Partial classes can be pretty cool because of the opportunities it gives in separating machine or designer generated code from user code. It does present a few issues, however.
One that we've been talking about recently is a scenario where you have some machine generated code that can function by itself, but sometimes your user wants to extend it. And, to extend it, the user needs to have some code run as part of the constructor.
That presents a problem. If we want the user to be able to use the class by itself, it needs to have a constructor in the generated code (assuming, of course, that the generated code needs a user-written constructor, which is true the majority of the time), but that prevents the user from writing a constructor themselves.
We talked about this from a design perspective, and explored some ways in which you could write “partial methods” - for want of a better term - in your partial classes. It is possible, but in all the schemes we came up with, you end up with weird ordering constraints (“I want the user constructor code to be inserted after the first 2 lines in the constructor but before the remaining 10 lines...”)
So, basically, those schemes were all untenable.
How can you do this? Well, the best way is to provide some designer support to add a call to a user method at the appropriate place.