Arrays & Lists in C#

In real-world applications, you rarely work with single variables. You work with collections of data—lists of users, arrays of prices, or grids of pixels. C# provides powerful tools to manage these collections efficiently.

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1. Arrays: Fixed-Size Collections

An array is a contiguous block of memory that holds a fixed number of elements of the same type.

Types of Arrays

1. Single-Dimensional: A simple row of data.

   int[] numbers = new int[5] { 1, 2, 3, 4, 5 };
   

2. Multi-Dimensional (Rectangular): A grid with fixed rows and columns.

   int[,] matrix = new int[2, 3] { { 1, 2, 3 }, { 4, 5, 6 } };
   Console.WriteLine(matrix[0, 1]); // Output: 2
   

3. Jagged Arrays: An "array of arrays" where each sub-array can have a different length.

   int[][] jagged = new int[3][];
   jagged[0] = new int[] { 1, 2 };
   jagged[1] = new int[] { 3, 4, 5, 6 };
   jagged[2] = new int[] { 7 };
   

Practical Example: Matrix Operations

Multi-dimensional arrays are perfect for mathematical matrix operations.

1. Matrix Addition

To add two matrices, they must have the same dimensions. You simply add the corresponding elements.

int[,] matrixA = { { 1, 2 }, { 3, 4 } };
int[,] matrixB = { { 5, 6 }, { 7, 8 } };
int[,] result = new int[2, 2];

for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 2; j++) {
        result[i, j] = matrixA[i, j] + matrixB[i, j];
    }
}
// Result: { { 6, 8 }, { 10, 12 } }

2. Matrix Multiplication

Multiplication is more complex: the value at result[i, j] is the sum of the products of the i-th row of A and the j-th column of B.

int[,] A = { { 1, 2 }, { 3, 4 } };
int[,] B = { { 5, 6 }, { 7, 8 } };
int[,] C = new int[2, 2];

for (int i = 0; i < 2; i++) { // Rows of A
    for (int j = 0; j < 2; j++) { // Columns of B
        C[i, j] = 0;
        for (int k = 0; k < 2; k++) { // Shared dimension
            C[i, j] += A[i, k] * B[k, j];
        }
    }
}
// Result: { { 19, 22 }, { 43, 50 } }

Important Array Class Methods

The System.Array class provides static methods to manipulate arrays.

• Array.Sort(): Sorts elements in ascending order.
• Array.Reverse(): Reverses the order of elements.
• Array.IndexOf(): Returns the index of the first occurrence of a value.
• Array.Clear(): Sets a range of elements to their default value (e.g., 0 for int).
• Array.Copy(): Copies a range of elements from one array to another.

int[] scores = { 45, 12, 98, 33, 75 };

Array.Sort(scores);     // { 12, 33, 45, 75, 98 }
Array.Reverse(scores);  // { 98, 75, 45, 33, 12 }

int index = Array.IndexOf(scores, 45); // 2

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2. List: Dynamic Collections

List<T> is part of System.Collections.Generic. Unlike arrays, lists can grow and shrink dynamically as you add or remove items.

Common List Operations

List<string> tasks = new List<string> { "Coding", "Testing" };

tasks.Add("Deploying");             // Add to end
tasks.Insert(1, "Reviewing");       // Insert at specific index
tasks.AddRange(new[] { "Doc", "Fix" }); // Add multiple

tasks.Remove("Testing");            // Remove by value
tasks.RemoveAt(0);                  // Remove by index

bool hasFix = tasks.Contains("Fix"); // Check existence
int count = tasks.Count;             // Get current size

When to use List vs Array?

• Use Array: When the size is fixed and known at compile time (e.g., days of the week) or for high-performance memory-critical applications.
• Use List: In 95% of standard application development where data size changes at runtime.

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3. LINQ & Extension Methods

LINQ (Language Integrated Query) allows you to perform powerful queries on arrays and lists using extension methods found in the System.Linq namespace.

Common LINQ Methods

Practical Examples

using System.Linq;

int[] numbers = { 5, 10, 15, 20, 25, 30 };

// 1. Filter: Numbers greater than 15
var largeNumbers = numbers.Where(n => n > 15).ToArray();

// 2. Transform: Convert numbers to strings with a prefix
var labels = numbers.Select(n => $"Value: {n}").ToList();

// 3. Aggregation
int total = numbers.Sum();
double avg = numbers.Average();

// 4. Quick Checks
bool hasEvens = numbers.Any(n => n % 2 == 0); // true
bool allPositive = numbers.All(n => n > 0);   // true

// 5. Finding specific items
int firstLarge = numbers.First(n => n > 20); // 25

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4. Iterating Collections

The foreach Loop

The cleanest way to read every item. It works for both arrays and lists.

foreach (var number in numbers) 
{
    Console.WriteLine(number);
}

The for Loop

Use this if you need the index of the item during iteration.

for (int i = 0; i < numbers.Length; i++) 
{
    Console.WriteLine($"Index {i} has value {numbers[i]}");
}