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 WindowLongParam
{
     /// <summary>Sets a new address for the window procedure.</summary>
     /// <remarks>You cannot change this attribute if the window does not belong to the same process as the calling thread.</remarks>
     GWL_WNDPROC = -4,

     /// <summary>Sets a new application instance handle.</summary>
     GWLP_HINSTANCE = -6,

     GWLP_HWNDPARENT = -8,

     /// <summary>Sets a new identifier of the child window.</summary>
     /// <remarks>The window cannot be a top-level window.</remarks>
     GWL_ID = -12,

     /// <summary>Sets a new window style.</summary>
     GWL_STYLE = -16,

     /// <summary>Sets a new extended window style.</summary>
     /// <remarks>See <see cref="ExWindowStyles"/>.</remarks>
     GWL_EXSTYLE = -20,

     /// <summary>Sets the user data associated with the window.</summary>
     /// <remarks>This data is intended for use by the application that created the window. Its value is initially zero.</remarks>
     GWL_USERDATA = -21,

     /// <summary>Sets the return value of a message processed in the dialog box procedure.</summary>
     /// <remarks>Only applies to dialog boxes.</remarks>
     DWLP_MSGRESULT = 0,

     /// <summary>Sets new extra information that is private to the application, such as handles or pointers.</summary>
     /// <remarks>Only applies to dialog boxes.</remarks>
     DWLP_USER = 8,

     /// <summary>Sets the new address of the dialog box procedure.</summary>
     /// <remarks>Only applies to dialog boxes.</remarks>
     DWLP_DLGPROC = 4
}

Notes:

None.