Accessing a COM component

Tools Required:
TlbImp.exe: (Type Library Importer) Imports (converts, generates wrapper DLL) the type defination found in COM component into equivalent .net definations (or metadata), understandable by Common Language Runtime (CLR). The metadata generated, by TlbImp can be viewed by Ildasm.exe. If you are using the Visual Studio development environment, you only need to add a reference to the COM type library and the conversion is done automatically.

Let's Start:
To use exisiting COM+ application we need create Runtime Callable Wrapper (RCW) using TlbImp.Exe, with VS.Net it is created automatically, when we reference the exisiting COM Component. The difference is that, VS.Net create the RCW with the same name as the original DLL, which may be confusing, and with TlbImp.Exe, we can specify the different name using /out: parameter.

Let us assume that we have a COM Component with a method Add, which takes two parameter A and B and returns the Sum.

'CompAdd.Dll 
(class1) 
Public Function Add(A As Long, B As Long) As Long

    Add = A + B

End Function

Now we will run TlbImp.Exe in our existing DLL i.e. CompAdd.Dll

This generated a wrapper dll CompAddRcw.dll. we can view this DLL using IlDasm.Exe

The discussion in IlDasm.Exe is beyond the scope of this article. Now we will simply write code to use the Wrapper DLL, which in turn will be calling the actual DLL. Now let us look into the C# code for doing it.

//Written By Imtiaz Alam
using CompAddRcw;
using System;

namespace Imtiaz
{
   class EarlyBinding
   {
       public static void Main()
       {
           CompAddRcw.Class1 objCar = new CompAddRcw.Class1();

           long c;

           int x=100;
           int y=200;
      
           c=objCar.Add( ref x, ref y);

           Console.WriteLine(c);
      
       }
   }
}

As we can see that we are getting the same kind of tooltip as we used to get with VB when we do early binding. Compile the program with /r: switch e.g.csc TestClient.cs /r:CompAddRcw.dll. Execute it and we just called COM component with .net.