/*
 * Combo Box styles
 */ 
#define CBS_SIMPLE            0x0001L
#define CBS_DROPDOWN          0x0002L
#define CBS_DROPDOWNLIST      0x0003L
#define CBS_OWNERDRAWFIXED    0x0010L
#define CBS_OWNERDRAWVARIABLE 0x0020L
#define CBS_AUTOHSCROLL       0x0040L
#define CBS_OEMCONVERT        0x0080L
#define CBS_SORT              0x0100L
#define CBS_HASSTRINGS        0x0200L
#define CBS_NOINTEGRALHEIGHT  0x0400L
#define CBS_DISABLENOSCROLL   0x0800L

 /*
 * Combo Box Notification Codes
 */ 
#define CBN_ERRSPACE        (-1)
#define CBN_SELCHANGE       1
#define CBN_DBLCLK          2
#define CBN_SETFOCUS        3
#define CBN_KILLFOCUS       4
#define CBN_EDITCHANGE      5
#define CBN_EDITUPDATE      6
#define CBN_DROPDOWN        7
#define CBN_CLOSEUP         8
#define CBN_SELENDOK        9
#define CBN_SELENDCANCEL    10

[DllImport("user32.dll")] 
static extern int SendMessage(int hWnd,uint msg, int wParam, int lParam); 
....
SendMessage(MyGridCombo.hWnd, 0x0003, 0, 0);