The problem with assembly "parsing" is that we have less informations about line and file (the informations is based on .pdb file, and Pdb contains lines informations only for methods)
Consider to use reflection on a built binary instead of parsing the C# code directly. The reflection API is really easy to use and perhaps you can get all the information you need?
Have a look at Gold Parser. It has a very intuitive IU that lets you interactively test your grammar and generate C# code. There are plenty of examples available with it and it is completely free.
I've implemented just what you are asking (AST Parsing of C# code) at the OWASP O2 Platform project using SharpDevelop AST APIs.
In order to make it easier to consume I wrote a quick API that exposes a number of key source code elements (using statements, types, methods, properties, fields, comments) and is able to rewrite the original C# code into C# and into VBNET.
For example this is how you process a C# source code text and populate a number of TreeViews & TextBoxes:
public void updateView(string sourceCode)
{
var ast = new Ast_CSharp(sourceCode);
ast_TreeView.show_Ast(ast);
types_TreeView.show_List(ast.astDetails.Types, "Text");
usingDeclarations_TreeView.show_List(ast.astDetails.UsingDeclarations,"Text");
methods_TreeView.show_List(ast.astDetails.Methods,"Text");
fields_TreeView.show_List(ast.astDetails.Fields,"Text");
properties_TreeView.show_List(ast.astDetails.Properties,"Text");
comments_TreeView.show_List(ast.astDetails.Comments,"Text");
rewritenCSharpCode_SourceCodeEditor.setDocumentContents(ast.astDetails.CSharpCode, ".cs");
rewritenVBNet_SourceCodeEditor.setDocumentContents(ast.astDetails.VBNetCode, ".vb");
}
The example on ascx_View_SourceCode_AST.cs.o2 also shows how you can then use the information gathered from the AST to select on the source code a type, method, comment, etc..
For reference here is the API code that wrote (note that this is my first pass at using SharpDevelop's C# AST parser, and I am still getting my head around how it works):
DMS provides a vast infrastructure for parsing, tree building, construction of symbol tables and flow analyses, source-to-source transformation, and regeneration of source code from the (modified) ASTs. (It also handles many other languages than just C#.)
EDIT (September) 2013: This answer hasn't been updated recently. DMS has long handled C# 5.0
We have recently released a C# parser that handles all C# 4.0 features plus the new async feature: C# Parser and CodeDOM
This library generates a semantic object model which retains comments and formatting information and can be modified and saved. It also supports the use of LINQ queries to analyze source code.
You should definitely check out Roslyn since MS just opened (or will soon open) the code with an Apache 2 license here. You can also check out a way to parse this info with this code from GitHub.