WIX: Set a property based on a condition

Question

This should be easy, but after several hours I’m coming up blank. ;(

I do a Registry Search (actually 2), because I need to check for either of 2 previous installs and then install my new files to the location of the prior install that was found.

• Only one of these prior installs will actually exist.

I then need to install my new files to the 'InstallLocation' of which 'PROD#' was found.

<!—Look for the UnInstall key of the 1st possible product -->
<!— GUID = {E928E024-DEFE-41A7-8469-D338212C4943} -->
<Property Id='PROD1'>
  <RegistrySearch Id='PROD_REG1' Type='raw'
              Root='HKLM' Key='$(var.REGKEY_PROD1)' Name='InstallLocation' />
</Property>

<!—Look for the UnInstall key of the 2nd possible product -->
<!— GUID = {A40A9018-DB9D-4588-A591-F012600C6300} -->
<Property Id='PROD2'>
  <RegistrySearch Id='PROD_REG2' Type='raw'
              Root='HKLM' Key='$(var.REGKEY_PROD2)' Name='InstallLocation' />
    </Property>

<!-- ? How to set INSTALL_HERE Property to whichever ‘InstallLocation’ was found ? -->

<!-- Define the directory structure -->
<Directory Id="TARGETDIR" Name="SourceDir">
  <Directory Id="INSTALL_HERE">
    <Directory Id="MAIN_DIRECTORY" Name="MyProduct"/>  
    <Directory Id="HELP_DIRECTORY" Name="Help"/>      
  </Directory>
</Directory>

Answer 1

The following will set properties A and B to the result of two different registry searches. If the search for B was successfull, it overrides the value of A with the value of B.

  <Property Id="A">
     <!-- registry search 1 here -->
  </Property>

  <Property Id="B">
     <!-- registry search 2 here -->
  </Property>

  <SetProperty Id="A" After="AppSearch" Value="[B]">
     B
  </SetProperty>

Note how the SetProperty element uses the value of B twice: once as Value="[B]" to override the value of A, and once as the condition text of the custom action. The After="AppSearch" ensures that the custom action is executed right after the registry searches.

See also this answer of Rob Mensching