No, you can't write a true ISAPI filter in C#. ISAPI requires that the .DLL export the functions required by the interface, but managed code .DLL's do not export functions in a manner compatible with the requirement. HttpHandlers offer the equivilent functionality.
A guide to posting questions on CodeProject[^]
Dave Kreskowiak Microsoft MVP Visual Developer - Visual Basic
2006, 2007, 2008