Question

I need to be able to GZip compress a file in an Excel VBA function. Specifically I need to be able to use the ‘deflate’ algorithm.

Is there a way to do this without having to exec a command line application? With no dependency on external tools the code will be more robust.

Ideally the code would make use of pre-installed VBA or COM library functions – I don’t want to have to implement this logic myself or install DLLs etc.

If possible, I want installation of the function to be as simple as adding a .xla to the available Excel Add-Ins. No DLLs, EXEs, registry entries etc. required.

Edit Can I make use of the .NET GZipStream to do this?

Answer 1

VBA (which is really a dialect of VB6) is slow for these kind of applications. I remember I once implemented Shannon-Fano algorithm on VB6 and on C, the C version was about 10 times faster, even after being turned into a DLLMain and called from there rather than on a command-line executable.

There are lots of COM DLLs that provide compression services, both open source and shareware, and some of them implement GZIP’s deflate algorithm. It’d be really simple to just call one function from such a DLL from your VBA code to do the compression on your behalf.

I understand your being reluctant on using something external to your application, though in this case you might have to apply an exception for performance’s sake.

In an effort to completely spoil your fun, examine file ZIPFLDR.DLL on windows\system32. you may also like to take a look at these links:

• This has an example of how to do what you want (zipping using windows built-in ZIP capabilities) from VB.NET, it shouldn’t be much different from VBA or VB6: Transparent ZIP with DLL call
• This one has a sample application on VB6 using windows built-in capabilities to zip (in ZIP rather than GZIP format, of course): Using Windows XP "Compressed Folder" shell extension to work with .zip files

Found both thru googling, you should be able to find more/better examples.