Here's an excerpt of code to dock a .NET form inside a viewport:

[Flags]
        internal enum WindowStyles : uint
        {
            WS_OVERLAPPED = 0x00000000,
            WS_POPUP = 0x80000000,
            WS_CHILD = 0x40000000,
            WS_MINIMIZE = 0x20000000,
            WS_VISIBLE = 0x10000000,
            WS_DISABLED = 0x08000000,
            WS_CLIPSIBLINGS = 0x04000000,
            WS_CLIPCHILDREN = 0x02000000,
            WS_MAXIMIZE = 0x01000000,
            WS_BORDER = 0x00800000,
            WS_DLGFRAME = 0x00400000,
            WS_VSCROLL = 0x00200000,
            WS_HSCROLL = 0x00100000,
            WS_SYSMENU = 0x00080000,
            WS_THICKFRAME = 0x00040000,
            WS_GROUP = 0x00020000,
            WS_TABSTOP = 0x00010000,

            WS_MINIMIZEBOX = 0x00020000,
            WS_MAXIMIZEBOX = 0x00010000,

            WS_CAPTION = WS_BORDER | WS_DLGFRAME,
            WS_TILED = WS_OVERLAPPED,
            WS_ICONIC = WS_MINIMIZE,
            WS_SIZEBOX = WS_THICKFRAME,
            WS_TILEDWINDOW = WS_OVERLAPPEDWINDOW,

            WS_OVERLAPPEDWINDOW = WS_OVERLAPPED | WS_CAPTION | WS_SYSMENU | WS_THICKFRAME | WS_MINIMIZEBOX | WS_MAXIMIZEBOX,
            WS_POPUPWINDOW = WS_POPUP | WS_BORDER | WS_SYSMENU,
            WS_CHILDWINDOW = WS_CHILD,
        }

        public const Int32 GWL_STYLE = -16;

        [DllImport( "user32.dll", CharSet = CharSet.Auto, ExactSpelling = true )]
        internal static extern IntPtr SetParent( IntPtr hWnd, IntPtr hWndParent );
        [DllImport( "user32.dll", CharSet = CharSet.Auto, ExactSpelling = true )]
        internal static extern IntPtr GetParent( IntPtr hWnd );
        [DllImport( "user32.dll" )]
        internal static extern WindowStyles GetWindowLong( IntPtr hWnd, Int32 nIndex );
        [DllImport( "user32.dll" )]
        internal static extern UInt32 SetWindowLong( IntPtr hWnd, Int32 nIndex, UInt32 dwNewLong );
        //[DllImport( "user32.dll" )]
        //internal static extern IntPtr SetParent( IntPtr child, IntPtr newParent );

        internal static WindowStyles GetWindowStyle( IntPtr hWnd )
        {
            return (WindowStyles)GetWindowLong( hWnd, GWL_STYLE );
        }

        internal static void SetWindowStyle( IntPtr hWnd, WindowStyles windowStyle )
        {
            SetWindowLong( hWnd, GWL_STYLE, (UInt32)windowStyle );
        }

        class MainViewWindow : global::Autodesk.Max.Plugins.ViewWindow
        {
            IntPtr previousParent;
            WindowStyles previousStyle;
            public override IntPtr CreateViewWindow( IntPtr hParent, int x, int y, int w, int h )
            {
                this.previousParent = GetParent( Plugin.Instance.MainFrame.Handle );
                this.previousStyle = GetWindowStyle( Plugin.Instance.MainFrame.Handle );

                MainFrame mainFrame = Plugin.Instance.LaunchDefault_( hParent, Plugin.LayoutType.General );
                SetWindowStyle( mainFrame.Handle, WindowStyles.WS_CHILD | WindowStyles.WS_VISIBLE | WindowStyles.WS_CLIPCHILDREN );
                SetParent( mainFrame.Handle, hParent );

                mainFrame.dockedInsideExtendedView = true;
                return mainFrame.Handle;
            }

            public override void DestroyViewWindow( IntPtr hWnd )
            {
                Plugin.Instance.MainFrame.dockedInsideExtendedView = false;
                SetWindowStyle( Plugin.Instance.MainFrame.Handle, this.previousStyle );
                SetParent( Plugin.Instance.MainFrame.Handle, this.previousParent );
                Plugin.Instance.MainFrame.FormBorderStyle = FormBorderStyle.Sizable;
                // NOTE: Do not call close here because when resetting a scene the layout is saved correctly, then this
                // method is called by Max and Close(...) here saves the layuout again, with an empty view thus causing
                // a blank layout to be saved
                Plugin.Instance.MainFrame.Hide();
            }

            public override string Name
            {
                get { return ProjectName + " Main Window"; }
            }

            /// <summary>
            /// Can only have one mainframe!
            /// </summary>
            public override int NumberCanCreate
            {
                get { return 1; }
            }

            public override bool CanCreate
            {
                get
                {
                    // Cannot put Mainframe into another view without first undocking it from the previous one
                    return !Plugin.Instance.MainFrame.dockedInsideExtendedView;
                }
            }
        }

Marsel Khadiyev (Software Developer, EPHERE Inc.)