If Count already equals the capacity, the capacity of the Stack is increased by automatically reallocating the internal array, and the existing elements are copied to the new array before the new element is added.
a null reference (Nothing in Visual Basic) can be pushed onto the Stack as a placeholder, if needed. It occupies a slot in the stack and is treated like any object.
If Count is less than the capacity of the stack, Push is an O(1) operation. If the capacity needs to be increased to accommodate the new element, Push becomes an O(n) operation, where n is Count.