getting error while converting target framework of an asp.net web application from 2.0 to 3.5

While converting the taget framework of an asp.net web application from 2.0 to 3.5 in visual studio 2008, i am getting开发者_StackOverflow中文版 the error : Exception of type 'System.Runtime.InteropServices.ExternalException' was thrown

I solved the issue, my web application didn't have a web.config.I added a deafult web.config and then converted it.It worked fine.

I recently upgraded the target framework in the my web project's (Asp.NET web forms) properties from 4.8 to 4.8.1.
I searched the web.config for any "targetFramework" where it was referencing older versions and changed them to "4.8.1". Save. Recompile.