#region 分段测量多段线长度
    private static double textHight = 10;

    [CommandMethod(nameof(PolylineDemo))]
    public void PolylineDemo()
    {
       using var tr = new DBTrans();
        if(!tr.LayerTable.Has("标注"))
        {
            tr.LayerTable.Add("标注",1);
        }
        var pso = new PromptSelectionOptions()
        {
            MessageForAdding = "\n选择要测量的多段线或【设置字高（S）】"
        };
        pso.Keywords.Add("S"," ");
        OpFilter sf = OpFilter.Build(e=>e.Dxf(0)== "LWPOLYLINE");
       
        pso.KeywordInput += (o, s) =>
        {
            switch (s.Input.ToUpper())
            {
                case "S":
                    var r2 = Env.Editor.GetDouble("\n请输入字高<"+textHight+">");
                    if (r2.Status == PromptStatus.OK && r2.Value>0)
                    {
                        textHight = r2.Value;
                    }
                    break;
                case " ":
                    return;
            }
        };

        //开始选择
        var r1 = Env.Editor.GetSelection(pso,sf);
        if (r1.Status != PromptStatus.OK)
        {
            return;
        }
        //获取到所有的多段线
        var polylines = r1.Value.GetEntities<Polyline>();

        foreach (var polyline in polylines)
        {
            for (int i = 0; i < polyline.NumberOfVertices; i++)
            {
                //获取当前线段的类型
                var st = polyline.GetSegmentType (i); 
                if(st == SegmentType.Line)//直线
                {
                    var cur = polyline.GetLineSegmentAt(i).ToCurve();
                    AddText(cur, tr); //添加文字
                }else if(st == SegmentType.Arc)//圆弧
                {
                    var cur = polyline.GetArcSegmentAt(i).ToCurve();
                    AddText(cur, tr); //添加文字
                }   
            }
            if (polyline.Closed)
            {
                var ppr  = Env.Editor.GetPoint("\n选择多边形面积计算结果的位置");
                if (r1.Status != PromptStatus.OK)
                {
                    return;
                }
                var point3d =  ppr.Value;
                GetArea(polyline, point3d,tr);
            }
           
        }

    }

    private void AddText(Curve curve,DBTrans tr)
    {
        //拿到线段起点和终点
        var pt1 = curve.StartPoint;
        var pt2 = curve.EndPoint;
        var length = curve.GetLength();
        //计算角度
        var angle1 = pt1.GetAngle(pt2);
        //计算文字偏移角度
        var angle2 = angle1 + Math.PI*0.5;

        var textPoint = curve.GetPointAtDist(length*0.5).Polar(angle2,textHight);
        var text = new DBText()
        {
           Position = textPoint,
           TextString = (length/1000).ToString("0.00"),
           HorizontalMode = TextHorizontalMode.TextCenter,
           VerticalMode = TextVerticalMode.TextVerticalMid,
           AlignmentPoint = textPoint,
           WidthFactor = 0.7,
           Layer = "标注",
           Height = textHight
        };
        //设置文字角度和线平行并向上
        text.Rotation =  angle1 > Math.PI*0.5 && angle1 <= Math.PI*1.5 ? angle2 + Math.PI : angle1;
        tr.CurrentSpace.AddEntity(text);

    }

    /// <summary>
    /// 计算多边形的面积
    /// </summary>
    /// <param name="polyline">要计算的多边形对象</param>
    /// <param name="point">放置的点</param>
    /// <param name="tr"></param>
    public void GetArea(Polyline polyline,Point3d point,DBTrans tr)
    {
        if (polyline.Closed)
        {
            List<Point2d> pointList = polyline.GetPoints()
                .Select(point => point.Point2d()).ToList();
            double area = Math.Abs(GeometryEx.GetArea(pointList));
            var text = new DBText()
            {
                Position = point,
                TextString = "当前多边形的面积为：" + (area / 1000).ToString("0.00"),
                HorizontalMode = TextHorizontalMode.TextCenter,
                VerticalMode = TextVerticalMode.TextVerticalMid,
                AlignmentPoint = point,
                WidthFactor = 0.7,
                Layer = "标注",
                Height = textHight,
            };
            var sts = tr.TextStyleTable;
            if (sts.Has("宋体"))
            {
                var textStyle = tr.TextStyleTable.GetRecord("宋体");
                if (textStyle != null)
                {
                    text.TextStyleId = textStyle.Id;
                }
            }
           
            tr.CurrentSpace.AddEntity(text);
        }
        
    }

    #endregion

如果遇到中文字体乱码，可以如下操作，选择字体为宋体。