Sometimes we use some third party tools that create external dependencies to our SharePoint projects. For example if you use Pattern and Practice SharePoint Guidance Library in your SharePoint project in visual studio and build WSP, then WSP package will include these SharePoint Guidance libraries also. However, this might create problems related to deployment. To explain the issue, let’s consider how SharePoint solution retraction/removal might affects other WSP package deployed in the same SharePoint environment.
Scenario: Two WSPs sharing Common Dependency
Let’s consider you have two Visual studio SharePoint project: SharePoint Project 1 and SharePoint Project 2. Both projects are using (i.e., referencing) a third party dll (i.e., thirdparty.dll). Now if you generate WSP from two different Visual Studio projects, both WSP will include the thirdparty.dll in their WSP package.
Figure 1: A single thirdparty.dll is shared by two WSP solutions
As shown in the image above, a single third party dll is referenced by two Visual studio SharePoint projects. Now when you 'will build two different WSP solutions the same third party dll will be included to two different WSPs. If these two WSPs are deployed in a single SharePoint server/farm with GAC deployment, then both WSP will deploy the same ThirdParty.dll in the GAC. Everything will work fine till now.
Problem: retracting/removing one solution will fail other WSP to work
Now consider a scenario. After deploying the two WSPs in the production with GAC deployment, you have found a problem with Project 2 WSP. So you want to remove the Project 2 WSP while you will keep Project 1 WSP. So you will remove the Project 2 WSP. Removing the Project 2 WSP will remove all the assemblies from GAC/BIN related to the WSP. So removing the Project 2 WSP will remove thirdparty.dll from GAC. As soon as you remove the Project 2 WSP from SharePoint server/farm, Project 1 WSP will fail to work as it’ll not find the referenced thirdparty.dll in GAC. So removing Project 1 WSP (or project 2 WSP) will remove the thirdparty.dll from GAC and other WSPs which are using the same dll will fail to work.
Solution: Package your external dependency to new WSP
The solution to this problem is to exclude thirdparty.dll from both WSPs and create a new WSP dependency/prerequisite solution for thirdparty.dll. You can do it easily with SharePoint 2010 Package editor. As a result the thridparty.dll will not be included in any of the two WSPs.To package a new WSP for the thirdparty.dll you need to create another new Visual Studio SharePoint projects which will just include the thirdparty.dll (or any other prerequisites). The new architecture is shown below:
Figure 2: Shared thirdparty.dll is included in separate WSP
As shown in the figure 2, now the shared dll (thirdparty.dll) is excluded from Project 1 and Project 2 WSP. Whether you use WSPBuilder or SharePoint 2010 project template in Visual studio, you can exclude one more dlls from the WSP. Then create a new Visual Studio project (WSPBuilder or SharePoint) which will include the thirdparty.dll and the output WSP package will include the shared/external dll. Now you will have three WSPs and the new WSP will be prerequisite WSP for other two WSPs.
Now with this model, you can remove Project 1 WSP or Project 2 WSP from your server without affecting each other. Even if you remove Project 1 WSP from the Farm/Server, the thirdparty.dll will not be removed from GAC as it’s not deployed as part of Project 1 WSP. Later if you want to redeploy the Project 1 WSP again then you don’t need to redeploy the prerequisite WSP as it’s already deployed in the server. If needed you can package your external dependency in more than one WSP packages.
If you are developing a SharePoint custom product that will be deployed in some client’s environment, then you should care about creating one or more dependency WSP packages. If your SharePoint solution is developed for a specific client or as an in-house product then you might not face the problem even if you don’t create a prerequisite WSP package. However if you are developing a SharePoint solution as a custom product and you don’t know the client yet, you should create prerequisite WSP package. And then when you will deploy your product in client server, you can deploy the perquisite or not depending on if the client has the prerequisite dlls install or not. If the client has the dlls/prerequisites installed in the farm, you don’t need to deploy the prerequisite WSP. This will make your SharePoint product more independent and will have less impact on other SharePoint solution deployed in the farm.