如图：最终效果

polyline连接：

代码如下：

public void joinentities()
{
    Curve pLine = Z.db.SelectEntities<Curve>().First().Clone() as Curve;
    pLine.ChangeEntityColor(1);
    Curve pLine1 = Z.db.SelectEntities<Curve>().First().Clone() as Curve;
    Curve pLine2 = Z.db.SelectEntities<Curve>().First().Clone() as Curve;
    Entity[] otherEntities = new Entity[2];
    otherEntities[0] = pLine1;
    otherEntities[1] = pLine2;
    // join the other entities
    try
    {
        IntegerCollection joinedEntities = pLine.JoinEntities(otherEntities);//调用函数的主题必须polyline
        if (joinedEntities.Count > 0) Application.DocumentManager.MdiActiveDocument.Editor.WriteMessage("nSome or all entities joined!");
        Z.db.AddEntityToModeSpace(pLine);
        Z.Zoom();
    }
    catch (System.Exception)
    {
        Z.ed.WriteMessage("连接失败!\n");
    } 
}

 polyline3d连接：

 public void joinentitiesPolyline3d()
 {
     Point3d[] p3d = new Point3d[2] { new Point3d(10, 10, 0), new Point3d(0, 0, 0) };
     Polyline3d p3 = new Polyline3d(Poly3dType.SimplePoly,new Point3dCollection(p3d),false);
     Curve firstPl = p3;//polyline3d可连接范围广，包括圆弧，椭圆弧，样条曲线，直线，多段线
    // Curve firstPl = Z.db.SelectEntities<Curve>().First().Clone() as Curve;//连接的线不能是数据库中已有的线
     if (firstPl is null) return;    
     firstPl.ChangeEntityColor(1);
     List<Curve> curveslist = Z.db.SelectEntities<Curve>();
     Curve[] curvesarray = new Curve[curveslist.Count];
     for (int i = 0; i < curveslist.Count; i++)
     {
         curvesarray[i] = curveslist[i].Clone()as Curve;
     }
     try
     {
         IntegerCollection resultPl = firstPl.JoinEntities(curvesarray);//调用函数的主题必须polyline
         if (resultPl.Count > 0) Application.DocumentManager.MdiActiveDocument.Editor.WriteMessage("nSome or all entities joined!");
         Z.db.AddEntityToModeSpace(firstPl);
         Z.Zoom();
     }
     catch (System.Exception)
     {
         Z.ed.WriteMessage("连接失败!\n");
     } 
 }

 Polyline连接(可连接弧，不可连spline和ellipse)

 public void joinentitiesPolyline3d()
 {
     Point3d[] p3d = new Point3d[2] { new Point3d(10, 10, 0), new Point3d(0, 0, 0) };
     Polyline3d p3 = new Polyline3d(Poly3dType.SimplePoly,new Point3dCollection(p3d),false);
     Curve firstPl = p3;//polyline3d可连接范围广，包括圆弧，椭圆弧，样条曲线，直线，多段线
    // Curve firstPl = Z.db.SelectEntities<Curve>().First().Clone() as Curve;//连接的线不能是数据库中已有的线
     if (firstPl is null) return;    
     firstPl.ChangeEntityColor(1);
     List<Curve> curveslist = Z.db.SelectEntities<Curve>();
     Curve[] curvesarray = new Curve[curveslist.Count];
     for (int i = 0; i < curveslist.Count; i++)
     {
         curvesarray[i] = curveslist[i].Clone()as Curve;
     }
     try
     {
         IntegerCollection resultPl = firstPl.JoinEntities(curvesarray);//调用函数的主题必须polyline
         if (resultPl.Count > 0) Application.DocumentManager.MdiActiveDocument.Editor.WriteMessage("nSome or all entities joined!");
         Z.db.AddEntityToModeSpace(firstPl);
         Z.Zoom();
     }
     catch (System.Exception)
     {
         Z.ed.WriteMessage("连接失败!\n");
     } 
 }

封装后结果：

 public void xx()
 {

     Polyline polyline = Z.PolyJoin();
 }

 public static Polyline PolyJoin()
 {
     //Point3d[] p3d = new Point3d[2] { new Point3d(10, 10, 0), new Point3d(0, 0, 0) };
     //Polyline3d p3 = new Polyline3d(Poly3dType.SimplePoly,new Point3dCollection(p3d),false);
     //Curve firstPl = p3;//polyline3d可连接范围广，包括圆弧，椭圆弧，样条曲线，直线，多段线
     //Curve firstPl = Z.db.SelectEntities<Curve>().First().Clone() as Curve;//连接的线不能是数据库中已有的线
     //if (firstPl is null) return;    
     Polyline firstPl = new Polyline();
     List<Curve> curveslist = Z.db.SelectEntities<Curve>();
     Curve[] curvesarray = new Curve[curveslist.Count];
     for (int i = 0; i < curveslist.Count; i++)
     {
         curvesarray[i] = curveslist[i].Clone() as Curve;
     }
     try
     {
         firstPl = curvesarray.Where(x => x is Polyline).First() as Polyline;
         IntegerCollection resultPl = firstPl.JoinEntities(curvesarray);//调用函数的主题必须polyline
         if (resultPl.Count > 0) Application.DocumentManager.MdiActiveDocument.Editor.WriteMessage("nSome or all entities joined!");
         firstPl.ChangeEntityColor(1); Z.db.AddEntityToModeSpace(firstPl);
         //Z.Zoom();
     }
     catch (System.Exception)
     {
         Z.ed.WriteMessage("连接失败!\n");
     }
     return firstPl;
 }