How can I draw a rounded rectangle as the border for a rounded Form?

Question

I'm creating a Form that has a rounded border (as shown in this question ).
As this person also seems to have an issue with, I can't seem to draw a rounded border.

This is code I have used for setting up the actual border shape:

// ... within InitializeComponent ...
this.FormBorderStyle = FormBorderStyle.None;
IntPtr handle = CreateRoundRectRgn(0, 0, Width, Height, 20, 20);
Region = System.Drawing.Region.FromHrgn(handle);
DeleteObject(handle);

this.ResizeRedraw = true;

This is code that overrides OnPaint and draws the border outline.

protected override void OnPaint(PaintEventArgs e)
{
     // I've tried modifying the parameters here.
     GraphicsPath path = MyRoundedRectangle.Create(0, 0, Width, Height, 10, MyRoundedRectangle.RectangleCorners.All);

     Pen p = new Pen(Brushes.Black, 3f);
     e.Graphics.DrawPath(p, path);
}

The content of MyRoundedRectangle is identical to the code provided in this question , in which the answer linked to this page , which contains the code for MyRoundedRectangle .

I'd like to have a full surrounding border, but instead I get this:

Answer 1

A base implementation of what is described in the comments.
The Form frmRoundCorners provides some properties that allow to draw its rounded area with a custom BackColor, a custom BorderColor and a custom inner border color, acting as a shadow for the internal side of the Form's border.

The Form itself is implemented using a base class, baseForm , derived from Form , so the Form's properties can be set in the Form designer.

The transparency is activated setting the Form's original BackColor equal to its TrasparencyKey , making its ClientArea completely transparent, but drawable.
The Form's original border is set to FormBorderStyle.None in the base class constructor.
I didn't set a specific BackColor/TransparencyKey Color (it must be set in the Form's designer) because I think it's something one need to experiment with. I'ld suggest a medium gray color. Avoid red components.

The Form can be moved, clicking on any point of its ClientArea and dragging it.

The minimum/maximum curvature of the Form and its custom Border is set to 15 and 180 degrees. It cannot be changed to a different range using the PropertyGrid.
The rounded area ot the Form and its border are drawn using the GraphicsPath.AddArc() method, then applying a Matrix transformation to the Graphics object, both in the Scale and the Transform (position) components. The Size component is untouched.

This is what it looks like:

using System.ComponentModel;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;

[ToolboxItem(false)]
public partial class frmRoundCorners : baseForm
{
    private GraphicsPath pathRegion = new GraphicsPath(FillMode.Winding);
    private GraphicsPath pathBorder;
    Point pMousePosition = Point.Empty;

    public frmRoundCorners()
    {
        SetStyle(ControlStyles.AllPaintingInWmPaint |
                 ControlStyles.UserPaint |
                 ControlStyles.OptimizedDoubleBuffer |
                 ControlStyles.ResizeRedraw, true);
        InitializeComponent();
    }

    protected override void OnMouseDown(MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left) {
            pMousePosition = e.Location;
        }
        base.OnMouseDown(e);
    }

    protected override void OnMouseMove(MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left) {
            Point screenPos = PointToScreen(e.Location);
            this.Location = new Point(screenPos.X - pMousePosition.X, screenPos.Y - pMousePosition.Y);
        }
        base.OnMouseMove(e);
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);

        e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
        e.Graphics.PixelOffsetMode = PixelOffsetMode.Half;
        RoundedCornerRectangle(this.ClientRectangle);
        RectangleF rect = pathRegion.GetBounds();
        float scaleX = 1 - (BorderSize / rect.Width);
        float scaleY = 1 - (BorderSize / rect.Height);
        using (Pen pen = new Pen(BorderColor, BorderSize))
        using (Pen penBorder = new Pen(InternalBorderColor, 2))
        using (Brush brush = new SolidBrush(FillColor))
        using (Matrix mx = new Matrix(scaleX, 0, 0, scaleY, (pen.Width / 2), (pen.Width / 2)))
        {
            e.Graphics.Transform = mx;
            e.Graphics.FillPath(brush, pathRegion);
            e.Graphics.DrawPath(penBorder, pathBorder);
            e.Graphics.DrawPath(pen, pathRegion);
        }
    }

