I recently upgraded from Visual Web Developer 2008 to Visual Web Developer 2010 and have run into an issue, I started seeing a configuration error. In two cases I had been working on web applications in Visual Web developer 2010.

Case 1 appeared after I had opened an existing application and when prompted, do I want to configure the site for use with ASP.NET 4.0, I must have said yes.

Case 2 came when I created a new application and my system is setup to use Framework ASP.NET 4.0

In both situations I got the following error <compilation debug="true" targetFramework="4.0" /> pointing to my web.config file. I like working in Visual Web Developer 2010, but my hosting server is not yet setup for .Net Framework 4.0, so I needed to find out how to down-grade from 4.0 to 3.5 framework.

The following is what I did to change the target Framework from ASP.NET 4.0 to ASP.NET 3.5.

1) Ensure IIS and the ASP.NET properties are configured for Framework 2.0. Note: Framework 3.5 will not show up in the list of installed options due to the fact that framework 3.5 is an extension of 2.0 and not a stand alone release

 2) Configure you web application to use target Framework 4.0 by right clicking your website in the solution explorer >> Property Pages >> Build >> Change "Target Framework" to .NET Framework 3.5.