C# Arrays and Lists

Three variables named score1, score2, and score3 can hold three scores, but the structure breaks when a fourth arrives and forces duplicated code for every operation. A collection stores related values under one name and lets a loop apply the same operation to each element.

Arrays and generic lists are strongly typed: an int[] holds integers and a List<string> holds strings. That restriction catches accidental mixtures before runtime and makes every retrieved item useful without a cast.

An array's length is chosen when it is created. The elements can be replaced, but the array cannot gain another slot. Indexes start at zero, so an array of length three has indexes 0, 1, and 2. Reading index 3 throws IndexOutOfRangeException.

Arrays fit fixed-shape data, low-level APIs, and cases where an exact size is known. They are also the foundation beneath many collection concepts. Use Length in loop boundaries so the code adapts when the array changes.

using System;

class Program
{
  static void Main()
  {
      string[] weekdays = { "Mon", "Tue", "Wed" };
      weekdays[1] = "Tuesday";

      for (int index = 0; index < weekdays.Length; index++)
      {
          Console.WriteLine($"{index}: {weekdays[index]}");
      }
  }
}

0: Mon
1: Tuesday
2: Wed

List<T> is a generic type; T stands for the element type supplied between angle brackets. Add appends an item, Insert places one at an index, Remove removes the first equal item, RemoveAt removes by index, and Count reports the number of elements currently present.

Removing by value and removing by position express different intentions. Remove returns false when the value was not found. RemoveAt throws when the index is invalid. Validate externally supplied indexes rather than letting a user select an arbitrary position unchecked.

List<string> tasks = new List<string>();
tasks.Add("Reply to email");
tasks.Add("Review invoice");
tasks.Insert(1, "Book meeting room");

tasks.Remove("Reply to email");

foreach (string task in tasks)
{
  Console.WriteLine(task);
}

Book meeting room
Review invoice

Use an array when size is fixed or an API specifically expects one. Use List<T> for most ordered application data that changes over time. Neither is ideal for every lookup: repeatedly scanning thousands of items by key may call for a dictionary, which comes next.

Both arrays and lists are reference types. Assigning one variable to another does not clone the elements; both variables reach the same collection. To make a shallow list copy, construct a new List<T> from the existing sequence. If elements are mutable objects, both collections can still reference the same element objects.