Accessing COM+ component using C#


This article explains step by step of accessing COM+ application using C#. The code is compiled using beta2. Microsoft (R) Visual C# Compiler Version 7.00.9254 [CLR version v1.0.2914]. It can be used with Beta1 with some minor modification.

If we want need to access the existing COM+ application with any .net supported language, we don't need to modify the single line of exisiting COM+ application, despite of the fact their execution model is completely different. Here is the step-by-step easy example of doing the same. We can access it in two ways, Early Binding and Late binding (remembered the old VB concepts). We will first look at Early binding example.

Early Binding:

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.

Note: We have to register COM DLL first before using it...

Public Function Add(A As Long, B As Long) As Long
Add = A + B

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.


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 TestClient.cs /r:CompAddRcw.dll. Execute it and we just called COM component with .net.

Late Bindings:

For Late binding we have to use System.Reflection namespace, which allows us to programmaticaly access the types contained in any assembly. We will see how we can do late bindings in four easy steps...

  • We have Get IDispatch Interface using Type.GetTypeFromProgID("Project1.Class1").
  • We have to create instance using the type ID Activator.CreateInstance(objAddType)
  • We have to make array of arguments (if required)
  • Invoke the Method using objAddType.InvokeMember function. 


The method Type.GetTypeFromProgID is used to load the type information of the COM object.The call to Activator.CreateInstance returns an instance of the COM object. And Finally InvokeMember function is usedto call the Method of COM object.


This article gives very basic idea of using the old COM Application, for the purpose of simplicity and to focus on the purpose of the article no error handling has been done. In my next article we will see how can we use .net component with VB.