继承System.Windows.Forms.Form窗体类时，重载其画窗体形状的事件OnPaint,就可以画出各种自定义的窗体形状。例子代码是画一个圆形窗体。
protected override void OnPaint(PaintEventArgs e)
{
GraphicsPath gp = new GraphicsPath();
gp.AddEllipse(20,20,110,110);
//gp.AddLine(30,30,110,110);
Matrix RotationTransform = new Matrix(1,0, 0,1,1,1);// rotation matrix
//Matrix TranslationTransform = new Matrix(1, 0, 0, 1, 0, 0); // translation matrix
PointF TheRotationPoint = new PointF(110.0f, 110.0f);// rotation point
// rotate the rotation transformation matrix f degrees around TheRotationPoint
RotationTransform.RotateAt(f, TheRotationPoint);
//Call the Transform method of the Graphics Path in order to multiply it by the rotation matrix and rotate the balloon
gp.Transform(RotationTransform);
e.Graphics.DrawPath(Pens.Black, gp);
f=f+10;
this.Region=new Region(gp);//这句是很重要的，设定窗体显示的区域为你所画的区域。
}
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