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.

Practice

1. What does the list pattern [] match in C#?

easy

A. An empty list with no elements

B. A list with exactly one element

C. A list with two or more elements

D. Any list regardless of elements

Solution

1. Step 1: Understand the empty list pattern

   The pattern [] is used to match lists that have no elements at all.

2. Step 2: Compare with other list patterns

   Patterns like [var x] match one element, and [var x, .. var rest] match lists with multiple elements, so they don't match empty lists.

3. Final Answer:

   An empty list with no elements -> Option A

4. Quick Check:

   Empty list = [] pattern [OK]

Hint: Empty list pattern is just empty brackets [] [OK]

Common Mistakes:

• Confusing [] with single element pattern
• Thinking [] matches any list
• Assuming [] matches lists with elements

2. Which of the following is the correct syntax to match a list with exactly one element using list patterns in C#?

easy

A. [var x, .. var rest]

B. [var x]

C. [.. var rest]

D. []

Solution

1. Step 1: Identify the pattern for one element

   The pattern [var x] matches a list with exactly one element, binding it to x.

2. Step 2: Understand other patterns

   [var x, .. var rest] matches lists with at least one element plus the rest, [.. var rest] matches any list, and [] matches empty lists.

3. Final Answer:

   [var x] -> Option B

4. Quick Check:

   One element list = [var x] [OK]

Hint: One element list uses [var x] syntax [OK]

Common Mistakes:

• Using [var x, .. var rest] for single element
• Using [] for single element list
• Confusing .. var rest with single element

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

var list = new List<int> { 5, 10, 15 };
switch (list)
{
    case []:
        Console.WriteLine("Empty");
        break;
    case [var x]:
        Console.WriteLine($"One element: {x}");
        break;
    case [var x, .. var rest]:
        Console.WriteLine($"First: {x}, Rest count: {rest.Count}");
        break;
}

medium

A. Empty

B. One element: 5

C. No match

D. First: 5, Rest count: 2

Solution

1. Step 1: Analyze the list contents

   The list has three elements: 5, 10, and 15.

2. Step 2: Match the list pattern cases

   The first case matches empty list, which is false. The second case matches exactly one element, which is false. The third case matches a list with at least one element: [var x, .. var rest]. Here, x is 5, and rest contains 10 and 15, so count is 2.

3. Final Answer:

   First: 5, Rest count: 2 -> Option D

4. Quick Check:

   List with 3 elements matches [x, ..rest] pattern [OK]

Hint: More than one element matches [x, ..rest] pattern [OK]

Common Mistakes:

• Choosing empty or one element case incorrectly
• Miscounting rest elements
• Ignoring pattern order in switch

4. Identify the error in this C# code using list patterns:

var numbers = new List<int> { 1, 2, 3 };
if (numbers is [var first, var second])
{
    Console.WriteLine($"First: {first}, Second: {second}");
}

medium

A. The variable names first and second are invalid

B. List patterns cannot be used with List<int>

C. The pattern [var first, var second] requires exactly two elements, but list has three

D. Missing else block after if

Solution

1. Step 1: Understand the pattern [var first, var second]

   This pattern matches lists with exactly two elements, binding them to first and second.

2. Step 2: Check the list size

   The list has three elements: 1, 2, and 3, so it does not match the pattern requiring exactly two elements.

3. Final Answer:

   The pattern [var first, var second] requires exactly two elements, but list has three -> Option C

4. Quick Check:

   Pattern length must match list length exactly [OK]

Hint: Pattern length must match list length exactly [OK]

Common Mistakes:

• Assuming pattern matches partial list
• Thinking variable names cause error
• Ignoring list size in pattern matching

5. Given a list of integers, how can you use list patterns in C# to check if the list starts with 1 and has at least two elements? Choose the correct pattern.

hard

A. [1, var second, .. var rest]

B. [var first, 1, .. var rest]

C. [1, .. var rest]

D. [.. var rest, 1]

Solution

1. Step 1: Understand the requirement

   The list must start with 1, then have a second element, and then zero or more elements after that (at least two elements total).

2. Step 2: Analyze each pattern

   [1, var second, .. var rest]: [1, var second, .. var rest] matches a list starting with 1, then a second element, then the rest (zero or more). This fits the requirement.
   [var first, 1, .. var rest]: [var first, 1, .. var rest] matches lists where the second element is 1, not the first.
   [1, .. var rest]: [1, .. var rest] matches lists starting with 1 but allows zero or more elements after, so it matches lists with only one element too.
   [.. var rest, 1]: [.. var rest, 1] matches lists ending with 1, not starting.

3. Final Answer:

   [1, var second, .. var rest] -> Option A

4. Quick Check:

   Start with 1 and at least two elements = [1, var second, .. rest] [OK]

Hint: Start with 1 and two elements uses [1, var second, .. rest] [OK]

Common Mistakes:

• Confusing element positions in pattern
• Using patterns that allow only one element
• Matching end of list instead of start