How to Resize a Borderless Windows Form?

Handling resizing of borderless forms

Moving a borderless Windows Form is easy, but resizing such a form presents a unique challenge. By setting the "FormBorderStyle" property to "None", the default border disappears, making resizing impossible.

Solution:

To overcome this problem, use a custom drawn control in the lower right corner of the form, simulating a resize handle. Additionally, implement the "WndProc" method to intercept the "WM_NCHITTEST" message and determine the cursor's position relative to the form. If the cursor is inside the simulation's title bar or handle, the "m.Result" value is updated accordingly.

Here is a sample code snippet:

public partial class Form1 : Form {
    public Form1() {
        InitializeComponent();
        this.FormBorderStyle = FormBorderStyle.None;
        this.DoubleBuffered = true;
        this.SetStyle(ControlStyles.ResizeRedraw, true);
    }
    private const int cGrip = 16;      // 控制柄大小
    private const int cCaption = 32;   // 标题栏高度；

    protected override void OnPaint(PaintEventArgs e) {
        // 在右下角绘制控制柄
        Rectangle rc = new Rectangle(this.ClientSize.Width - cGrip, this.ClientSize.Height - cGrip, cGrip, cGrip);
        ControlPaint.DrawSizeGrip(e.Graphics, this.BackColor, rc);
        // 绘制模拟的标题栏
        rc = new Rectangle(0, 0, this.ClientSize.Width, cCaption);
        e.Graphics.FillRectangle(Brushes.DarkBlue, rc);
    }

    protected override void WndProc(ref Message m) {
        if (m.Msg == 0x84) {  // 捕获 WM_NCHITTEST
            Point pos = new Point(m.LParam.ToInt32());
            pos = this.PointToClient(pos);
            if (pos.X >= this.ClientSize.Width - cGrip && pos.Y >= this.ClientSize.Height - cGrip) {
              m.Result = (IntPtr)17; // HTBOTTOMRIGHT
              return;
            }
        }
        base.WndProc(ref m);
    }
}

With these modifications, your borderless window can now be easily moved and resized.

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