Wix: moving <Upgrade> into <Fragment> - wix

Question

I'm working on a suite of related applications that all have their own WiX installer. As you can imagine, there's a lot of duplication between these installers, and I'm trying to do something about that. To this end, I've created a WiX Library Project with some .wxs files, and I've been trying to shove as much code as possible into these .wxs files. Which has worked mostly fine, so far. However, there's one thing that I can't seem to move to a .wxs: the Upgrade element.
This is my upgrade code:
<Upgrade Id="MY-UPGRADE-CODE-GUID">
<!-- If an older version is found replace it -->
<UpgradeVersion OnlyDetect="no" Property="OLDERFOUND"
Maximum="$(var.ProductVersion)" IncludeMaximum="no"/>
<!-- If the same version is found do nothing (OnlyDetect) -->
<UpgradeVersion OnlyDetect="yes" Property="SELFFOUND"
Minimum="$(var.ProductVersion)" IncludeMinimum="yes"
Maximum="$(var.ProductVersion)" IncludeMaximum="yes"/>
<!-- If a newer version is found do nothing (OnlyDetect) -->
<UpgradeVersion OnlyDetect="yes" Property="NEWERFOUND"
Minimum="$(var.ProductVersion)" IncludeMinimum="no"/>
</Upgrade>
<CustomAction Id="AlreadyUpdated" Error="This version of !(wix.Param.Upgrade.ProductName) is already installed. You will have to uninstall it manually before this installer can continue."/>
<CustomAction Id="NoDowngrade" Error="A newer version of !(wix.Param.Upgrade.ProductName) is already installed. You will have to uninstall that version manually before this installer can continue."/>
<InstallExecuteSequence>
<RemoveExistingProducts After="InstallInitialize"/>
<!-- If the same version is found execute AlreadyUpdated -->
<Custom Action="AlreadyUpdated" After="FindRelatedProducts">SELFFOUND</Custom>
<!-- If a newer version is found execute NoDowngrade -->
<Custom Action="NoDowngrade" After="FindRelatedProducts">NEWERFOUND</Custom>
</InstallExecuteSequence>
For Upgrade/#Id, I use the same value as for Product/#UpgradeCode. (I'm not sure whether that's necessary, or even good practice.) I'd like to put that in a WixVariable at some point but for now I'm trying to make it work with literal GUIDs.
I have already moved the CustomActions and the InstallExecuteSequence to a Fragment in my .wxs file. I have defined a WixVariable to contain the ProductName, so I can have each app's installer display its own name. This works exactly the way I want and expect.
However, when I move the Upgrade element into a Fragment (it doesn't matter if it's in the app's Product.wxs file or in one of the library .wxs files, and it also doesn't matter if the Fragment is the same one that contains the InstallExecuteSequence), the following problems occur:
If I first run the old version of the installer, and then the new one with Upgrade moved into a separate Fragment, but with identical Product/#Version, Product#/UpgradeCode, and Upgrade/#Id, I end up with 2 installs of the same product, which is obviously not what I want.
If I install the new installer twice, it won't install the second time (which is what I want), but it won't show my error message either.
I would replace the whole thing with a MajorUpgrade, were it not for the facts that 1) it doesn't seem to want to give an error message when the same version is installed again, and, more importantly, 2) it's not allowed inside a Fragment for some reason, so I'm still stuck with duplication (albeit a lot less than previously).
So ... how can I move my Upgrade element into a .wxs library without losing the current functionality? What am I doing wrong?

Answer 1

As described in the wix Linker documentation, the linker will follow references between fragments while building a windows installer database. But this means that it will ignore any fragments that aren't referenced (directly or indirectly via other fragments) by the product wxs.
This behavior can be quite useful in your scenario, because it allows you to model the dependencies between all your reusable components, and the linker will make sure to include only the ones that are needed for a specific product.
To make sure that your upgrade logic is included, you have two alternatives:
Add a dummy Property to your fragment, and reference it with a PropertyRef in your product wxs.
Or put your upgrade logic in an Include element, and then instruct the wix preprocessor to include it in your product wxs. See the Wix Preprocessor documentation for an example.