    private void RoundedCornerRectangle(Rectangle r)
    {
        pathRegion = new GraphicsPath(FillMode.Alternate);
        float innerCurve = this.CurveAngle - this.m_PenSizeOffset;

        pathRegion.StartFigure();
        pathRegion.AddArc(r.X, r.Y, CurveAngle, CurveAngle, 180, 90);
        pathRegion.AddArc(r.Right - CurveAngle, r.Y, CurveAngle, CurveAngle, 270, 90);
        pathRegion.AddArc(r.Right - CurveAngle, r.Bottom - CurveAngle, CurveAngle, CurveAngle, 0, 90);
        pathRegion.AddArc(r.X, r.Bottom - CurveAngle, CurveAngle, CurveAngle, 90, 90);
        pathRegion.CloseFigure();

        pathBorder = new GraphicsPath();
        pathBorder.StartFigure();
        pathBorder.AddArc(r.X + m_PenSizeOffset, r.Y + m_PenSizeOffset, innerCurve, innerCurve, 180, 90);
        pathBorder.AddArc(r.Right - innerCurve - m_PenSizeOffset, r.Y + m_PenSizeOffset, innerCurve, innerCurve, 270, 90);
        pathBorder.AddArc(r.Right - innerCurve - m_PenSizeOffset, r.Bottom - innerCurve- m_PenSizeOffset, innerCurve, innerCurve, 0, 90);
        pathBorder.AddArc(r.X + m_PenSizeOffset, r.Bottom - innerCurve - m_PenSizeOffset, innerCurve, innerCurve, 90, 90);
        pathBorder.CloseFigure();
    }
}

public class baseForm : Form
{
    private Color m_InternalBorderColor = Color.FromArgb(128, 128, 128);
    private Color m_BorderColor = Color.Red;
    private Color m_FillColor = Color.WhiteSmoke;
    private float m_PenSize = 6f;
    private float m_CurveAngle = 60.0f;
    internal float m_PenSizeOffset = 3f;

    public baseForm() => InitializeComponent();
    private void InitializeComponent() => FormBorderStyle = FormBorderStyle.None;

    [EditorBrowsable(EditorBrowsableState.Always), Browsable(true), Category("Appearance")]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
    [DefaultValue(60.0f)]
    public virtual float CurveAngle
    {
        get => this.m_CurveAngle;
        set {
            this.m_CurveAngle = Math.Max(Math.Min(value, 180), 15);
            Invalidate();
        }
    }
    [EditorBrowsable(EditorBrowsableState.Always), Browsable(true), Category("Appearance")]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
    [DefaultValue(6.0f)]
    public virtual float BorderSize
    {
        get => this.m_PenSize;
        set {
            this.m_PenSize = value;
            this.m_PenSizeOffset = value / 2.0f;
            this.Invalidate();
        }
    }

    [EditorBrowsable(EditorBrowsableState.Always), Browsable(true), Category("Appearance")]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
    public virtual Color BorderColor
    {
        get => this.m_BorderColor;
        set {
            this.m_BorderColor = value;
            this.Invalidate();
        }
    }

    [EditorBrowsable(EditorBrowsableState.Always), Browsable(true), Category("Appearance")]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
    public virtual Color FillColor
    {
        get => this.m_FillColor;
        set {
            this.m_FillColor = value;
            this.Invalidate();
        }
    }

    [EditorBrowsable(EditorBrowsableState.Always), Browsable(true), Category("Appearance")]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
    [Description("Get or Set the Color of the internal border")]
    public virtual Color InternalBorderColor
    {
        get => this.m_InternalBorderColor;
        set {
            this.m_InternalBorderColor = value;
            this.Invalidate();
        }
    }

    [EditorBrowsable(EditorBrowsableState.Never), Browsable(false)]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
    [DefaultValue(FormBorderStyle.None)]
    public new FormBorderStyle FormBorderStyle
    {
        get => base.FormBorderStyle;
        set => base.FormBorderStyle = FormBorderStyle.None;
    }
}