Can you mix .net languages within a single project? So pre-compiled, I would like to call classes and methods of other source files.
For both web and apps?
In particular I'd be interested in F# and C#.
CMS mentions an interesting approach, but in reality I would suggest you keep things simple and have different assemblies (projects) for the C# and F# code. There are well documented communication points between C# and F# (such as here and here) - I'd recommend them instead. The differences between C# and F# (especially with F# having a different runtime!) are quite large...