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".