I know there are different approaches such as GUI_UPLOAD, GUI_DOWNLOAD, and CL_GUI_FRONTEND_SERVICES, but I am not sure which one should be used in different scenarios.

I would like to understand the complete process of file upload and download in ABAP along with best practices and common troubleshooting steps.

Please help with the following points:

• What is the difference between GUI_UPLOAD, GUI_DOWNLOAD, and CL_GUI_FRONTEND_SERVICES?
• How can we upload data from a CSV or Excel file into an internal table?
• How can we validate uploaded data before processing it?
• How can we download an internal table to CSV or Excel format?
• How can we allow users to select a file using the file selection dialog?
• How can we handle file encoding issues such as UTF-8 and special characters?
• What are the common errors like “File not found”, “Access denied”, or “Frontend services not available”, and how can they be resolved?
• How can we make file upload and download programs work efficiently for large files?
• What are the best practices for error handling and performance?
• Can someone share a complete working ABAP example for both file upload and file download?

A beginner-friendly explanation with complete ABAP code, screenshots, and practical examples would be very helpful.