Drawing a rounded edge panel in C# windows forms doesn't load correctly

Question

I am trying to create a panel with rounded edges, but always comes out wrong picture:

what i tried was these two codes but none of them achieves my desired expectation

public class CustomPanel : Panel
{

    public int CornerRadius { get; set; } = 10;

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

        Graphics g = e.Graphics;
        Rectangle bounds = new Rectangle(0, 0, Width, Height);
        GraphicsPath path = GetRoundedRectangle(bounds, CornerRadius);

        using (SolidBrush brush = new SolidBrush(BackColor))
        {
            g.FillPath(brush, path);
        }

        using (Pen pen = new Pen(BorderColor, BorderWidth))
        {
            g.DrawPath(pen, path);
        }
    }

    private GraphicsPath GetRoundedRectangle(RectangleF rect, int radius)
    {
        GraphicsPath path = new GraphicsPath();
        float r = radius;
        path.StartFigure();
        path.AddArc(rect.X, rect.Y, r, r, 180, 90);
        path.AddArc(rect.Right - r, rect.Y, r, r, 270, 90);
        path.AddArc(rect.Right - r, rect.Bottom - r, r, r, 0, 90);
        path.AddArc(rect.X, rect.Bottom - r, r, r, 90, 90);
        path.CloseFigure();
        return path;
    }

    public Color BorderColor { get; set; } = Color.Black;
    public int BorderWidth { get; set; } = 1;
}

This method works only if the panel's back color is different from the form's back color, so i need a different solution.

[DllImport("Gdi32.dll", EntryPoint = "CreateRoundRectRgn")]
public static extern IntPtr CreateRoundRectRgn
    (
    int top,
    int bot,
    int left,
    int right,
    int widthelipse,
    int heightelipse
    );

Answer 1

Seems like the right and bottom border are out of painting area. I copied your code and made it work simply by modifying this line:

From:

Rectangle bounds = new Rectangle(0, 0, Width, Height);

To:

Rectangle bounds = new Rectangle(0, 0, Width - 1, Height - 1);

I reduced the panel's painting Rectangle Width and Height by 1 pixel to ensure it inside the painting area.

Hope this helps.