Default Capacity of List

Question

What is the default capacity of a List?

Answer 1

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);
    }
}

Answer 2

Why don't you just try it?

Console.WriteLine("Default capacity of a List: " + new List<int>().Capacity);

This answer will work on all versions of .NET that have List. On my version, it happens to be 0.