CloseSocket is a virtual procedure used to close the socket connection, and to reset the handle for the socket descriptor. 

When HandleAllocated indicates the socket handle is valid, CloseSocket will reset both the Handle and the HandleAllocated flag by calling Disconnect on the global protocol stack. CloseSocket also sets Handle to Id_INVALID_SOCKET to prevent subsequent access using the now invalid socket descriptor. Access to the socket handle and the allocated flag in the method is a thread-safe operation; they are protected using an internal TIdCriticalSection instance. 

TIdSocketHandle.HandleAllocated, TIdSocketHandle.Handle, GStack, TIdCriticalSection, Id_INVALID_SOCKET