COM INTEROPERABILITY in .Net

COM Interoperabillity

•COM à Reusable

•COM à written in one language (c++) and consumed by other language (VB)

•COM has Metadata : stores all info stored inside type library like name, Id of interface, methods and argumetns.

•COM is Not Extensible à drawback

•COM   Issues

    –METADATA       à Type Library – TLB 

      –FREEING MEMORY  à Com relies on Ref. Count, .Net  on GC

            •IUnknown interface required to be implemented by every com obj.

              –AddRef(), Release() for destruction of object

–INTERFACE à IUnknown, IDispatch,ICollectionEvent

–METHOD BINDING- .Net uses Reflection, Com uses IDispatch

–DATA TYPES à Variant type   , .Net class has Object

–REGISTRATION à CLSID – 128 bit, ProgID (Excel.Application)

–THREADING   à STA and MTA

–ERROR HANDLING à HResult , .Net uses Exception

–MARSHALLING à  COMà SafeArray  à Array  in .net             Variant in COM  à object in .net     BStr  à String  , IUnknown,IDispatch  à Object