Because Microsoft has made it part of their core business to extract patent royalties from distributors of free software (particularly on smartphones), I decided to take a look at the patent safety offered by the licence of the parts of their .NET framework that they’re releasing as free software:
The first limit is that you’re only protected if you’re distributing the code “as part of either a .NET Runtime or as part of any application designed to run on a .NET Runtime“. So if you add any of the code to another project, then you lose protection and MS reserves the right to use their patents against you.
Secondly, the protection only applies to a “compliant implementation” of .NET. So if you want to remove some parts and make a streamlined framework for embedded devices, then your implementation won’t be compliant and the protection doesn’t apply to you.
Form and context
Another question is about the impact of Microsoft calling this a “personal promise” instead of a “licence”: Can Microsoft sell the patents to a troll and watch the troll sue everyone? Probably not. This has been tested in court once that I’m aware of. It’s the 2012 “in re Spansion” case in the USA and the judge ruled that a promise is the same as a licence. (See The value of promises and estoppel defences)
And last comes the question of an implicit patent licence. Since Microsoft have released the software under a free software licence (the MIT/X11 licence), giving all users the freedoms to use, study, improve, and share the software, have they thus implicitly promised not to sue the users for doing this things? As far as I know, there’s no case law on this. I’ve heard that this idea might work in the USA but might not work in the UK, but I got it from an unreliable source and I don’t know his sources. In short, it’s not something you can rely on, but if you’re accused of patent infringement it might be worth a try.
How is this different to the GPL?
Code distributed under the GNU GPLv3, comes with a patent grant which basically says the contributors can’t use their patents against the users for exercising the freedoms granted in the licence:
Each contributor grants you a non-exclusive, worldwide, royalty-free patent license under the contributor’s essential patent claims, to make, use, sell, offer for sale, import and otherwise run, modify and propagate the contents of its contributor version.
This patent licence looks fine for users of the code published by Microsoft, but its protections disappear very quickly for those who wish to modify or re-use the code.