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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of positional pattern matching with tuples

What is the output of this C# code using positional patterns with tuples?

C Sharp (C#)

var point = (X: 3, Y: 4);
string result = point switch
{
    (0, 0) => "Origin",
    (var x, 0) => $"X={x}, on X-axis",
    (0, var y) => $"Y={y}, on Y-axis",
    (var x, var y) => $"Point at ({x},{y})"
};
Console.WriteLine(result);

A X=3, on X-axis

B Y=4, on Y-axis

C Point at (3,4)

D Origin

💡 Hint

Think about which pattern matches the tuple (3,4) first.

The tuple (3,4) does not match (0,0), nor (var x, 0) because Y is 4, not 0, nor (0, var y) because X is 3, not 0. So it matches the last pattern (var x, var y) and outputs "Point at (3,4)".

❓ Predict Output

intermediate

Pattern matching with positional records

Given the record and code below, what is the output?

C Sharp (C#)

record Person(string FirstName, string LastName);

var person = new Person("Alice", "Smith");
string greeting = person switch
{
    ("Alice", "Smith") => "Hello Alice Smith!",
    ("Bob", _) => "Hello Bob!",
    _ => "Hello stranger!"
};
Console.WriteLine(greeting);

A Hello Alice Smith!

B Hello Bob!

C Hello stranger!

D Compilation error

💡 Hint

Check which positional pattern matches the Person record.

The person is Alice Smith, which matches the first pattern exactly, so the output is "Hello Alice Smith!".

🔧 Debug

advanced

Positional pattern matching with guards on records

What is the output of this code?

C Sharp (C#)

record Point(int X, int Y);

var p = new Point(1, 2);
string result = p switch
{
    (var x, var y) when x > y => "X is greater",
    (var x, var y) when y > x => "Y is greater",
    (var x, var y) when x == y => "Equal",
    _ => "Unknown"
};
Console.WriteLine(result);

A Output: Y is greater

B Compilation error: positional patterns not supported on records

C Runtime error: NullReferenceException

D Output: X is greater

💡 Hint

Records support positional patterns via generated Deconstruct methods. For (1,2), evaluate the when conditions in order.

Records automatically generate Deconstruct methods, so the code compiles and runs successfully. For Point(1,2), it deconstructs to x=1, y=2. The first guard (1 > 2) is false, the second (2 > 1) is true, so it outputs "Y is greater".

❓ Predict Output

advanced

Output of nested positional patterns

What is the output of this code with nested positional patterns?

C Sharp (C#)

record Rectangle(Point TopLeft, Point BottomRight);
record Point(int X, int Y);

var rect = new Rectangle(new Point(0, 0), new Point(5, 5));
string description = rect switch
{
    (var (x1, y1), var (x2, y2)) when x1 == 0 && y1 == 0 && x2 == 5 && y2 == 5 => "Standard 5x5 rectangle",
    _ => "Other rectangle"
};
Console.WriteLine(description);

A Runtime error

B Other rectangle

C Compilation error

D Standard 5x5 rectangle

💡 Hint

Look at how nested positional patterns match the record properties.

The Rectangle has two Points. The pattern matches the nested Points by decomposing them with (var (x1, y1), var (x2, y2)). The when clause checks the coordinates. Since they match exactly, the output is "Standard 5x5 rectangle".

❓ Predict Output

expert

Output of positional pattern with deconstruction and discard

What is the output of this code using positional patterns with discard (_) and deconstruction?

C Sharp (C#)

record Color(int R, int G, int B);

Color c = new(255, 0, 0);
string result = c switch
{
    (255, 0, 0) => "Red",
    (0, 255, 0) => "Green",
    (0, 0, 255) => "Blue",
    (var r, var g, var b) when r == g && g == b => "Gray",
    _ => "Unknown color"
};
Console.WriteLine(result);

A Gray

B Red

C Unknown color

D Green

💡 Hint

Check which positional pattern matches the color (255,0,0).

The color is (255,0,0), which matches the first pattern exactly, so the output is "Red".

Practice

1.

What is the main purpose of positional patterns in C#?

record Point(int X, int Y);
Using case Point(var x, var y): inside a switch statement helps to:

easy

A. Extract values of X and Y from a Point object easily

B. Create a new Point object with given X and Y

C. Change the values of X and Y in the Point object

D. Call a method named Deconstruct on the Point object

Solution

1. Step 1: Understand positional pattern usage

   Positional patterns allow matching an object and extracting its parts in one step, like case Point(var x, var y):.

2. Step 2: Identify what the pattern does with Point

   It extracts the X and Y values from the Point object into variables x and y for use inside the case block.

3. Final Answer:

   Extract values of X and Y from a Point object easily -> Option A

4. Quick Check:

   Positional pattern extracts values [OK]

Hint: Positional patterns extract parts of an object [OK]

Common Mistakes:

• Thinking positional patterns create or modify objects
• Confusing Deconstruct method call with pattern matching
• Assuming positional patterns only check types without extracting

2.

Which of the following is the correct syntax for using a positional pattern with a record Person(string Name, int Age) inside a switch expression?

switch(person) {
    case ???:
        // code
        break;
}

easy

A. Person(var name, var age)

B. Person[name, age]

C. Person{name, age}

D. Person(name: name, age: age)

Solution

1. Step 1: Recall positional pattern syntax

   Positional patterns use parentheses with variables: Type(var1, var2).

