Ran across this one today, and because it’s weird and annoying I thought I’d post a solution. You can thank me later!
The problem goes like this: You’re using Visual Studio 2010 and trying to add a control to an APSX page in an ASP.NET Web Application targeted to .NET 4.0, and some controls don’t show up in IntelliSense. When you type the tag manually, Visual Studio complains that
Element 'ListView' is not a known element. This can occur if there is a compilation error in the Web site, or the web.config file is missing.
The application builds fine, and doesn’t throw any errors when you run it.
After a bit of Googling, I found the solution on the ASP.NET forums:
Close Visual Studio, delete the schema cache, and re-open Visual Studio. You can find the schemas under something like:
It is safe to delete all files in this folder.
Delete the contents of that above folder, and all is well.