List patterns in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a list pattern in C#?

A list pattern in C# is a way to match elements inside a list or array by their order and values. It lets you check if a list has specific elements in a certain sequence.

beginner

How do you match the first two elements of a list using list patterns?

You write a pattern like [var first, var second, ..]. This matches a list where the first element is stored in first, the second in second, and .. means any remaining elements.

beginner

What does the .. symbol mean in a list pattern?

The .. symbol means 'match the rest of the list'. It allows the pattern to match lists longer than the elements explicitly listed.

intermediate

Can list patterns be used to check for an exact list length?

Yes. If you write a pattern without .., like [1, 2, 3], it matches only lists with exactly three elements: 1, 2, and 3 in order.

intermediate

Give an example of using list patterns in a switch expression.

Example:

return list switch {
  [1, 2, 3] => "Exact match",
  [1, 2, ..] => "Starts with 1, 2",
  _ => "Other"
};

This checks the list and returns a string based on the pattern matched.

What does the list pattern [var x, var y, ..] match?

A A list with exactly three elements

B A list with exactly two elements

C A list with no elements

D A list with at least two elements, capturing the first two

The pattern matches lists with two or more elements, capturing the first two into variables x and y, and the rest is ignored by '..'.

Which list pattern matches only a list with exactly three elements 1, 2, and 3?

A [1, 2, 3]

B [1, 2, ..]

C [.., 1, 2, 3]

D [1, .., 3]

The pattern [1, 2, 3] matches only lists with exactly those three elements in order.

In C# list patterns, what does the underscore (_) pattern mean?

A Match a list with one element

B Match an empty list only

C Match any list

D Match a list with exactly two elements

The underscore (_) is a discard pattern that matches anything, including any list.

Which of these is a valid list pattern in C#?

A {var a, var b, var c}

B [var a, var b, var c]

C (var a, var b, var c)

D <var a, var b, var c>

List patterns use square brackets [] to match elements in order.

What happens if you use [1, 2, ..] to match the list [1, 2]?

A It matches successfully

B It does not match

C It throws an error

D It matches only if the list has more than two elements

The pattern matches lists with two or more elements starting with 1 and 2. The list [1, 2] matches because '..' can match zero or more elements.

Explain how list patterns work in C# and how you can use them to check list contents.

Think about how you check items in a list one by one.

Describe a scenario where list patterns can simplify your code compared to traditional if-else checks.

Imagine you want to do different things based on the first few items in a list.

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