Why is Sharepoint 2010 restricted to .Net 3.5?

I just came in contact with development for Sharepoint 2010. What I don't understand is: why are all Sharepoint project restricted to use the .Net 3.5 framework?

I understand that Sharepoint 2010 is developed on top of framework 3.5 but why cant i reference Sharepoint 2010 dlls in a .Net 4.0 project? For example the Client Object Model?

As I understand it a .Net 4.0 project can use .Net 3.5 Assemblies without problems normally. Why does开发者_C百科n't this work for Sharepoint 2010 assemblies?

Thanks for your help! Rocko

What kind of project is this?

If it is a WebPart or WorkFlow project then it would be running under a SharePoint 2010 instance, and that would therefore require .NET 3.5.

A partial solution I have seen is to create a .NET 4.0 WorkFlow hosted on Windows AppFabric ( http://msdn.microsoft.com/en-us/windowsserver/ee695849.aspx ) , with which SharePoint 2010 can interact with fairly easily.