Can I develop for .NET Framework 4 in Visual Studio 2008?

Question

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?

Answer 1

You cannot use Visual Studio 2008 to develop 4.0 applications in a supported manner. Currently only Visual Studio 2010 can be used to develop 4.0 apps. Starting with 2008 you can use it to develop for earlier versions of the framework but not later ones.

Answer 2

From ScottGu's blog:

There isn't any way to target .NET 4 from VS08 and use new features. Having said that, .NET4 is upwards comaptible with .NET 3.5 - so applications you build targeting .NET 3.5 with VS08 should work fine on top of .NET 4.