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Exposing .NET Components to COM

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29 Sep 2004CPOL6 min read 1.2M   13.9K   286   245
A method of calling .NET functions from a COM enabled non .NET environment through a COM callable wrapper

Table of Contents


Working as a developer prior to the advent of the .NET Framework really makes one appreciate the rich number of classes the Framework supports right out of the box. Many times however, one may need to be working in the unmanaged world and would like to invoke one of those readily available classes provided in the managed world. Since I have been playing around with .NET for a while, I have learned many of the great classes available within the .NET Framework. SimonS recently posted a question in the C# forum that made me finish a little research and I wanted to share my finding with everyone here. I had actually been tinkering with this idea for a while but never actually had enough time to finish. Here it is in all its glory.

The problem is, suppose that I have written a nice library, set of utility functions, etc. running under the .NET Framework, however I want to use this under pre .NET development environments. For SimonS, he would like to use VB6 specifically. Enter the COM Callable Wrapper (CCW) to create a proxy that will provide access to our functions through interface pointers. This can be accomplished through the use of those fun little attribute tags (I keep finding them more and more useful everyday) and with an interface of course.

To begin, you will need to include the System.Runtime.InteropServices; namespace. The first thing we will do is create an interface with the same name as the class prefixed with an _ (underscore). Within this interface, we will need to include all functions we want to "export" from within our .NET assembly. It is important that we apply the InterfaceType attribute to our interface we declare; this will expose our interface to COM. Next, above our class declaration, we will include a ClassInterface attribute which exposes all public methods, properties, fields, and events from a .NET class. Previously, I was using AutoDual, however Heath Stewart[^] pointed out this was not the best method to use as it can create version-related problems in the long run. After reading a little more, I changed the code to use ClassInterfaceType.None which forces our class to gain access only through our interface. This keeps everything viable during changes to the class in the future. The only other item to note is to go ahead and inherit your interface you defined above in your new class.

C# Source Code

using System;
using System.Runtime.InteropServices;

namespace Tester
    public interface _Numbers
        int GetDay();
        int GetMonth();

        int GetYear();

        int DayOfYear();

    public class Numbers : _Numbers
        public Numbers(){}
        public int GetDay()

        public int GetMonth()

        public int GetYear()

        public int DayOfYear()

VB.NET Source Code

Imports System
Imports System.Runtime.InteropServices

Namespace Tester

    <Guid("89439AD1-756F-4f9c-BFB4-18236F63251E"), _
    InterfaceType(ComInterfaceType.InterfaceIsIDispatch)> _
   Public Interface _Tester
        <DispId(1)> Function GetMonth() As Integer
        <DispId(2)> Function GetDay() As Integer
        <DispId(3)> Function GetYear() As Integer
        <DispId(4)> Function DayOfYear() As Integer
    End Interface

    <Guid("1376DE24-CC2D-46cb-8BF0-887A9CAF3014"), _
     ClassInterface(ClassInterfaceType.None), _
     ProgId("Tester.Numbers")> Public Class Tester
        Implements _Tester

        Public Tester()

        Public Function GetMonth() As Integer Implements _Tester.GetMonth
            GetMonth = DateTime.Now.Month
        End Function

        Public Function GetDay() As Integer Implements _Tester.GetDay
            GetDay = DateTime.Now.Day
        End Function

        Public Function GetYear() As Integer Implements _Tester.GetYear
            GetYear = DateTime.Now.Year
        End Function

        Public Function DayOfYear() As Integer Implements _Tester.DayOfYear
            DayOfYear = DateTime.Now.DayOfYear
        End Function

    End Class

End Namespace

More on Attributes - Automation

You may be wondering what all those different attributes are being used for. The short answer is simply Automation, more exactly, an Automation Server. Visual Basic is an Automation client, which simply means it consumes COM components that expose their functionality through an IDispatch interface. Visual Basic can't do vtable lookup's to get the address of an interface pointer so IDispatch steps in to help. IDispatch identifies several important methods, the two we will discuss here are; GetIDOfNames and Invoke. As you may have guessed, GetIDOfNames returns the DispId defined within our interface which can then be passed to the Invoke method along with any parameters of the "calling" function from the client. With the use of attributes, we can define all of our DispId's and even the ProgId for our class. This methodology is what allows certain scripting clients such as VBScript to talk to COM components.

Before Compiling

Property Page

Doing It By Hand Or Not...

