[wix-users] Merge changes for Config files and IIS settings

[Ven H]

I have a business requirement. I know many eyebrows will be raised, but
unfortunately, I need to achieve this requirement.

There is an application folder that already exists (which was probably
installed by some other MSI). Now, I have to install the same application
folder using my MSI. The application folder has a lot of config files.
Since the end customer would have made some changes to this config, my MSI
should not overwrite these files. But, the config file changes from my MSI
should be merged from my MSI. For this, I think I may have to use a Custom
Action. However, I have the following challenges / questions.

1. If a file already exists and my MSI also has the same file to be
installed, will the file from MSI overwrite the existing file? If so, can
this be prevented selectively ( I don't want some selected files like
config files to be overwritten) by using NeverOverwrite attribute (Will
NeverOverwrite work on files outside of MSI, i.e., the files already
present and not installed by MSI or it will work 2nd time when the same MSI
is being installed or upgraded)?

2. Let's say this file is not overwritten and merged using Custom Action,
what will happen when I run an Upgrade again using my MSI? Will it
overwrite the other files (config file components will have NeverOverwrite
attribute and hence they will be spared, I believe)?

3. I have similar requirement for IIS (web sites and app pools). They might
have been created manually or using another MSI. First time, when I run my
MSI, they should not be recreated since the end customer could have updated
some settings manually. So, I should be able to merge the changes.

Please help / advise.