2. Step 2: Match syntax to options

   Person(var name, var age) uses parentheses and variables correctly: Person(var name, var age). Others use invalid syntax for positional patterns.

3. Final Answer:

   Person(var name, var age) -> Option A

4. Quick Check:

   Positional pattern syntax uses parentheses [OK]

Hint: Positional patterns always use parentheses, not brackets or braces [OK]

Common Mistakes:

• Using square brackets or curly braces instead of parentheses
• Trying to use named arguments syntax inside patterns
• Omitting var keyword or variable names

3.

What is the output of the following code?

record Point(int X, int Y);

var p = new Point(3, 4);
switch (p)
{
    case Point(0, 0):
        Console.WriteLine("Origin");
        break;
    case Point(var x, var y) when x == y:
        Console.WriteLine("On diagonal");
        break;
    case Point(var x, var y):
        Console.WriteLine($"Point at ({x}, {y})");
        break;
}

medium

A. Origin

B. Point at (3, 4)

C. On diagonal

D. No output

Solution

1. Step 1: Check first case Point(0, 0)

   p is (3,4), so it does not match (0,0).

2. Step 2: Check second case with condition x == y

   x=3, y=4, so x != y; condition fails.

3. Step 3: Third case matches any Point and prints coordinates

   It matches and prints "Point at (3, 4)".

4. Final Answer:

   Point at (3, 4) -> Option B

5. Quick Check:

   Pattern matches last case with output [OK]

Hint: Check cases top to bottom; first match runs [OK]

Common Mistakes:

• Assuming (3,4) matches (0,0)
• Ignoring the when condition in second case
• Thinking no case matches and no output

4.

Identify the error in this code snippet using positional patterns:

record Rectangle(int Width, int Height);

var rect = new Rectangle(5, 10);
switch(rect)
{
    case Rectangle(var w, var h):
        Console.WriteLine($"Width: {w}, Height: {h}");
        break;
    case Rectangle(w, h):
        Console.WriteLine("Invalid pattern");
        break;
}

medium

A. Switch expression cannot use records

B. First case should use named arguments instead of positional pattern

C. Second case uses variables without var keyword, causing a syntax error

D. No error, code runs fine

Solution

1. Step 1: Review positional pattern variable declaration

   Variables in positional patterns must be declared with var or a type.

2. Step 2: Identify error in second case

   Second case uses Rectangle(w, h) without var, which is invalid syntax.

3. Final Answer:

   Second case uses variables without var keyword, causing a syntax error -> Option C

4. Quick Check:

   Variables need var keyword in patterns [OK]

Hint: Always declare variables with var in positional patterns [OK]

Common Mistakes:

• Omitting var keyword for variables in patterns
• Thinking records can't be used in switch
• Confusing named arguments with pattern variables

5.

Given the record Person(string Name, int Age), write a switch expression using positional patterns that returns:

• "Child" if Age < 13
• "Teen" if Age is between 13 and 19 inclusive
• "Adult" otherwise

Which of the following switch expressions correctly implements this?

hard

A. person switch {
Person(_, var age) when age < 13 => "Child",
Person(_, var age) when age > 13 && age <= 19 => "Teen",
Person(_, _) => "Adult"
};

B. person switch {
Person(var name, var age) when age < 13 => "Child",
Person(var name, var age) when age > 13 && age < 19 => "Teen",
_ => "Adult"
};

C. person switch {
Person(var name, var age) when age <= 13 => "Child",
Person(var name, var age) when age >= 13 && age <= 19 => "Teen",
Person(var name, var age) => "Adult"
};

D. person switch {
Person(var name, var age) when age < 13 => "Child",
Person(var name, var age) when age >= 13 && age <= 19 => "Teen",
Person(var name, var age) => "Adult"
};

Solution

1. Step 1: Check age ranges and conditions

   Child: age < 13, Teen: 13 ≤ age ≤ 19, Adult: otherwise.

2. Step 2: Analyze each option's conditions

   person switch { Person(var name, var age) when age < 13 => "Child", Person(var name, var age) when age >= 13 && age <= 19 => "Teen", Person(var name, var age) => "Adult" }; correctly uses age < 13 for Child, age >= 13 && age <= 19 for Teen, and default Adult case. person switch { Person(_, var age) when age < 13 => "Child", Person(_, var age) when age > 13 && age <= 19 => "Teen", Person(_, _) => "Adult" }; misses age=13 (falls to Adult) because Teen condition uses > 13. person switch { Person(var name, var age) when age < 13 => "Child", Person(var name, var age) when age > 13 && age < 19 => "Teen", _ => "Adult" }; excludes age == 13 and 19 for Teen. person switch { Person(var name, var age) when age <= 13 => "Child", Person(var name, var age) when age >= 13 && age <= 19 => "Teen", Person(var name, var age) => "Adult" }; overlaps Child and Teen at age 13.

3. Step 3: Confirm the correct implementation

   person switch { Person(var name, var age) when age < 13 => "Child", Person(var name, var age) when age >= 13 && age <= 19 => "Teen", Person(var name, var age) => "Adult" }; matches all conditions correctly and clearly.

4. Final Answer:

   Option D code correctly implements the age groups -> Option D

5. Quick Check:

   Check age ranges carefully in when clauses [OK]

Hint: Check boundary conditions carefully in when clauses [OK]

Common Mistakes:

• Overlapping or missing age boundaries
• Using _ wildcard instead of variables when needed
• Incorrect logical operators in conditions