Recursive pattern matching in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of recursive pattern matching with nested tuples

What is the output of this C# code using recursive pattern matching on nested tuples?

C Sharp (C#)

using System;

class Program {
    static string Describe((int, (int, int)) data) => data switch {
        (0, (0, 0)) => "All zeros",
        (var x, (var y, var z)) when x == y && y == z => "All equal",
        (var x, (_, var z)) when x > z => "First greater than last",
        _ => "Other"
    };

    static void Main() {
        Console.WriteLine(Describe((0, (0, 0))));
        Console.WriteLine(Describe((3, (3, 3))));
        Console.WriteLine(Describe((5, (1, 2))));
        Console.WriteLine(Describe((1, (2, 3))));
    }
}

A

All zeros
All equal
Other
First greater than last

B

All zeros
All equal
First greater than last
Other

C

Other
All equal
First greater than last
Other

D

All zeros
Other
First greater than last
Other

💡 Hint

Look carefully at the pattern conditions and the order they are checked.

The method Describe uses recursive pattern matching on a tuple containing an int and another tuple of two ints.

1. (0, (0, 0)) matches the first pattern exactly, so prints "All zeros".

2. (3, (3, 3)) matches the second pattern where all three values are equal, so prints "All equal".

3. (5, (1, 2)) matches the third pattern because 5 > 2, so prints "First greater than last".

4. (1, (2, 3)) matches none of the first three patterns, so prints "Other".

🧠 Conceptual

intermediate

Understanding recursive pattern matching with lists

Given this C# recursive pattern matching on a list, what is the value of result after execution?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static int SumList(List<int> list) => list switch {
        [] => 0,
        [var head, .. var tail] => head + SumList(tail)
    };

    static void Main() {
        var numbers = new List<int> {1, 2, 3};
        var result = SumList(numbers);
        Console.WriteLine(result);
    }
}

A 3

B 0

C 6

D Compilation error

💡 Hint

Think about how the list is split into head and tail recursively.

The method SumList uses recursive pattern matching on a list.

It matches an empty list [] returning 0.

Otherwise, it matches a list with a head and tail, sums the head with the recursive sum of the tail.

For the list [1, 2, 3], the sum is 1 + 2 + 3 = 6.

🔧 Debug

advanced

Identify the runtime error in recursive pattern matching

What runtime error will this C# code produce when executed?

C Sharp (C#)

using System;

class Program {
    static int Factorial(int n) => n switch {
        0 => 1,
        > 0 => n * Factorial(n - 1),
        _ => throw new ArgumentException("Negative input")
    };

    static void Main() {
        Console.WriteLine(Factorial(-1));
    }
}

A System.ArgumentException: Negative input

B StackOverflowException

C No output, program runs fine

D System.InvalidOperationException

💡 Hint

Check which pattern matches negative input and what it does.

The method Factorial uses pattern matching on n.

For n == 0, returns 1.

For n > 0, recursively calculates factorial.

For any other value (negative), it throws ArgumentException with message "Negative input".

Calling Factorial(-1) triggers this exception.

📝 Syntax

advanced

Which option causes a syntax error in recursive pattern matching?

Which of the following C# code snippets will cause a syntax error due to incorrect recursive pattern matching syntax?

A int Sum((int head, int tail) data) => data switch { var (h, t) => h + t };

B int Sum((int head, int tail) data) => data switch { (var h, var t) => h + t };

C int Sum((int head, (int, int) tail) data) => data switch { (var h, (var t1, var t2)) => h + t1 + t2 };

D int Sum((int head, int tail) data) => data switch { (var h, var t) => h + t, };

💡 Hint

Check the pattern syntax for tuples inside switch expressions.

Option A uses var (h, t) which is invalid syntax in C# pattern matching.

Correct tuple patterns use parentheses with var for each element, like (var h, var t).

Options B, C, and D use valid tuple patterns.

🚀 Application

expert

Calculate depth of nested tuples using recursive pattern matching

What is the output of this C# program that calculates the depth of nested tuples using recursive pattern matching?

C Sharp (C#)

using System;

class Program {
    static int Depth(object obj) => obj switch {
        (var l, var r) => 1 + Math.Max(Depth(l), Depth(r)),
        _ => 0
    };

    static void Main() {
        var nested = ((1, (2, 3)), (4, 5));
        Console.WriteLine(Depth(nested));
    }
}

A Compilation error

B 2

C 4

D 3

💡 Hint

Count how many levels of nested tuples are inside the variable.

The method Depth uses recursive positional pattern matching on 2-tuples.

If yes, it adds 1 plus the maximum depth of its two elements.

The nested tuple ((1, (2, 3)), (4, 5)) has depth 3:

• Level 1: outer tuple
• Level 2: first item is a tuple (1, (2, 3))
• Level 3: inside that, second item is (2, 3)

The right branch (4, 5) reaches depth 2, so max is 3.