You may find it useful to set the Register for COM interop option to True. This will create a type library and make the correct registry entries for you. You don't have to do this, and in fact, you can use the Assembly Registration Tool (Regasm.exe)[^] that comes with the .NET SDK, however it is much simpler to just check it in the property window. One caveat, if you are doing this by hand, without the IDE to register for COM Interop, prior to running the reasm.exe tool to create a type library you will need to use the Strong Name tool (sn.exe)[^] to sign your assembly and thus allowing the assembly to be placed in the Global Assembly Cache (GAC)[^]. The following screen shot shows the creation of a strong named key file via the command line.

Image 2

The choice to install the assembly to another location other than the GAC is ultimately up to you. If you wish to place your assembly in another location other than the GAC, you should include the /codebase flag when using regasm.exe on the command line. Either location that you choose, be it the GAC or your own directory with the /codebase flag will require you to have a strong-named assembly. Once you have created this file, simply edit your AssemblyInfo.cs file and change the AssemblyKeyFile property to reflect the new strong name key file name. In case you decide to just use the .NET SDK, the following command line should work fine, creating your type library and making the registry additions.

regasm Tester.dll /tlb:Tester.tlb

Image 3

To copy your assembly over to the GAC, you can use the Global Assembly Cache Tool (Gacutil.exe)[^] with the following command:

gacutil /i tester.dll

Here Comes VB

Open VB, create a new Standard EXE, select Project ---> References and your class should be listed. Place a check mark on your class and add a button to your form. Click on the button and add the following "code".

Private Sub Command1_Click()

    Dim i As Tester.Numbers
    Set i = New Tester.Numbers

    MsgBox "The date is: " & i.GetMonth & "/" & i.GetDay & "/" & i.GetYear

End Sub

Throw in a Little MFC Too

Ok, I know most everyone here, or at least a lot of you use MFC on a regular basis. Here, we can see the implementation in MFC is rather simple. I removed the code for the About box on initialization just to shorten it up. You will need to include the #import "Tester.tlb" statement in your header file as well as using namespace Tester; assuming you want scope for everything in the example.

BOOL CNickDlg::OnInitDialog()

    CString strMessage;
    Tester::_Numbers *com_ptr;
    Tester::_NumbersPtr p(__uuidof(Tester::Numbers));
    com_ptr = p;

    int m_iDay = com_ptr->GetDay();
    int m_iMonth = com_ptr->GetMonth();
    int m_iYear = com_ptr->GetYear();

    strMessage.Format("Today is: %d/%d/%d", m_iMonth, m_iDay, m_iYear);
    MessageBox(strMessage, "Today's Date", MB_ICONASTERISK | 
    SetIcon(m_hIcon, TRUE);            
    SetIcon(m_hIcon, FALSE);        
    return TRUE; 

Final Thoughts

There have been many questions posted regarding the concept that is being applied in this article. Again, the purpose of creating a COM Callable Wrapper (CCW) is strictly to provide a bridging mechanism between .NET and COM. The .NET Framework is still required to be installed on the client machine or server to work. We are identifying an interface in .NET that is exposed as an IDispatch interface in turn to COM (via our attributes). IDispatch interfaces are what Automation clients such as VB use to enable COM support. So with just a few attributes applied in the correct location, we are able to quickly create an assembly that exposes its methods to COM enabled non .NET applications. Even though COM and the CLR work under different architectural structures, they still work well when integrated. Hope this is of some help to someone somewhere thinking about things like this. If anyone else has any feedback, I am open, just post a thread below.

Update History

  • 1/15/2003 - Initial release
  • 1/16/2003 - Switched ClassInterfaceType from AutoDual to None to correct versioning problems
  • 1/16/2003 - Added VB.NET Source by request
  • 1/18/2003 - Added MFC example for additional fun
  • 2/10/2003 - Included more information on the process
  • 9/15/2003 - Included more information about manual registration without the IDE
  • 10/28/2003 - Included a more overall detailed explanation of what happens in the Final Thoughts section.
  • 11/26/2003 - Update included coverage of a ProgId, Guids, and DispIds


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Written By
Software Developer (Senior)
United States United States
Nick graduated from Iowa State University with a B.S. in Management Information System and a minor in Computer Science. Nick works for Zetetic.

Nick has also been involved with the Iowa .NET User Group since it's inception, in particular giving presentations over various .NET topics. Nick was awarded the Visual C# MVP award from Microsoft for four years in a row.

