Aspect Form

AspectForm.cs

    using System;
    using System.Runtime.InteropServices;
    using System.Windows.Forms;

    public partial class Form1 : Form
    {
        private int ncHeight;
        private int ncWidth;
        private int hitTest;
        private double aspect = 0;

        public Form1() {
            InitializeComponent();
        }

        protected override void OnCreateControl() {
            base.OnCreateControl();
            var cs = ClientSize;
            aspect = (double)cs.Width / cs.Height;
        }

        protected override void OnSizeChanged(EventArgs e) {
            base.OnSizeChanged(e);
            var cs = ClientSize;
            ncHeight = Height - cs.Height;
            ncWidth = Width - cs.Width;
        }

        protected override void WndProc(ref Message m) {
            switch (m.Msg) {
                case WM_NCHITTEST:
                    WmNcHitTest(ref m);
                    break;
                case WM_WINDOWPOSCHANGING:
                    WmWindowPosChanging(ref m);
                    break;
                default:
                    base.WndProc(ref m);
                    break;
            }
        }

        private void WmNcHitTest(ref Message m) {
            base.WndProc(ref m);
            hitTest = m.Result.ToInt32();
        }

        private void WmWindowPosChanging(ref Message m) {
            if (aspect != 0) {
                var wp = (WINDOWPOS)m.GetLParam(typeof(WINDOWPOS));
                if ((wp.flags & SWP_NOSIZE) == 0) {
                    switch (hitTest) {
                        case HTTOP:
                        case HTBOTTOM:
                            wp.cx = (int)((wp.cy - ncHeight) * aspect) + ncWidth;
                            break;
                        default:
                            wp.cy = (int)((wp.cx - ncWidth) / aspect) + ncHeight;
                            break;
                    }
                    Marshal.StructureToPtr(wp, m.LParam, false);
                }
            }
        }

        private const int WM_WINDOWPOSCHANGING = 0x0046;
        private const int WM_NCHITTEST = 0x0084;
        private const int SWP_NOSIZE = 0x0001;
        private const int HTTOP = 12;
        private const int HTBOTTOM = 15;

        [StructLayout(LayoutKind.Sequential)]
        public struct WINDOWPOS
        {
            public IntPtr hwnd;
            public IntPtr hwndInsertAfter;
            public int x;
            public int y;
            public int cx;
            public int cy;
            public int flags;
        }
    }