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pluginsmefversioning

MEF Plugin update strategies for winforms

I'm developing an extensible application with MEF. The application will have many types of plugins to collect and process data in different ways.

I'm thinking about building a versioned online repository for the plugins, that will enable the user to download new versions of the plugins when they become available.

It would be nice if MEF could load different versions of the same plugin simultaneously, though from what I understand this isn't possible (correct me if I'm wrong).

So I've resigned myself to the fact I will need to update the plugin and archive the previous version.

What would be the best strategy for doing this?

Example 1

The Application downloads a new version of a loaded running plugin. I can't place the plugin in the plugin directory as there is already DLL with the same name. So I could rename the new plugin with a version suffix. I can't load the same assembly, so I guess I'll have to force a restart. So on restart, it achieves the old plugin and loads the new one.

--- This seems solutions seems a little messy

Example 2

The Application downloads a new version of a loaded running plugin. The plugin is encased in some type of installer. The installer closes the host gracefully and archives the existing plugin. The installer installs new plugin and restarts the host app.

--- this also seems a little messy

I am seeking any correction of my assumptions, or any insight into a successful strategy to my achieve my goal.

Solution

  • The .NET Framework has a fearure called Shadow Copy which allows you to update loaded assebmlies. Basically it copies the assemblies to a temp folder and loads them from there. This way the assemblies located in your application's installation folder will not be locked by the OS and you will be able to replace them. ASP.NET, unit test framweorks and many other applications use shadow copying.

    To enable this feature you will need to load your application in a new AppDomain since you cannot enable shadow copying on the main AppDomain. You can create a simple loader that will create an AppDomain and execute your application there. This is very straight forward. For an example of MEF + Shadow Copy have a look at Glenn Block's Way of MEF and in particullar the PartUpdatesInPlace sample.

    Now as far as versioning is concerned you will need to be able to have two or more versions of an assembly loaded at the same time in the same application domain. There are two ways to do this:

    1. Strong named assemblies in the GAC.
    2. Assemblies with the version included in their name (like Plugin.v1.dll). Strong naming is optional in this case but a good idea nonetheless. The advantage of this approach is that two or more versions of a plugin can coexist in the same directory.

    Have a look at this answer for an example of MEF + Versioning.

    You can even use the recomposition feature of MEF and have your plugin container updated after:

    • A new plugin assembly is added
    • A plugin assembly is deleted
    • A plugin assembly is replaced

    Have a look at this question for an example.