My ASP.NET application runs in IIS on my web server and uses Microsoft .NET Framework 4 Beta 2. (Its Application Pool is set to .NET Framework version .NET Framework v4.0.21006.)

It gives this new error:

A potentially dangerous Request.Form value was detected from the client...

This is due to a breaking change in .NET 4.

To revert to the behavior of the ASP.NET 2.0 request validation feature, I added the following setting in the Web.config file:

<httpRuntime requestValidationMode="2.0" />

Now Visual Studio 2008 throws a compile-time error:

The 'requestValidationMode' attribute is not declared.

And I can no longer debug on my development machine using the ASP.NET Development Server that comes with Visual Studio.

I need Visual Studio and its ASP.NET Development Server to recognize the new .NET Framework 4 requestValidationMode attribute.

How can I debug my application in .NET 4? Must I switch from Visual Studio 2008 to Visual Studio 2010 Beta 2?