Vertically (only) resizable windows form in C#

You need to set the form’s MinimumSize and MaximumSize properties to two sizes with different heights but equal widths.

If you don’t want the horizontal resize cursor to appear at all, you’ll need to handle the WM_NCHITTEST message, like this:

protected override void WndProc(ref Message m) {
    base.WndProc(ref m);
    switch (m.Msg) {
        case 0x84: //WM_NCHITTEST
            var result = (HitTest)m.Result.ToInt32();
            if (result == HitTest.Left || result == HitTest.Right)
                m.Result = new IntPtr((int)HitTest.Caption);
            if (result == HitTest.TopLeft || result == HitTest.TopRight)
                m.Result = new IntPtr((int)HitTest.Top);
            if (result == HitTest.BottomLeft || result == HitTest.BottomRight)
                m.Result = new IntPtr((int)HitTest.Bottom);

            break;
    }
}
enum HitTest {
    Caption = 2,
    Transparent = -1,
    Nowhere = 0,
    Client = 1,
    Left = 10,
    Right = 11,
    Top = 12,
    TopLeft = 13,
    TopRight = 14,
    Bottom = 15,
    BottomLeft = 16,
    BottomRight = 17,
    Border = 18
}