GroupBox 重绘圆角边框和文字

        private void GroupBox_Paint(object sender, PaintEventArgs e)
        {
            if (sender != null && sender is GroupBox)
            {
                GroupBox gbx = sender as GroupBox;
                e.Graphics.Clear(gbx.BackColor);
                Color color = Color.Black;
                Pen p = new Pen(color, 1);
                int w = gbx.Width;
                int h = gbx.Height;
                Brush b = null;
                if (gbx.Parent != null)
                    b = new SolidBrush(gbx.Parent.BackColor);
                else
                    b = new SolidBrush(this.BackColor);
                //绘制直线
                e.Graphics.DrawLine(p, 3, h - 1, w - 4, h - 1);                     //bottom
                e.Graphics.DrawLine(p, 0, h - 4, 0, 12);                            //left
                e.Graphics.DrawLine(p, w - 1, h - 4, w - 1, 12);                    //right
                e.Graphics.FillRectangle(b, 0, 0, w, 8);
                e.Graphics.DrawLine(p, 3, 8, 10, 8);                                //lefg top
                e.Graphics.DrawLine(p,                                              //right top
                    e.Graphics.MeasureString(gbx.Text,
                    gbx.Font).Width + 8, 8, w - 4, 8);
                //绘制文字
                e.Graphics.DrawString(gbx.Text, gbx.Font, Brushes.Blue, 10, 0);     //title
                //绘制弧线
                e.Graphics.DrawArc(p, new Rectangle(0, 8, 10, 10), 180, 90);        //left top
                e.Graphics.DrawArc(p, new Rectangle(w - 11, 8, 10, 10), 270, 90);   //right top
                e.Graphics.DrawArc(p, new Rectangle(0, h - 11, 10, 10), 90, 90);    //left bottom
                e.Graphics.DrawArc(p, new Rectangle(w - 11, h - 11, 10, 10), 0, 90);//right bottom
            }
        }

因为 GroupBox 自带的灰色边框太过难看，所以尝试着对其重新绘制了一个新的边框。最开始不带圆角，感觉不太舒服，然后又添加了圆角，不过四个圆角的外侧还是与GroupBox外的颜色不太一样。试着用过一次路径来做，但是GroupBox内侧会有污点，不知道是怎么回事。

上面是没有污点，但是四个圆角外有细微颜色不同的，下面是对四个圆角外侧也做了着色，但是 GroupBox 内却出现污点的。如果有哪位高人能够帮忙解释一下污点的问题和提出一些解决方法，本人感激不尽

        private void GroupBox_Paint(object sender, PaintEventArgs e)
        {
            if (sender != null && sender is GroupBox)
            {
                GroupBox gbx = sender as GroupBox;
                e.Graphics.Clear(gbx.BackColor);
                Color color = Color.Black;
                Pen p = new Pen(color, 1);
                int w = gbx.Width;
                int h = gbx.Height;
                Brush b = null;
                if (gbx.Parent != null)
                    b = new SolidBrush(gbx.Parent.BackColor);
                else
                    b = new SolidBrush(this.BackColor);
                e.Graphics.FillRectangle(Brushes.PapayaWhip, new Rectangle(0, 0, w, h));
                GraphicsPath gp = new GraphicsPath();
                gp.AddArc(new Rectangle(0, 8, 10, 10), 180, 90);                //left top
                gp.AddLine(0, h - 4, 0, 12);                                    //left
                gp.AddArc(new Rectangle(0, h - 11, 10, 10), 90, 90);            //left bottom
                gp.AddLine(3, h - 1, w - 4, h - 1);                             //bottom
                gp.AddArc(new Rectangle(w - 11, h - 11, 10, 10), 0, 90);        //right bottom
                gp.AddLine(w - 1, h - 4, w - 1, 12);                            //right
                gp.AddArc(new Rectangle(w - 11, 8, 10, 10), 270, 90);           //right top
                gp.AddLine(3, 8, w - 4, 8);                                     //top
                e.Graphics.FillRegion(Brushes.White, new Region(gp));
                //绘制直线
                e.Graphics.DrawLine(p, 3, h - 1, w - 4, h - 1);                     //bottom
                e.Graphics.DrawLine(p, 0, h - 4, 0, 12);                            //left
                e.Graphics.DrawLine(p, w - 1, h - 4, w - 1, 12);                    //right
                e.Graphics.FillRectangle(b, 0, 0, w, 8);
                e.Graphics.DrawLine(p, 3, 8, 10, 8);                                //lefg top
                e.Graphics.DrawLine(p,                                              //right top
                    e.Graphics.MeasureString(gbx.Text,
                    gbx.Font).Width + 8, 8, w - 4, 8);
                //绘制文字
                e.Graphics.DrawString(gbx.Text, gbx.Font, Brushes.Blue, 10, 0);     //title
                //绘制弧线
                e.Graphics.DrawArc(p, new Rectangle(0, 8, 10, 10), 180, 90);        //left top
                e.Graphics.DrawArc(p, new Rectangle(w - 11, 8, 10, 10), 270, 90);   //right top
                e.Graphics.DrawArc(p, new Rectangle(0, h - 11, 10, 10), 90, 90);    //left bottom
                e.Graphics.DrawArc(p, new Rectangle(w - 11, h - 11, 10, 10), 0, 90);//right bottom
            }
        }