There are several reasons .NET Framework accommodates interoperability between old and new technologies. First, many companies have extensive libraries of legacy code that they cannot simply rework into new libraries as a result a mechanism that allow to bridge old legacy code with new applications requires. Second, many companies trying to do migration one piece at a time as a result there is a need to integrate existing libraries with new libraries while migrating entire application. Windows based libraries of the past relied mostly on COM architecture and as a result interoperability was designed to bridge .NET framework and COM specifically. This bridge is the Runtime Callable Wrapper (RCW) which helps to marshal data, handle interfaces and events.

We can import libraries via Visual Studio 2005 via Add Reference option that provides us with the tab called COM and the list of all registered COM components available. Alternatively we can use TlbImp.exe. There is one important thing to remember, COM does not allow parameter overloading, so you need to pass all COM parameters and since they use references, null cannot be passed. In C# we need to create object variables and then pass them or the better way is to use Type.Missing field instead.

class Program
    {
        private static Object OptionalParamHandler = Type.Missing;
        static void Main(string[] args)
        {
            Application NewExcelApp = new Application();
            NewExcelApp.Worksheets.Add(ref OptionalParamHandler,
                ref OptionalParamHandler, ref OptionalParamHandler,
                ref OptionalParamHandler);
        }
    }

There are many tools that allow .NET Framework to work with COM

 

Name	Application Name
Type Library Importer	TlbImp.exe
Type Library Exporter	TlbExp.exe
Registry Editor	Regedit.exe
Intermediate Language Disassembler	Ildasm.exe
Assembly Registration Tool	Regasm.exe

Example of COM usage within .NET Code

axAcroPDF1.LoadFile(@"SamplePDFDocument.pdf");
axAcroPDF1.Print();

If we need to handle COM Exceptions we need to employ RuntimeWrappedException class with following Properties

 

Name	Description
Data	Collection of key/value pairs
HelpLink	Sets a link to the help file
InnerException	Manages the Exception
Message	Manages a message about exception
Source	Manages or sets the name of the application
StackTrace	Manages a string representation of the frames
TargetSite	Manages the method that throws the current exception
WrappedException	Manages the object that was wrapped by the RuntimeWrappedException

There are several limitations in COM Interop .NET Framework

• No support for static or shared members
• COM does not allow parameterized constructors
• Limits in using inheritance
• Non Windows OS don’t have registries that limits .NET environments