根据三点创建圆弧

private Point3d GetArcCenter(Point3d pt1,Point3d pt2,Point3dpt3,ref double radius)
       {
           double xysm, xyse, xy;
           double[] m_ArcCenter = new double[3];
           xy = Math.Pow(pt1.X, 2) + Math.Pow(pt1.Y, 2);
           xyse = xy - Math.Pow(pt3.X, 2) - Math.Pow(pt3.Y, 2);
           xysm = xy - Math.Pow(pt2.X, 2) - Math.Pow(pt2.Y, 2);
           xy = (pt1.X - pt2.X) * (pt1.Y - pt3.Y) - (pt1.X - pt3.X) * (pt1.Y -pt2.Y);
           // 判断参数有效性
           if (Math.Abs(xy)<0.000001)
           {
               MessageBox.Show("所输入的参数无法创建圆形!");
               return Point3d.Origin;
           }

           // 获得圆心和半径
           m_ArcCenter[0] = ((xysm * (pt1.Y - pt3.Y)) - (xyse * (pt1.Y -pt2.Y))) / (2 * xy);
           m_ArcCenter[1] = ((xyse * (pt1.X - pt2.X)) - (xysm * (pt1.X -pt3.X))) / (2 * xy);
           m_ArcCenter[2] = 0;
           radius = Math.Sqrt((pt1.X - m_ArcCenter[0]) * (pt1.X -m_ArcCenter[0]) + (pt1.Y - m_ArcCenter[1]) * (pt1.Y -m_ArcCenter[1]));
           if (radius<0.000001)
           {
               MessageBox.Show("半径过小");
               return Point3d.Origin;
           }

           //函数返回圆心的位置，而半径在参数中通过引用方式返回
           Point3d arcCenter = new Point3d(m_ArcCenter[0], m_ArcCenter[1],m_ArcCenter[2]);
           return arcCenter;
       }

       public Arc AddArcFromPt(Point3d ptSt,Point3d ptSc,Point3dptEn,ObjectId objid,string featurename)
       {
           // 三点法创建圆弧
           Point3d ptCenter;
           double radius = 0.0;
           Arc objArc;
           ptCenter = GetArcCenter(ptSt, ptSc, ptEn, ref radius);
           if (isClockWise(ptCenter, ptSt, ptSc, ptEn))
               objArc = AddArcCSEP(ptCenter, ptSt, ptEn,radius,objid,featurename);
           else
               objArc = AddArcCSEP(ptCenter, ptEn, ptSt,radius,objid,featurename);
           return objArc;
       }

       private Arc AddArcCSEP(Point3d ptCenter,Point3d ptSt,Point3dptEn,double radius,ObjectId objid,string featurename)
       {
           double radiusent;
           double stAng, enAng;
           // 计算半径
           radiusent = radius;
           // 计算起点角度和终点角度
           stAng = CalLineAngle(ptCenter, ptSt);
           enAng = CalLineAngle(ptCenter, ptEn);
           Arc objArc = new Arc(ptCenter, radius, stAng, enAng);
           objArc.LinetypeId = objid;
           objArc.Layer = featurename;
           objArc.SetDatabaseDefaults();
           return objArc;
       }

       private bool isClockWise(Point3d ptCenter,Point3d ptSt,Point3dptSc,Point3d ptEn)
       {
           double a1, a2, a3;
           a1 = CalLineAngle(ptSt, ptCenter);
           a2 = CalLineAngle(ptSc, ptCenter);
           a3 = CalLineAngle(ptEn, ptCenter);
           bool isClock = (a1 < a2) | (a2 < a3)| (a1 < a3);
           return isClock;
       }

       private double CalLineAngle(Point3d vStartPt,Point3d vEndPt)
       {
           double X, Y;
           double CalculateLineAngle = 0.0;
           double dbJudgeAngle = 0.001;
           X = vEndPt.X - vStartPt.X;
           Y = vEndPt.Y - vStartPt.Y;
           if ((X > 0 && Y> 0)||(X > 0&& Y < 0))
           {
               CalculateLineAngle = Math.Atan(Y / X);
           }
           else if ((X < 0 && Y> 0) || (X < 0&& Y < 0))
           {
               CalculateLineAngle = Math.PI + Math.Atan(Y / X);
           }
           else if(X == 0)
           {
               if (Y < 0)
               {
                   CalculateLineAngle = -Math.PI / 2;
               }
               else if (Y > 0)
               {
                   CalculateLineAngle = Math.PI / 2;
               }
           }
           else if (Y == 0)
           {
               if (X < 0)
               {
                   CalculateLineAngle = Math.PI;
               }
               else if (X > 0)
               {
                   CalculateLineAngle = 0;
               }
           }
           else
           {
               ;
           }

           if (Math.Abs(CalculateLineAngle - 0) <dbJudgeAngle)
           {
               CalculateLineAngle = 0;
               return CalculateLineAngle;
           }
           else if (CalculateLineAngle < 0)
           {
               CalculateLineAngle = 2 * Math.PI + CalculateLineAngle;
               return CalculateLineAngle;
           }
           else
               return CalculateLineAngle;
       }

#autocad二次开