Dual-Stream File Formats

In addition to having the ability to open and save files created by earlier versions of Microsoft Excel, Microsoft Excel 97 offers a new type of file format called a dual-stream file format. This is a file format that can be opened in both Microsoft Excel 5/95 and Microsoft Excel 97, and that independently supports the features of each.

This format allows users of previous versions of Microsoft Excel to open your workbooks and sometimes run your code. This depends on the features you use and how you write your project. If you use Save As, with the file format set to Microsoft Excel Add-in, you do not get a dual-stream file.

If you use the dual-stream format for your workbooks, you'll notice the following issues:

• Dual-stream files are significantly larger than workbooks saved in one format or the other. (This should be obvious as you are actually saving two versions of the workbook in one file.)
• If a dual-stream workbook is saved using Microsoft Excel 5/95, the Microsoft Excel 97 stream will be lost and all Microsoft Excel 97 – specific features will be lost.

For most add-in developers the dual-stream format isn't really an issue. Its primary function is to provide backward compatibility for worksheets. Your add-ins will generally take advantage of the newer features and will be saved in Microsoft Excel 97 format. Add-ins are never dual-stream; the two formats are mutually exclusive. Note, however, that a Microsoft Excel 5/95 add-in will load and run in Microsoft Excel 97.