When trying the example using VS 2010 and .net 4 I received the following error on the definition of au3 "Interop type 'AutoItX3Lib.AutoItX3Class' cannot be embedded. Use the applicable interface instead". This is exactly what I did: changed AutoItX3Lib.AutoItX3Class (class) to AutoItX3Lib.AutoItX3 (interface). Problem solved.