VS 2010 compiler error: Interop type XXX cannot be embedded. Use the applicable interface instead.

In most cases (such as error for the usage of UPnPNATClass as noted in one of the comments) this error is the result of code which tries to instantiate a COM object e.g. here piece of code starting up Excel:

Excel.ApplicationClass xlapp = new Excel.ApplicationClass();

Here it is enough to say that Excel.ApplicationClass derives from Excel.Application interface and one can even instantiate Excel using Excel.Application interface. Rewriting this code as below produces exact same results:

Excel.Application xlapp = new Excel.Application();

Ref: http://blogs.msdn.com/b/mshneer/archive/2009/12/07/interop-type-xxx-cannot-be-embedded-use-the-applicable-interface-instead.aspx