List<T> — the growing array

Arrays that grow

var names = new List<string>();     // empty
names.Add("an");
names.Add("binh");
Console.WriteLine(names.Count);     // 2 — actual elements
names[0] = "An";                    // index like an array
names.Remove("binh");               // removes first equal element
Console.WriteLine(names.Contains("An"));   // True

List<T> wraps an array it reallocates behind your back: Add is amortized O(1), Count is the real element count. Capacity is the internal array's size — you almost never need to touch it. Count, not Length, is the beginner tell that a list isn't an array.

Removal, insertion, searching

names.Insert(0, "cuong");       // O(n) — everything shifts
names.RemoveAt(names.Count - 1);   // remove last
int at = names.IndexOf("An");   // -1 when absent, like arrays
names.Sort();                   // in-place, same contract as Array.Sort

Remove and RemoveAt shift everything after the removed slot — cheap at the end, expensive at the front. If you find yourself doing front-heavy insertion, the list may be the wrong shape (a deque/stack model fits better).

Iterating safely

foreach over a list while Add/Remove-ing throws InvalidOperationException — the list is being modified during enumeration. Collect the changes and apply after the loop, or use a for loop backwards when removing by index.