You Cannot Use SharePoint Foundation 2010 Objects with .NET 4

The SharePoint Foundation 2010 object model is not accessible using the .NET Framework 4 (or later).  Calling any object or method in the object model using any framework version other than .NET 3.5 will throw the following exception: 

System.PlatformNotSupportedException
Microsoft SharePoint is not supported with version 4.0.30319.1 of the Microsoft .Net Runtime.

Of course, you can always access the SharePoint objects using the web services interface from your .NET 4 application, but this will only allow limited functionality of the complete object model.