C# Enumeration:

public enum ComboBoxMessages
{

    CB_GETEDITSEL = 0x0140,
    CB_LIMITTEXT = 0x0141,
    CB_SETEDITSEL = 0x0142,
    CB_ADDSTRING = 0x0143,
    CB_DELETESTRING = 0x0144,
    CB_DIR = 0x0145,
    CB_GETCOUNT = 0x0146,
    CB_GETCURSEL = 0x0147,
    CB_GETLBTEXT = 0x0148,
    CB_GETLBTEXTLEN = 0x0149,
    CB_INSERTSTRING = 0x014A,
    CB_RESETCONTENT = 0x014B,
    CB_FINDSTRING = 0x014C,
    CB_SELECTSTRING = 0x014D,
    CB_SETCURSEL = 0x014E,
    CB_SHOWDROPDOWN = 0x014F,
    CB_GETITEMDATA = 0x0150,
    CB_SETITEMDATA = 0x0151,
    CB_GETDROPPEDCONTROLRECT = 0x0152,
    CB_SETITEMHEIGHT = 0x0153,
    CB_GETITEMHEIGHT = 0x0154,
    CB_SETEXTENDEDUI = 0x0155,
    CB_GETEXTENDEDUI = 0x0156,
    CB_GETDROPPEDSTATE = 0x0157,
    CB_FINDSTRINGEXACT = 0x0158,
    CB_SETLOCALE = 0x0159,
    CB_GETLOCALE = 0x015A,
    CB_GETTOPINDEX = 0x015B,
    CB_SETTOPINDEX = 0x015C,
    CB_GETHORIZONTALEXTENT = 0x015D,
    CB_SETHORIZONTALEXTENT = 0x015E,
    CB_GETDROPPEDWIDTH = 0x015F,
    CB_SETDROPPEDWIDTH = 0x0160,
    CB_INITSTORAGE = 0x0161
    /*  CB_MSGMAX    = 354*/
}

C# Constants:

public const int CB_GETEDITSEL        = 0x0140;
public const int CB_LIMITTEXT         = 0x0141;
public const int CB_SETEDITSEL        = 0x0142;
public const int CB_ADDSTRING         = 0x0143;
public const int CB_DELETESTRING      = 0x0144;
public const int CB_DIR           = 0x0145;
public const int CB_GETCOUNT          = 0x0146;
public const int CB_GETCURSEL         = 0x0147;
public const int CB_GETLBTEXT         = 0x0148;
public const int CB_GETLBTEXTLEN      = 0x0149;
public const int CB_INSERTSTRING      = 0x014A;
public const int CB_RESETCONTENT      = 0x014B;
public const int CB_FINDSTRING        = 0x014C;
public const int CB_SELECTSTRING      = 0x014D;
public const int CB_SETCURSEL         = 0x014E;
public const int CB_SHOWDROPDOWN      = 0x014F;
public const int CB_GETITEMDATA       = 0x0150;
public const int CB_SETITEMDATA       = 0x0151;
public const int CB_GETDROPPEDCONTROLRECT = 0x0152;
public const int CB_SETITEMHEIGHT     = 0x0153;
public const int CB_GETITEMHEIGHT     = 0x0154;
public const int CB_SETEXTENDEDUI     = 0x0155;
public const int CB_GETEXTENDEDUI     = 0x0156;
public const int CB_GETDROPPEDSTATE       = 0x0157;
public const int CB_FINDSTRINGEXACT       = 0x0158;
public const int CB_SETLOCALE         = 345;
public const int CB_GETLOCALE         = 346;
public const int CB_GETTOPINDEX       = 347;
public const int CB_SETTOPINDEX       = 348;
public const int CB_GETHORIZONTALEXTENT   = 349;
public const int CB_SETHORIZONTALEXTENT   = 350;
public const int CB_GETDROPPEDWIDTH       = 351;
public const int CB_SETDROPPEDWIDTH       = 352;
public const int CB_INITSTORAGE       = 353;
public const int CB_MSGMAX        = 354;;

VB Constants:

TODO

Notes:

The value of CB_MSGMAX may change depending on the version of windows