Resize form by only specific sizes

Question

I want to allow any form height, but only specific widths (300, 550 , 800, ...(+250)). I tried this code:

Resize += (s, a) => {
    if (Width < 425)
        Width = 300;
    else if (Width < 675)
        Width = 550;
    else if (Width < 925)
        Width = 800;
    else ... 
};

When I try to resize my form, border fluctuates between mouse position and expected width. How can I achieve that behaviour?

Answer 1

Sizing or BeforeResize event for Form

To control size of the form, ResizeEnd or SizeChanged are a bit late and if you want to change the size in those events, it results in flickering.

To handle sizing before the new size applies, you can hanlde WM-SIZING message which includes the proposed size and the edge of the form which has triggered the resize.

In the following example, I've set a tile size for the form and tried to resize based on the tiles:

private int widthGridSize = 200;
private int heightGridSize = 1;

private const int WM_SIZING = 0x0214;
enum EdgeType : int
{
    WMSZ_LEFT = 1,
    WMSZ_RIGHT = 2,
    WMSZ_TOP = 3,
    WMSZ_TOPLEFT = 4,
    WMSZ_TOPRIGHT = 5,
    WMSZ_BOTTOM = 6,
    WMSZ_BOTTOMLEFT = 7,
    WMSZ_BOTTOMRIGHT = 8
}
[StructLayout(LayoutKind.Sequential)]
public class RECT
{
    public int L, T, R, B;
    public int W => R - L;
    public int H => B - T;
}
protected override void WndProc(ref Message m)
{
    if (m.Msg == WM_SIZING)
    {
        var flag = (EdgeType)m.WParam;
        var rect = new RECT();
        Marshal.PtrToStructure(m.LParam, rect);
        var w = (int)Math.Round((double)rect.W / widthGridSize) * widthGridSize;
        var h = (int)Math.Round((double)rect.H / heightGridSize) * heightGridSize;
        switch (flag)
        {
            case EdgeType.WMSZ_LEFT:
                rect.L = rect.R - w;
                break;
            case EdgeType.WMSZ_RIGHT:
                rect.R = rect.L + w;
                break;
            case EdgeType.WMSZ_TOP:
                rect.T = rect.B - h;
                break;
            case EdgeType.WMSZ_TOPLEFT:
                rect.T = rect.B - h;
                rect.L = rect.R - w;
                break;
            case EdgeType.WMSZ_TOPRIGHT:
                rect.T = rect.B - h;
                rect.R = rect.L + w;
                break;
            case EdgeType.WMSZ_BOTTOM:
                rect.B = rect.T + h;
                break;
            case EdgeType.WMSZ_BOTTOMLEFT:
                rect.B = rect.T + h;
                rect.L = rect.R - w;
                break;
            case EdgeType.WMSZ_BOTTOMRIGHT:
                rect.B = rect.T + h;
                break;
            default:
                break;
        }
        Marshal.StructureToPtr(rect, m.LParam, true);
        m.Result = (IntPtr)1;
    }
    else
        base.WndProc(ref m);
}