CAD2007版本有GeometricExtents取范围函数的bug，2008以上版本修复，可用以下代码代替

Autodesk.AutoCAD.Interop.Common.AcadAttributeReference acadAtt =
   lstAtt[i].AcadObject as Autodesk.AutoCAD.Interop.Common.AcadAttributeReference;              
object objMin; object objMax; double[] ptMin; double[] ptMax;
acadAtt.GetBoundingBox(out objMin, out objMax);
ptMin = (double[])objMin;
ptMax = (double[])objMax;