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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of list pattern matching with nested lists

What is the output of this C# code using list patterns?

C Sharp (C#)

using System;

class Program {
    static void Main() {
        var list = new[] {1, 2, 3, 4};
        string result = list switch {
            [1, 2, 3, 4] => "Exact match",
            [1, 2, ..] => "Starts with 1, 2",
            [.., 4] => "Ends with 4",
            _ => "No match"
        };
        Console.WriteLine(result);
    }
}

A Exact match

B Starts with 1, 2

C Ends with 4

D No match

💡 Hint

Look carefully at the order of patterns and the exact list content.

The list exactly matches the first pattern [1, 2, 3, 4], so the output is "Exact match". The switch stops at the first matching pattern.

❓ Predict Output

intermediate

Result of list pattern with slice and discard

What will be printed by this C# program?

C Sharp (C#)

using System;

class Program {
    static void Main() {
        int[] numbers = {5, 10, 15, 20, 25};
        string output = numbers switch {
            [5, .., 25] => "Starts with 5 and ends with 25",
            [5, ..] => "Starts with 5",
            _ => "No match"
        };
        Console.WriteLine(output);
    }
}

A No match

B Starts with 5

C Compilation error

D Starts with 5 and ends with 25

💡 Hint

Check the slice pattern and the first and last elements of the array.

The array starts with 5 and ends with 25, so it matches the first pattern [5, .., 25].

🔧 Debug

advanced

Identify the error in list pattern usage

This code uses a list pattern. Which option is correct about it?

C Sharp (C#)

int[] arr = {1, 2, 3};
string res = arr switch {
    [1, 2, 3] => "Match",
    [1, 2] => "Partial",
    _ => "No match"
};

A The pattern [1, 2] is invalid because it does not match the array length.

B The code is valid and compiles without errors.

C List patterns require the array to be a List<T>, not an array.

D The switch expression must be inside a method, not at top level.

💡 Hint

List patterns support arrays and top-level code/statements in C# 11+.

The code is valid and compiles without errors in C# 11+. List patterns work on arrays (not just List), top-level statements support switch expressions, and the [1,2] pattern is syntactically valid (though it won't match the length-3 array at runtime).

📝 Syntax

advanced

Which list pattern syntax is correct?

Which option shows a valid list pattern syntax in C# 12+?

A var result = arr switch { [1, 2, ..] rest => rest.Length, _ => 0 };

B var result = arr switch { [1, 2, ..rest] => rest.Length, _ => 0 };

C var result = arr switch { [1, 2, .. var rest] => rest.Length, _ => 0 };

D var result = arr switch { [1, 2, ..rest] => rest, _ => 0 };

💡 Hint

Look for correct use of slice pattern with variable capture.

Option C correctly uses the slice pattern with variable capture: [1, 2, .. var rest]. The 'var rest' is required syntax for capturing the slice. Option C misplaces 'rest'. Options C and D omit 'var', causing syntax error; D also has type mismatch (rest is array slice, not int).

🚀 Application

expert

Count how many lists match a pattern

Given a list of integer arrays, how many match the pattern [_, _, .. var tail] where tail starts with 5?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        int[][] lists = {
            new[] {1, 2, 5, 6},
            new[] {3, 4, 5},
            new[] {7, 8, 9},
            new[] {0, 0, 5, 1}
        };
        int count = lists.Count(arr => arr is [_, _, .. var tail] && tail.Length > 0 && tail[0] == 5);
        Console.WriteLine(count);
    }
}

A 3

B 2

C 1

D 4

💡 Hint

Check each array if it has at least two elements and the third element is 5.

The pattern [_, _, .. var tail] captures elements from index 2 in tail. With tail.Length > 0 && tail[0] == 5: - {1,2,5,6}: tail={5,6}, yes - {3,4,5}: tail={5}, yes - {7,8,9}: tail={9}, no - {0,0,5,1}: tail={5,1}, yes Three match, so count=3.