What do you want to do in your C# functions? If you need to call a method from a report property (i.e. calulation, formatting, etc.): Create a class library project and define a class with static methods, build the DLL and copy it to the the ReportServer\bin folder. In the Report Properties dialog for a report, set a reference to the assembly and then refer to each method as =AssemblyNameSpaceName.ClassName.MethodName(YadaYa da)
This is covered (briefly) at the end chapter 5 in the SSRS 2000 book and there will be more examples in the 2005 book (due out next month.) There are also some examples of writing custom extensions in C#.
Paul Turley, MCSD, MCDBA, MCT, MSF Practitioner