Merge Modules

In my previous post, I mentioned a method for automating the addition of features (specifically properties) to a Windows Installer package generated by Visual Studio. After doing some more research, I encountered the wonderful land of merge modules.

Merge modules are just what they sound like; they allow you to merge features into a Windows Installer package that you are creating. Among other things, you can set default properties by including a Property table in the merge module.

Creating a merge module is not difficult but requires a bit of knowledge. Once again, you need the lovely Orca tool from the Windows Platform SDK.

Orca, from what I found, doesn't seem to be able to create a blank merge module template. So I had to do it by hand. Starting with a blank new file, you will need to add the following schema:

Table	Column	PK	Type	Nullable
Component	Component	X	String (72)	N
	ComponentId		String (38)	Y
	Directory		String (72)	N
	Attributes		Short Int	N
	Condition		String (255)	Y
	Key Path		String (72)	Y
Directory	Directory	X	String (72)	N
	Directory_Parent		String (72)	Y
	Default_Dir		Local String (255)	N
FeatureComponents	Feature_	X	String (38)	N
	Component_		String (72)	N
ModuleComponents	Component	X	String (72)	N
	ModuleID	X	String (72)	N
	Language	X	Short Int	N
ModuleSignature	ModuleID	X	String (72)	N
	Language	X	Short Int	N
	Version		String (32)	N

Some of these tables are predefined in Orca, so you won't need to enter all of them by hand. Most of the tables don't actually have to have data, they just need to exist for the file to be considered a valid merge module.

The only table that requires data is the ModuleSignature table. It must contain at least one row identifying the merge module. The format for the ModuleID is a GUID separated by underscores (_). The Language is 1033 for US English; you will need to look this code up if you are using other locales. Version is the typical Windows Installer version format: n.nn.nnnn.

Once you have that basic structure in place, you can enter your additions into the merge module. The merge module can have pretty much anything a full install package can have. Once you're done, save the file with a .msm extension. You can then go in to Visual Studio and add the merge module to your deployment project.

Technically, there is one more thing you really should have in your merge module: a _Validation table. While not strictly necessary for use, this table is used by Orca to run validation against the merge module.

One caveat I did find with using a merge module to preset properties in deployment project builds: you can't use a merge module to override existing entries in the Property table. You can only create new properties. I was able to get the majority of the setup I wanted done automatically, I still have to go in a manually edit my final .msi file to accommodate all of my requirements.