It's not just publicly accessible, it's MIT licensed with a patent grant. You could take the code, modify it in literally any way you want, wrap it in a commercial product and resell it. It doesn't come more free or open source without actually being public domain.
What you're asking for appears to be some sort of 'super open source' where not only are free to view, modify, and distribute your code, but also have to a right to have your code merged into the mainline of the code. What you're asking for goes well beyond the four freedoms.[0]
The PATENTS.txt file seems to suggest otherwise, you must stay within the bounds of "covered code". IANAL so I do not know what the side effects of this seemingly intersection of their patent/license coverage curtails, but it does seem like you lose protection and could have a Android/Oracle situation if you innovate something on top of the core CLR outside of the 'covered' bounds and then decided to sell it.
There's a slightly older writeup about it here http://endsoftpatents.org/2014/11/ms-net/
In any case, as the name suggests, it's only a promise, not a guarantee of rights.
It's FOSS.
