Default Capacity of List

Actually, it starts with a Capacity of 0. When you add the first element, the current implementation allocates a capacity of 4. After that, the capacity keeps doubling if expansion is needed, to guarantee amortized O(1) operation.

Keep in mind that this is the current behavior. You shouldn’t rely on it to be the case. This should demonstrate the current behavior:

List<int> list = new List<int>();
int capacity = list.Capacity;
Console.WriteLine("Capacity: " + capacity);

for (int i = 0; i < 100000; i++)
{
    list.Add(i);
    if (list.Capacity > capacity)
    {
        capacity = list.Capacity;
        Console.WriteLine("Capacity: " + capacity);
    }
}