Avoid the Managed Extensibility Framework.

Update: Microsoft has now changed the license to the MEF to the open source MS-PL license. Please see the follow up post for details.

As a .NET developer, you should avoid using the newly released Managed Extensibility Framework as its license prevents its use beyond the Windows platform. This will prevent your .NET software from running on Linux or MacOS in the future.

Luckily, there is a cross platform solution available today that has no platform limitations: Mono.Addins, a technology inspired by Eclipse's own plugin system. We have tutorials, reference manuals, API docs, our FAQ our public groups and multiple large applications with source code available that you can learn from (MonoDevelop, Banshee, F-Spot and Gnome-Do).

The rule obviously applies to any new APIs that are built for .NET as they are not immediately available for Mono. But unlike the binary-only APIs, these half-open source code releases pose additional problems for the open source CLI:

• More people are exposed to the source code, preventing them from working on a fully open source implementation of it.
• There is a smaller desire from the community to reimplement the code, as it is relatively easy to just use the existing implementation or ignore the issue for the time being.
• Some folks might not care about the license restriction, and ignore it altogether. A practice I do not endorse.

There are two additional issues that are worth pointing out. Should non-open source libraries be hosted on CodePlex to begin with? CodePlex is branded as the "Open Source Project Hosting" from Microsoft, yet, this license is clearly not open source and does not qualify as open source according to the definition and is a clear violation of Codeplex.Com's requirements:

The above link points to http://en.wikipedia.org/wiki/Open_source_licenses.

MEF should be pulled out of the CodePlex site along with all other platform-limiting software, like ASP.NET MVC (h/t to gblock for pointing out that MVC is also violating the terms). Unless CodePlex cooks up a special exception that some software and restrictions are more equal than others.

The second point that is worth making is that picking licenses like the MS-LPL for .NET software is shooting the .NET community in the foot. The LPL license is obviously an effort to tie things into the Windows platform, putting company first, community and developers second.

The MS-LPL is a poisonous license, do not use it (do not confuse with the MS-PL which is a decent license, the extra "L" makes a big difference).

Update: gblock points out that CodePlex technically only requires a "license" to be picked, but does not require it to be OSI compliant. But even if the wording on the FAQ does allow for that interpretation, it is misleading as there is a link to OSI licenses next to it, and the main page clearly states that Codeplex is an "Open Source Project Hosting" site. Capital "O" and capital "S".

The "CodePlex Basics: Getting Started" page also points to the Open Source License Definition, there are no words about custom licenses or about hosting proprietary code with source code available.

Update2: Glenn has reached out to the community and clearly wants to improve the status quo when it comes to MEF, open source, CodePlex and the official policies. Good luck to Glenn.

It is also worth pointing out that Glenn was one of the early adopters and advocates of the MS-PL at Microsoft (to clarify: the MS-PL is an open source license).