头部变量读取
Inheritance: Entity
 protected override Property ReadSonClass(Property prop)
 {
     if (prop.Value.Length > 0 && prop.Value[0] == '$')
     {
         H_Variables hv = new H_Variables(DXFData, prop);
         base.Sons.Add(hv);
         var lastProp = hv.ReadProperties();
         return lastProp;
     }
     else
     {
         return null;
     }
 }
 protected override Property ReadSonClass(Property prop)
 {
     if (prop.Value.Length > 0 && prop.Value[0] == '$')
     {
         H_Variables hv = new H_Variables(DXFData, prop);
         base.Sons.Add(hv);
         var lastProp = hv.ReadProperties();
         return(lastProp);
     }
     else
     {
         return(null);
     }
 }