Combining an absolute path with a relative path in WiX 3.7 - wix

Question

I'm trying to use RemoveFolderEx to delete a folder one level up from my install folder upon uninstall of the app. Here is what I have, but it's not working:
<Fragment>
<Directory Id="TARGETDIR" Name="SourceDir">
<Directory Id="ProgramFilesFolder">
<Directory Id="MyAppBaseFolder" Name="MyAppID">
<Directory Id="INSTALLFOLDER" Name="MyAppLauncher">
<Directory Id="UPDATESCRIPTSFOLDER" Name="Scripts" />
<Component Id="CleanupMainApplicationFolder" Guid="*">
<RegistryValue Root="HKLM" Key="SOFTWARE\MyApp ID\MyApp ID Windows Client" Name="Path" Type="string" Value="${path::getfullpath(path::combine([INSTALLFOLDER],'..\MyApp'))}" KeyPath="yes" />
<util:RemoveFolderEx On="uninstall" Property="APPLICATIONFOLDER" />
</Component>
</Directory>
</Directory>
</Directory>
<Directory Id="ProgramMenuFolder">
<Directory Id="ApplicationProgramsFolder" Name="MyApp ID"/>
</Directory>
</Directory>
</Fragment>
The MSI will build fine, but when I run the setup it seems to be failing on the RegistryValue element and complaining about the ".." in the path. The string I'm making is like: "C:\Program Files (X86)\MyApp ID\MyAppLauncher\..\MyApp". I need it to resolve to: "C:\Program Files (X86)\MyApp ID\MyApp".
I'm not too familiar with the ${path:: ... } stuff. I saw a couple examples with ${path::combine()} so I guess I assumed that would work with getfullpath in .NET. Perhaps it does and my syntax is just wrong?
Thank you!
EDIT: Sorry, I got that ${path::combine... stuff from BUILDING WIX without fully reading what that article is doing. It has nothing to do with a wxs file, it's for NAnt (as Rob mentioned below). So I guess my question simply is this: how can I combine and resolve an absolute and relative path like C:\Program files (X86)\MyApp ID\MyAppLauncher\..\MyApp. Thanks
EDIT2: Thanks again Rob, I was focusing so much on resolving the path I didn't even consider the most obvious solution, which was to simply reference a new <Directory element. I now have it cleaning up 2 folders with the following code:
<Property Id="APPLICATIONFOLDER">
<RegistrySearch Key="SOFTWARE\MyApp ID\MyApp ID Windows Client" Root="HKLM" Type="raw" Id="APPLICATIONFOLDER_REGSEARCH" Name="Path" />
</Property>
<Property Id="PRINTERFOLDER">
<RegistrySearch Key="SOFTWARE\MyApp ID\MyApp ID Printer" Root="HKLM" Type="raw" Id="PRINTERFOLDER_REGSEARCH" Name="Path" />
</Property>
...
<Fragment>
<Directory Id="TARGETDIR" Name="SourceDir">
<Directory Id="ProgramFilesFolder">
<Directory Id="MyAppBaseFolder" Name="MyAppID">
<Directory Id="INSTALLFOLDER" Name="MyAppLauncher">
<Directory Id="UPDATESCRIPTSFOLDER" Name="Scripts" />
<Component Id="CleanupMainApplicationFolder" Guid="*">
<RegistryValue Root="HKLM" Key="SOFTWARE\MyApp ID\MyApp ID Windows Client" Name="Path" Type="string" Value="[LM_INSTALLFOLDER]" KeyPath="yes" />
<util:RemoveFolderEx On="uninstall" Property="APPLICATIONFOLDER" />
<RegistryValue Root="HKLM" Key="SOFTWARE\MyApp ID\MyApp ID Printer" Name="Path" Type="string" Value="[LMP_INSTALLFOLDER]" />
<util:RemoveFolderEx On="uninstall" Property="PRINTERFOLDER" />
</Component>
</Directory>
<Directory Id="LM_INSTALLFOLDER" Name="MyApp" >
</Directory>
<Directory Id="LMP_INSTALLFOLDER" Name="MyAppPrinter" >
</Directory>
</Directory>
</Directory>
<Directory Id="ProgramMenuFolder">
<Directory Id="ApplicationProgramsFolder" Name="MyApp ID"/>
</Directory>
</Directory>
</Fragment>

Answer 1

The syntax you have in the RegistryValue/#Value looks like NAnt or something. It isn't clear what interprets that but the Windows Installer will not. You have a couple options to get that registry value correct. First, change the RegistryValue element to look like:
<RegistryValue Root="HKLM"
Key="SOFTWARE\MyApp ID\MyApp ID Windows Client"
Name="Path"
Type="string"
Value="[MyAppBaseFolder]MyApp"
KeyPath="yes" />
Alternatively, you could define "MyApp" in your Directory tree and reference it directly. That'd look a little like adding the following as a child of MyAppBaseFolder (peer of INSTALLFOLDER):
<Directory Id='MyAppFolder' Name='MyApp' />
Then updating your RegistryValue element to look like:
<RegistryValue Root="HKLM"
Key="SOFTWARE\MyApp ID\MyApp ID Windows Client"
Name="Path"
Type="string"
Value="[MyAppFolder]"
KeyPath="yes" />
I prefer the latter but that's just because I prefer using identifiers without concatenating strings.