Sometime in the (near?) future, dotnet (or just C#?) will have "partial classes", but from what I've seen, they're these rigidly textual file-based things. Essentially they allow "code generators" to generate some of the code for you.
The other thing to note is that there's no way in hell you'd be able to build this mechanism yourself. The language and it's tools are too complex, overdependent, and rigid. On the other hand...
Even 25 years ago, programmers have been able to add features such as multiple inheritance into Smalltalk by themselves. In the last couple of years, some programmers added a very expressive kind of "partial classes" called "traits" to Smalltalk, again by themselves. And they provided integrated tool support within the existing IDE.
Note that Microsoft Research is funding research for traits with C#. You can bet this is a pain in the ass. Give up and use Smalltalk!
I'm reminded of all this by James Robertson's item on traits being ported from Squeak to Cincom Smalltalk.