基本上你需要添加这个代码：

if (!pre_defined)
            {
                g.SetClip(new Rectangle(x,y,600,300));
            }

在drawline命令之前.其中x和y是父矩形的坐标.这很容易从你的功能.

这是完整的功能：

public void drawRectangle(double Width,double Height,int A,bool pre_defined)
    {
        Graphics g = pictureBox1.CreateGraphics();
        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
        g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;

        System.Drawing.Brush brush = new System.Drawing.SolidBrush(Color.FromArgb(r.Next(0,251),r.Next(0,251)));
        Pen myPen = new Pen(brush,2);
        myPen.Width = 2;
        int x = center.X;
        int y = center.Y;
        //top left
        P[0] = new PointF((float)Math.Round(x + (Width / 2) * Math.Cos(A) + (Height / 2) * Math.Sin(A)),(float)Math.Round(y - (Height / 2) * Math.Cos(A) + (Width / 2) * Math.Sin(A)));
        //top right
        P[1] = new PointF((float)Math.Round(x - (Width / 2) * Math.Cos(A) + (Height / 2) * Math.Sin(A)),(float)Math.Round(y - (Height / 2) * Math.Cos(A) - (Width / 2) * Math.Sin(A)));
        //bottom left
        P[2] = new PointF((float)Math.Round(x + (Width / 2) * Math.Cos(A) - (Height / 2) * Math.Sin(A)),(float)Math.Round(y + (Height / 2) * Math.Cos(A) + (Width / 2) * Math.Sin(A)));
        //bottom right
        P[3] = new PointF((float)Math.Round(x - (Width / 2) * Math.Cos(A) - (Height / 2) * Math.Sin(A)),(float)Math.Round(y + (Height / 2) * Math.Cos(A) - (Width / 2) * Math.Sin(A)));
        if (!pre_defined)
        {
            g.SetClip(new Rectangle(50,50,300));
        }
        g.DrawLine(myPen,P[0],P[1]);
        g.DrawLine(myPen,P[1],P[3]);
        g.DrawLine(myPen,P[3],P[2]);
        g.DrawLine(myPen,P[2],P[0]);
    }

编辑：这不是一个完整的例子,因为这个只会将Clip设置为父宽度和高度.您需要修改函数以提供每个元素的宽度和高度.但现在我正在看你提供的图片,它看起来比我想象的要复杂.您可能最终会存储所有随机值的数组,并按大小排序,然后绘制所有元素.