C# Constants:

static readonly int GWL_WNDPROC    = -4;
static readonly int GWL_HINSTANCE  = -6;
static readonly int GWL_HWNDPARENT = -8;
static readonly int GWL_STYLE      = -16;
static readonly int GWL_EXSTYLE    = -20;
static readonly int GWL_USERDATA   = -21;
static readonly int GWL_ID     = -12;

VB Constants:

Public Const GWL_WNDPROC    = -4
Public Const GWL_HINSTANCE  = -6
Public Const GWL_HWNDPARENT = -8
Public Const GWL_STYLE      = -16
Public Const GWL_EXSTYLE    = -20
Public Const GWL_USERDATA   = -21
Public Const GWL_ID     = -12

C# Enum:

public enum GetWindowLongConst
{
     GWL_WNDPROC    = -4,
     GWL_HINSTANCE  = -6,
     GWL_HWNDPARENT = -8,
     GWL_STYLE      = -16,
     GWL_EXSTYLE    = -20,
     GWL_USERDATA   = -21,
     GWL_ID     = -12
}

Notes:

None.