The container control sends a WM_CONTROL (in Container Controls) message with the CN_PICKUP notification code to its owner when a pickup and drop operation is initiated over a container.

param1
     USHORT        id            /*  Container control ID. */
     USHORT        CN_PICKUP     /*  Notification code. */

param2
     PCNRDRAGINIT  pCnrDragInit  /*  Pointer to the CNRDRAGINIT structure containing direct-manipulation information initiatedin a container. */

returns
     ULONG         ulReserved    /*  Reserved value, must be 0. */