntax error in switch case patterns

   Relational patterns in switch statements use syntax like case > 90:, without the is keyword. The code incorrectly uses case is > 90: and case is >= 80 and < 90:.

2. Step 2: Evaluate the options

   The 'and' keyword is not allowed in switch case patterns is incorrect; and is allowed to combine patterns like case >= 80 and < 90:. The pattern 'is > 90' should be 'case > 90:' without 'is' correctly identifies the misuse of is. Relational patterns cannot be used in switch cases is false; relational patterns are supported in switch cases. Missing braces around case blocks is wrong; braces are not required around case contents.

3. Final Answer:

   The pattern 'is > 90' should be 'case > 90:' without 'is' -> Option D

4. Quick Check:

   Switch case relational pattern: case > value, no is [OK]

Hint: No 'is' keyword needed for relational patterns in switch cases [OK]

Common Mistakes:

• Adding 'is' before relational operators in switch cases.
• Thinking 'and' cannot combine patterns in switch cases.
• Confusing switch statements with 'is' patterns.

5. You want to write a method that returns "Child", "Teen", or "Adult" based on age using relational patterns. Which of these switch expressions correctly implements this?

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

hard

A. Replace 'and' with '&&' in the second pattern

B. Correct as is

C. Use nested if statements instead of switch

D. Add parentheses around '>= 13 and < 20'

Solution

1. Step 1: Analyze the switch expression with relational patterns

   The switch expression uses relational patterns correctly: < 13 for "Child", combined pattern >= 13 and < 20 for "Teen", and default for "Adult".

2. Step 2: Confirm syntax correctness

   The 'and' keyword is valid in switch expressions for combining patterns. No parentheses or '&&' are needed here.

3. Final Answer:

   Correct as is -> Option B

4. Quick Check:

   Switch expressions support 'and' for combined relational patterns [OK]

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

Common Mistakes:

• Replacing 'and' with '&&' which is invalid in patterns.
• Adding unnecessary parentheses.
• Thinking switch expressions don't support relational patterns.