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COM Interoperability in .NET



Namespace: System.Runtime.CompilerServices

Why we need Interoperation?
A company may have a large application that is already tested, verified, and is stable. And they are interested to use new technology while keeping the old at its position.

Secondly, not every windows API has been wrapped in with .NET Framework.

.Net provides ample support for COM interoperation to import and use type libraries. Runtime Callable Wrapper (RCW) is used to bridge the communication between the COM and .NET.

Importing Type Libraries:
There are two ways to Import libraries.

1.       Using Add Reference Feature in Visual Studio 2005 and selecting appropriate COM component.
2.       Using tlbimp.exe. In command prompt go to the location where your assembly is, then execute the command “tlbimp.exe MyDll.dll [/out:ManagedMyDll.dll]

In C# optional parameters are not supported, so if you want to pass something as null, use Type.Missing. This is because COM Objects do not support parameter overloading, and they require you to pass in objects as a reference.

You can use: object obj = Type.Missing;

Tools used by COM Interop

TlbImp.exe (Type Library Import)
It imports a COM component to be consumed by the .Net Application.

TlbExp.exe
It exports a .Net Assembly sot that it can be consumed by the COM objects.

Regedit.exe
All COM components require to get registered in Registry, so it’s the right tool for them.

Ildasm.exe
Intermediate Language Disassembler to view intermediate language code.

Regasm.exe
It enables you to add/remove .NET assemblies from system registration database.


Handling Exceptions in COM Interop:
 In .Net framework 2.0, exceptions thrown from the .NET or from outside the .Net Framework are caught by catch (exception ex) {} as usual. But you should first look for the System.CompilerServices.RuntimeWrappedException, which have the property WrappedException that provides you the actual exception thrown outside the framework.

Limitations of the COM Interop:

Static/Shared Members: .Net framework does not support static/shared methods.
Parameterized Constructors: COM doesn’t allow parameterized constructors.
Inheritance: Shadowed members in COM component are not recognizable / callable.
Portability: COM components require registry that is not be available on platform other than windows.

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