Pass MSI Property before Install Finalize - wix

Question

I have a variable in my burn bundle as such:
<Variable Name="DemoVariable" Type="string" Value="ChangedProperty" bal:Overridable="yes"/>
Which is then used with an MSIProperty:
<MsiPackage SourceFile="testFile.msi" Id="DemoPackageId_1" Cache="yes" Visible="no">
<MsiProperty Name="PassedProperty" Value="[DemoVariable]"/>
</MsiPackage>
In my testFile.msi I have a property and custom action:
<Property Id="PassedProperty" Value="Unchanged"/>
<Binary Id="CustomActionDll"
SourceFile="CustomAction.CA.dll"/>
<InstallExecuteSequence>
<Custom Action="ShowMessageBoxCA" Before="CostFinalize"/>
</InstallExecuteSequence>
<CustomAction Id="ShowMessageBoxCA"
Return="check"
Execute="firstSequence"
BinaryKey="CustomActionDll"
DllEntry="ShowMessageBox"
HideTarget="no" />
The ShowMessageBox function literally just calls:
MessageBox.Show(session["PassedProperty"])
The problem is that the message box shows the message "unchanged". I looked through the log file for the installation and I can see that the PassedProperty is being changed sometime after InstallFinalize finishes. This is obviously too late for my custom action which takes place before CostFinalize.
Is there a way to make Burn change the MSIProperty earlier in the process rather than making the custom actions take place later?
Edit:
Okay so I found the answer but I can't post it yet so I'll leave the information here in an edit.
Basically MSI's have Public and Private properties. Only public properties are declared before the install phase takes place. Public properties must be all uppercase so to fix my issue I just replaced all instance of "Passed_Property" with "PASSED_PROPERTY" and it works fine.

Answer 1

So the problem I had was that I was creating my properties as private properties, which aren't available until after the installation. To make your properties public and therefore available throughout the whole install, they need to be Uppercase.
<Property Id="PASSED_PROPERTY" Value="Unchanged"/>
https://msdn.microsoft.com/en-us/library/aa370912(v=vs.85).aspx
As soon as I made them public the properties were changed by Burn.