I needed to create a Custom Activity that read from XML file and display the values on the Build log.
It’s very easy and I will not re-write how to do the basic Custom Activity, but if you are not familiar with that, you can see the following link:
Use and develop custom build process activities
After that, I built the project and distribute the library (DLL) under source control where the build controller point for the custom assemblies, see the previous link for the MSDN on how to do that.
The build log will be as the following image.
Make the activity return list of string