Question

How can I make a form have a fixed aspect ratio that is preserved when it is resized?

I know it can be done by overriding OnSizeChanged and manually modifying the [new] Height/Width, but that causes flicker since it is resized once before the event is called (to a size not matching the aspect ratio) and then resized again (to the correct aspect ratio). Is there a better way?

Answer 1

Some code to get you started. The key is to respond to the WM_SIZING message, it allows you to change the window rectangle. This sample is crude, you really want to pay attention to which corner or edge is being dragged by the user, available from m.WParam. The user interface will never be great, you can’t really do anything reasonable when the user drags a corner. Making your form’s layout flexible enough so you don’t care about aspect ration is the real solution. Displaying a scrollbar when the content doesn’t fit pretty much lets the user do the Right Thing automatically.

using System.Runtime.InteropServices;
// etc..

public partial class Form1 : Form {
    public Form1() {
        InitializeComponent();
    }
    protected override void WndProc(ref Message m) {
        if (m.Msg == 0x216 || m.Msg == 0x214) { // WM_MOVING || WM_SIZING
            // Keep the window square
            RECT rc = (RECT)Marshal.PtrToStructure(m.LParam, typeof(RECT));
            int w = rc.Right - rc.Left;
            int h = rc.Bottom - rc.Top;
            int z = w > h ? w : h;
            rc.Bottom = rc.Top + z;
            rc.Right = rc.Left + z;
            Marshal.StructureToPtr(rc, m.LParam, false);
            m.Result = (IntPtr)1;
            return;
        }
        base.WndProc(ref m);
    }
    [StructLayout(LayoutKind.Sequential)]
    public struct RECT {
        public int Left;
        public int Top;
        public int Right;
        public int Bottom;
    }
}