Exposing .NET Components to COM

• Download C# source files - 20.2 Kb
• Download VB.NET source files - 16.6 Kb
• Download MFC implementation example - 2.26 MB

• Introduction
• C# Source Code
• VB.NET Source Code
• More on Attributes - Automation
• Before Compiling
  • Doing It By Hand Or Not
• Here Comes VB
• Throw in a little MFC too
• Final Thoughts
• Update History

Introduction

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
{
	[Guid("D6F88E95-8A27-4ae6-B6DE-0542A0FC7039")]
	[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
	public interface _Numbers
	{
		[DispId(1)]
		int GetDay();
		
		[DispId(2)]
		int GetMonth();

		[DispId(3)]
		int GetYear();

		[DispId(4)]
		int DayOfYear();
	}

	[Guid("13FE32AD-4BF8-495f-AB4D-6C61BD463EA4")]
	[ClassInterface(ClassInterfaceType.None)]
	[ProgId("Tester.Numbers")]
	public class Numbers : _Numbers
	{
		public Numbers(){}
		
		public int GetDay()
		{
			return(DateTime.Today.Day);
		}

		public int GetMonth()
		{
			return(DateTime.Today.Month);
		}

		public int GetYear()
		{
			return(DateTime.Today.Year);
		}

		public int DayOfYear()
		{
			return(DateTime.Now.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

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 simplier 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. 

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 registery additions.

regasm Tester.dll /tlb:Tester.tlb

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()
{
	CDialog::OnInitDialog();

	CString strMessage;
	Tester::_Numbers *com_ptr;
	CoInitialize(NULL);
	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 | 
                   MB_ICONINFORMATION);
	
	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 it 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 - Including 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, Guid's, and DispId's.