Considering this question of SO, where whole C# in-memory compiler is being called. When only lexical and syntactic analyzing is required: parse text as a stream of lexemes, check them and exit.
Is it possible in current version of System.CodeDom.Compiler, if not - will it be?
If you can use Mono, I believe it has a C# parser/lexer you may be able to use.
Here's a link to look into. As for what the MS C# team is planning to do, there is some talk of at some point making the C# compiler into a "service" - but it's unclear what that means or when that will happen.