Getting an error while trying to debug an ASP.NET (WebForms) app that I've migrated

I'm trying to migrate an ASP.NET 2.0 app (WebForms) 开发者_开发知识库to ASP.NET 3.5. I've finally fixed a 500.19 IIS 7.5 error I was having. Now it will run, so long as I don't attempt to debug it. If I do attempt to debug it I get an error message. I've searched for help on this error and there's precious little out there. Here's the text of the error message:

Unable to start debugging on the web server. Check for one of the following.

1. The application you are trying to debug uses a version of the Microsoft .NET Framework that is not supported by the debugger.
2. The debugger has made an incorrect assumption about the Microsoft .NET Framework version your application is going to use.
3. The Microsoft .NET Framework version specified by you for debugging is incorrect.

Please see the Visual Studio .NET debugger documentation for correctly specifying the Microsoft .NET Framework version your application is going to use for debugging.

Well, as far as I know, I'm using the correct version of the .NET Framework (3.5) for this app. Although I am not as familiar with IIS 7.5 on Windows 7.

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I found that I had to get into IIS and change the application pool that my upgraded ASP.NET app was using. It had been assigned to an application pool that's associated with .NET 4.0. I changed it to one related to .NET Framework 2.0.