How can I get the file version for a .sln, if possible?
 

How can I retrieve the version number from an .sln file which I'm using to build with a 'Make VS.NET' action step? I also tried to get the number from an .msi file, which is created by the action, but nothing. I know how to do this for an .dll file, and this approach gives me no errors but no number as well.

I want to know what that number is to be able to increment it and set it to that value. Is there some other file that I should be looking for?

A .sln file doesn't have a version number; each individual project within the solution usually does, and you can use the Get VS.NET Version action to retrieve a project's version(s) into temporary macro(s). If you're using a Visual Studio setup project to create the .msi, you can retrieve the product version from the corresponding .vdproj file.
http://www.visualbuild.com/Manual/vs_netprojver.htm

The attached sample also demonstrates retrieving the product version from a .msi file and the file version of a .dll file into temporary macros and incrementing the version.