In his mystical spare time he is working on a development project called "DeveloperNotes" which integrates into Visual Studio .NET allowing developers easy access to common code pieces. He is also a fan of using dynamically typed languages to perform unit testing, not to mention how he loves to talk about himself in the third person.

Comments and Discussions

GeneralMaintaining GUID of the .NET COM Pin
Chiew Heng Wah20-Mar-05 18:20
Chiew Heng Wah20-Mar-05 18:20 
GeneralMade small change in VBA in Excel Pin
BillWerth19-Feb-05 10:38
BillWerth19-Feb-05 10:38 
GeneralVery helpfull, clear and to-the-point Pin
all-in-pita17-Feb-05 4:03
all-in-pita17-Feb-05 4:03 
QuestionHow can I .Net dll in Microsoft Navision Pin
S.Srinivasaga Perumal31-Jan-05 23:14
S.Srinivasaga Perumal31-Jan-05 23:14 
AnswerRe: How can I .Net dll in Microsoft Navision Pin
Anonymous10-Feb-05 22:19
Anonymous10-Feb-05 22:19 
QuestionHow cani use this with VBScript? Pin
User 13039330-Dec-04 3:33
User 13039330-Dec-04 3:33 
QuestionVersioning issues? Pin
John Goodwin17-Dec-04 10:34
John Goodwin17-Dec-04 10:34 
GeneralCompilation error when importing the Tester.tlb file Pin
pelos13-Dec-04 6:26
pelos13-Dec-04 6:26 
hi forum,

I have tested the example with an old C/C++ application and when importing the Tester.tlb file I get a lot of errors like this:

C:\Microsoft Visual Studio .NET\Vc7\include\comutil.h(1257) : error C2084: function '_variant_t::_variant_t(BOOL)' already has a body

I think i have built well the COM component, so, i think the problem could be related with the type of the unmanaged C++ project: it is just a VC++ 6 console application project, written in C and compiled as C++ code.

Could you help me please?

Thanks in advance!

GeneralCalling Controls from ActiveX Pin
Rein Petersen23-Nov-04 10:24
Rein Petersen23-Nov-04 10:24 
GeneralRe: Calling Controls from ActiveX Pin
Chiew Heng Wah20-Mar-05 18:29
Chiew Heng Wah20-Mar-05 18:29 
General.NET Interop not working Pin
bacro10-Oct-04 3:42
bacro10-Oct-04 3:42 
GeneralRe: .NET Interop not working Pin
Kristopher Cargile19-Oct-04 10:15
Kristopher Cargile19-Oct-04 10:15 
GeneralRe: .NET Interop not working Pin
Magne Ryholt8-Dec-04 3:08
Magne Ryholt8-Dec-04 3:08 
QuestionExposing .Net Events through CCW ? Pin
Jabba665-Oct-04 2:42
Jabba665-Oct-04 2:42 
AnswerRe: Exposing .Net Events through CCW ? Pin
Huisheng Chen4-Jul-05 22:15
Huisheng Chen4-Jul-05 22:15 
QuestionHow to use manifest.xml file Pin
SandeepN31-Aug-04 12:37
SandeepN31-Aug-04 12:37 
AnswerRe: How to use manifest.xml file Pin
E KuralMani1-Jul-05 15:30
E KuralMani1-Jul-05 15:30 
GeneralRe: How to use manifest.xml file Pin
SandeepN3-Jul-05 9:52
SandeepN3-Jul-05 9:52 
Generalusing with native C++... Pin
JAHA_30-Jun-04 12:59
JAHA_30-Jun-04 12:59 
QuestionHow to enable early Binding for .net Class? Pin
Anonymous18-Jun-04 12:12
Anonymous18-Jun-04 12:12 
AnswerRe: How to enable early Binding for .net Class? Pin
Nick Parker18-Jun-04 13:25
protectorNick Parker18-Jun-04 13:25 
GeneralInterface question Pin
kraquen12-May-04 12:10
kraquen12-May-04 12:10 
GeneralRe: Interface question Pin
Nick Parker1-Jun-04 11:51
protectorNick Parker1-Jun-04 11:51 
General'Compile Error: Variable uses an automation type not supported in Visual Basic Pin
nitincnarkar14-Apr-04 16:44
nitincnarkar14-Apr-04 16:44 
GeneralRe: 'Compile Error: Variable uses an automation type not supported in Visual Basic Pin
Nitin Narkar15-Apr-04 3:36
Nitin Narkar15-Apr-04 3:36 

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