Capacity is the number of elements that the ArrayList can store. Count is the number of elements that are actually in the ArrayList.
Capacity is always greater than or equal to Count. If Count exceeds Capacity while adding elements, the capacity is automatically increased by reallocating the internal array before copying the old elements and adding the new elements.
The capacity can be decreased by calling TrimToSize or by setting the Capacity property explicitly. When the value of Capacity is set explicitly, the internal array is also reallocated to accommodate the specified capacity.
Retrieving the value of this property is an O(1) operation; setting the property is an O(n) operation, where n is the new capacity.