Recommendation
ObjectARX and .NET applications
ObjectARX is not thread-safe and multithreading is not supported. Many APIs called from externally created threads will cause unexpected results or terminate AutoCAD. APIs that perform user interface functions (for example, acutPrintf()) are particularly problematic and should never be called from an external thread. If threading is absolutely required, use proper synchronization techniques to avoid unnecessary CPU utilization and potential conflicts between shared resources.