SQL CLR用户定义函数中的C＃支持程度？

This may be naive but I cannot get any confirmation of this: When I write a SQL function via the SQLCLR and as a C# SQL Server Project, can the SQL function include any method/class/namespace in the .NET BCL? Are there any restrictions on what the function can do? I have full control of the SQL Server and its hosting OS, so I can amend security settings at that level. I am more interested in the language support for C# in a SQLCLR user-defined function (SQL project).This may be naive but I cannot get any confirma