c# - The type 'T' is not compatible with the type 'T' -

i have 2 libraries written in c# i'd use in f# application. both libraries use same type, however, unable convince f#'s type checker of fact.

here's simple example using office interop types. f# seems particularly sensitive these type issues. casting on f# side not seem situation. 3 projects have reference same assembly ("microsoft.office.interop.excel" version 14.0.0.0).

in project "project1" (a c# project):

namespace project1 {     public class class1     {         public static microsoft.office.interop.excel.application getapp()         {             return new microsoft.office.interop.excel.application();         }     } } 

in project "project2" (a c# project):

namespace project2 {     public class class2     {         microsoft.office.interop.excel.application _app;          public class2(microsoft.office.interop.excel.application app)         {             _app = app;         }     } } 

in project "testapp" (an f# project):

[<entrypoint>] let main argv =      let c2 = project2.class2(project1.class1.getapp())     0  

any hints?

edit:

changing call class2's constructor following dynamic cast solves problem:

let c2 = project2.class2(project1.class1.getapp() :?> microsoft.office.interop.excel.application) 

however, unsatisfying, since 1) dynamic, , 2) still don't understand why original type check failed.

com interop. when reference com assembly generate com interop assembly marshal between .net , com. if have 2 assemblies both referencing same com assembly may have 2 identically generated interop assemblies.

one solution, , frankly better design in general, create interfaces in 1 of 2 assemblies (or shared 3rd assembly) expose features want make public , instead of using or consuming com types use these interfaces instead.

namespace project1 {     public interface iapplication     {         // application members here...     }      public class class1     {         public static iapplication getapp()         {             return new excelapplication(new microsoft.office.interop.excel.application());         }          private class excelapplication : iapplication         {             public excelapplication(microsoft.office.interop.excel.application app)             {                  this.app = app;             }              // implement iapplication here...         }     } }