The difference between meta programming and IL weaving
Ivan also posted a response to Jeremy's C# vNext features, and he said something that caught my eye:
5. Metaprogramming => no objection here although you can do it now with reflection emit
No, there is a big difference between reflection emit and byte code weaving. Reflection emit is done after compilation is completed, meta programming occurs during compilation. This matters because it means that your code cannot refer to the results of the change being made.
A case in point, I am using IL Weaving to add the Dispose method to a class, but because I am doing it after compilation is completed, I cannot call the Dispose() method on the class, or put it in a using statement, in the same assembly, I would get a compiler error, because the method (and the interface) are not there yet.
Using meta programming, the compiler will know that those now exist, and it can not start using it, because it happened during the compilation process.
The implication of that are pretty significant, if you are talking about what you can and can't do in terms of enriching the language.