How to modify the default install path in an MSI-based setup based on a registry value

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I got a question from a customer this week asking how they could modify the default installation path in their MSI-based setup package based on a value they wanted to retrieve from the registry. Typically this kind of modification is desired if your setup shares files/components with another MSI, and that other MSI can be installed to non-default locations by another setup package. The steps to accomplish this are roughly the following. Please note that I am basing this algorithm on how we accomplish this inside of Visual Studio setup (I also described what happens behind the scenes in VS setup in more detail here). There may be alternative ways to accomplish the same result.

  1. Create a new entry in the AppSearch table that contains the Signature_ you want to search for and a specific Property name that will store the data you retrieve
  2. Create an entry in the RegLocator table that has a Signature_ column name that matches the Signature_ column name you added to the AppSearch table in step 1 above and that contains the key/value you want to retrieve and use if it exists on the user's system
  3. Create an entry in the CustomAction table that has a Target value that matches the Property column name you added to the AppSearch table in step 1 above (and is in square brackets since this is an MSI property and it needs to be referenced in square brackets to resolve correctly here), and has a Source name that matches the name of the directory in the Directory table that you want to change the default installation path for
  4. Add an entry to the InstallExecuteSequence table that has an Action column name that matches the name of the custom action added in step 3 above and has a Condition column name that matches the Property column name you added to the AppSearch table in step 1 above. This will ensure that you will only try to change the installation directory if the registry value you are searching for exists on the user's system, and your MSI will use the default installation directory if the registry value is missing.