C# Signature:

[DllImport("user32.dll")]
static extern bool EnableMenuItem(IntPtr hMenu, uint uIDEnableItem,
   uint uEnable);

VB.NET Signature:

<DllImport("user32.dll", CallingConvention:=CallingConvention.Cdecl)> _
Public Function EnableMenuItem(ByVal hMenu As IntPtr, ByVal wIDEnableItem As UInteger, ByVal wEnable As UInteger) As IntPtr
End Function;

User-Defined Types:

Constants used with Add/Check/EnableMenuItem:

    internal const UInt32 MF_INSERT       =0x00000000;
    internal const UInt32 MF_CHANGE       =0x00000080;
    internal const UInt32 MF_APPEND       =0x00000100;
    internal const UInt32 MF_DELETE       =0x00000200;
    internal const UInt32 MF_REMOVE       =0x00001000;

    internal const UInt32 MF_BYCOMMAND    =0x00000000;
    internal const UInt32 MF_BYPOSITION       =0x00000400;

    internal const UInt32 MF_SEPARATOR    =0x00000800;

    internal const UInt32 MF_ENABLED      =0x00000000;
    internal const UInt32 MF_GRAYED       =0x00000001;
    internal const UInt32 MF_DISABLED     =0x00000002;

    internal const UInt32 MF_UNCHECKED    =0x00000000;
    internal const UInt32 MF_CHECKED      =0x00000008;
    internal const UInt32 MF_USECHECKBITMAPS  =0x00000200;

    internal const UInt32 MF_STRING       =0x00000000;
    internal const UInt32 MF_BITMAP       =0x00000004;
    internal const UInt32 MF_OWNERDRAW    =0x00000100;

    internal const UInt32 MF_POPUP        =0x00000010;
    internal const UInt32 MF_MENUBARBREAK     =0x00000020;
    internal const UInt32 MF_MENUBREAK    =0x00000040;

    internal const UInt32 MF_UNHILITE     =0x00000000;
    internal const UInt32 MF_HILITE       =0x00000080;

    internal const UInt32 MF_DEFAULT      =0x00001000;
    internal const UInt32 MF_SYSMENU      =0x00002000;
    internal const UInt32 MF_HELP         =0x00004000;
    internal const UInt32 MF_RIGHTJUSTIFY     =0x00004000;

    internal const UInt32 MF_MOUSESELECT      =0x00008000;
    internal const UInt32 MF_END          =0x00000080;  /* Obsolete -- only used by old RES files */

    internal const UInt32 MFT_STRING      =MF_STRING;
    internal const UInt32 MFT_BITMAP      =MF_BITMAP;
    internal const UInt32 MFT_MENUBARBREAK    =MF_MENUBARBREAK;
    internal const UInt32 MFT_MENUBREAK       =MF_MENUBREAK;
    internal const UInt32 MFT_OWNERDRAW       =MF_OWNERDRAW;
    internal const UInt32 MFT_RADIOCHECK      =0x00000200;
    internal const UInt32 MFT_SEPARATOR       =MF_SEPARATOR;
    internal const UInt32 MFT_RIGHTORDER      =0x00002000;
    internal const UInt32 MFT_RIGHTJUSTIFY    =MF_RIGHTJUSTIFY;

    internal const UInt32 MFS_GRAYED      =0x00000003;
    internal const UInt32 MFS_DISABLED    =MFS_GRAYED;
    internal const UInt32 MFS_CHECKED     =MF_CHECKED;
    internal const UInt32 MFS_HILITE      =MF_HILITE;
    internal const UInt32 MFS_ENABLED     =MF_ENABLED;
    internal const UInt32 MFS_UNCHECKED       =MF_UNCHECKED;
    internal const UInt32 MFS_UNHILITE    =MF_UNHILITE;
    internal const UInt32 MFS_DEFAULT     =MF_DEFAULT;

Notes:

None.

Tips & Tricks:

Please add some!

Sample Code:

Public Class Form1

    Private Const SC_CLOSE As Integer = &HF060
    Private Const MF_GRAYED As Integer = &H1

    <DllImport("user32.dll", CallingConvention:=Runtime.InteropServices.CallingConvention.Cdecl)> _
    Private Shared Function GetSystemMenu(ByVal hWnd As IntPtr, ByVal bRevert As Boolean) As IntPtr
    End Function

    <DllImport("user32.dll", CallingConvention:=Runtime.InteropServices.CallingConvention.Cdecl)> _
    Private Shared Function EnableMenuItem(ByVal hMenu As IntPtr, ByVal wIDEnableItem As UInteger, ByVal wEnable As UInteger) As IntPtr
    End Function


    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

    'Disable the form's 'X' button. Note that this does not disable the Alt+F4 functionality.
    EnableMenuItem(GetSystemMenu(Me.Handle, False), SC_CLOSE, MF_GRAYED)

    End Sub

End Class

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