How to sync version numbers for nuget packages when targeting multiple project files - versioning

Question

I have a project which has three project files targeting various .net versions (mylib.net20.csproj, mylib.net40.csproj, and mylib.net40-client.csproj). I have a single nuspec file named mylib.nuspec for packaging them all together.
It might help to see the files section of the nuspec file, so here it is:
<files>
<file src="bin\net20\*.dll" target="lib\net20\" />
<file src="bin\net40-client\*.dll" target="lib\net40-client\" />
<file src="bin\net40\*.dll" target="lib\net40\" />
</files>
Right now I use a static version number in the nuspec file and I can successfully package everything by running nuget pack mylib.nuspec. However, I would like to start using $version$ so I don't have to remember to update the version number in two places.
If I simply change the version number to $version$, and build in the same way I get the predictable error:
Attempting to build package from 'mylib.nuspec'.
The replacement token 'version' has no value.
If I use $version$ and package with nuget pack mylib.net40.csproj, it is successful, but I get a package that completely ignores the nuspec file.
Q: What can I do to get the $version$ variable to work?
Technically, I could rename my nuspec file to mylib.net20.nuspec and then package with nuget pack mylib.net20.nuspec. I really don't want to rename my nuspec file in this way though.

Answer 1

creating a package can be done two ways
Using nuspec directly
Using your csproj file , which in fact uses your nuspec file at run time.
First case is pretty straight forward where all metadata defined in nuspec file will be considered for package creation.
nuget pack nuspecfilename.nuspec
On executing above command all hard coded values in your nuspec file will be used for package creation
Above Procedure uses nuspec file directly.
Now let come to your requirement , Using token $version$ in your nuspec files.
In order to work with tokens rather than hard coded values , we need to use csproj file while running nuget pack command.
nuget pack nameofyourprojectfile.csproj -p "configuration=Release;platform=x64"
I'll explain the command i have used ,the concept of using csproj file while generating a package is it will replace all tokens in nuspec file at the run time.
Your corresponding assembly info file to your csproj file will contain required metadata of version which will be replaced at run time.
-p is parameters to be used at run time , here i have assumed that my proj file builds in release and x64 platform so i have passed that at run time so nuget which for artifacts to bundle accordingly.
please refer Here for more details on how to create nuget packages from csproj.
But Ideally when you package from csproj file your nuspec file will be used at run time so ,technically nuspec file should have same name as that of your csproj file.
so requirement of dealing with multiple project files with different frame work may not be achieved with single nuspec file. you need to have individual nuspec accordingly to achieve this.
But if you still want to stick with same nuspec file with out renaming it or making changes to it.
i would suggest passing version at run time
nuget pack nameofnuspec.nuspec -v 1.3.4.5
-v - is version , or you can use -version as well followed by the version of your choice.
after running above command nuget package will be created with the version specified.
to check nuget package created , rename it .zip file and check ...)