COM Interoperability in VB.NET Part 2

Build a .NET Server Callable from COM Clients

This article elucidates how to build and install-managed code that will be used from COM applications. A classic COM server is activated using the Service Control Manager (SCM). It looks up numerous information such as CLSIDs, IIDs, ProgIDs etc.

So what is the solution to use the .NET assemblies in the classic COM clients,

These are the steps concerned in the build process are as follows:

  1. Write and compile the managed code.

  2. Generate a COM type library (*.tlb) for the assembly using the tlbexp.exe utility so that allow the COM client to interact with the exposed types.

  3. Install and register the assembly so that COM SCM to locate it.

  4. Write and compile the COM code that references types in the assembly.

For demonstration purposes, I have created a .NET component in C# named Calculator. For client side, I have created a Visual Basic (VB) 6.0-based client.

Writing and Compiling the Managed Code:

To illustrate COM type communication with managed code, Let us see an example  in which I create a C# class library which has a class named Calculatorwhich supports three methods named Add() ,Subtract()and Hello(). Notice that we define another interface named Imuldiv.

namespace Simpleclasslib
using System;
using System.Runtime.InteropServices;
public interface Imuldiv
int Multiply(int x,int y);
int Division(int x,int y);
public class Calculator:Imuldiv
public Calculator(){}
public int Add(int x,int y)
return x+y;
public int Subtract(int x,int y)
return x-y;
int Imuldiv.Multiply(int x,int y)
return x*y;
int Imuldiv.Division(int x,int y)
return x/y;
public string Hello(string strName)
string str ;
str = "Hello " + strName ;
return str ; 

Once the managed code is written, the compilation process is the same as it would be for any other piece of managed code.

csc /out:Server.dll /target:library Calculator.cs

Now let us see the Server.dll in the ILDasm.exe.In that you can see the calculator class members and Imuldiv Interface members.


After compiling the project we have to create a type library file. So that only most unmanaged application development tools require a type library before you can make references to external types.

COM states that all server application that is programmed to share their features with other applications is to be register at a common location. The client needs to determine the exposed methods, properties and events. This is done via the libraries. The client reads the registry; determine the properties, methods, and event s of the object. COM requires up information such as CLSIDs, IIDs, and ProgIDs etc. But we know the assemblies are not registered.

.NET framework does not depend on the registry and uses metadata for this information. Hence, we have to produce the COM-compatible registry entries for our managed server so that the COM runtime could instantiate our server.

The .NET framework provides a couple of tools for this. You can use the Type Library Exporter utility (TLBEXP.exe) or the Assembly Registration Utility (Regasm.exe), both of which you'll find in the Bin directory of your .NET SDK installation.

REGASM is a superset of the TLBEXP utility in that it also does much more than generating a type library. It's also used to register the assembly, so that the appropriate registry entries are made to smooth the progress of the COM runtime and the .NET runtime to fastener up the COM aware client to the .NET component.

Usually a type library can be generated from an assembly using the regasm.exe utility. The regasm.exe utility not only registers an assembly and it also creates the required type library file, as shown here.

regasm Server.dll /tlb:Netserver.tlb

Now let us see the NetServer.tlb in the OLE viewer.

After creating the type library file you have to add a reference this to your project. For example, with Visual Basic 6.0, you can reference the .tlb file or dll file from the Project/References dialog. In Visual C++ 6.0, you can use the #import statement to import the type definitions from the type library directly into C++. Once the reference to the type library is added to the project, the types defined within that library can be referenced from unmanaged code.


In order to actually create managed types from unmanaged code, the assembly needs to be installed in the global assembly cache (GAC) and registered for use from COM. 
You can install an assembly in the global assembly cache using gacutil.exe utility. Assemblies can be uninstalled using the /u option.

gacutil /i simpleserver.dll

Writing and Compiling the Unmanaged Code:

Once the assembly is registered and properly installed, the types defined within the assembly can be used from COM as though they were normal COM types.

Now you can create a simple VB 6 Standard EXE project type and as I already said confirm whether you set a reference to the new generated type Library.

Place this code in the code section and now you see the usage of our .NET type in Classic COM.

Private Sub Command1_Click()
Dim a As New Calculator
MsgBox(a.Add(100, 100), vbInformation, "ADDITION (100,100)")
MsgBox(a.GetType, vbInformation, "INFORMATION-TYPE")
MsgBox(a.Hello("India"), vbInformation, "STRING")
MsgBox(a.Subtract(100, 1), vbInformation, "SUBTRACTION(100-1)")
Dim i As Imuldiv 'INTERFACE
i = a
MsgBox(i.Multiply(100, 100), vbInformation, "MULTIPLICATION(100*100)")
MsgBox(i.Division(100, 100), vbInformation, "DIVISION(100/100)")
End Sub


A lot of companies have spent a great amount of money and time in COM components. With introduction of .NET people are worried about the future of COM. Microsoft has recognized this and provide means to use classic COM components in .NET code and vice versa.

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