An application sends a ZGM_ALLOCATEROWS message to allocate memory for the specified number of rows. This only needs to be done if you expect the grid to have more than 2000 rows. This message should be sent immediately after the grid is dimensioned with ZGM_DIMGRID.
ZGM_ALLOCATEROWS wParam = (WPARAM)(int) iRows; //number of rows to allocate lParam = 0; //not used; set to zero
This message does not return a value.