Creating an ActiveX control in .Net using C#

A while back I had a client request that I write an ActiveX control for use on their corporate intranet. I had never done this before, and most of the examples I could find online were either really old, incomplete or based on using C++ and MFC. It's safe to say that my C++ skills are not quite up to the job, so for me it was really a requirement to be able to do this with C#. It took me about an hour or two to write the code for the control, but it took almost three days to successfully package and deploy it as a .cab file... So to save others from wasting their time like I did, I'll document my findings in this post.

Steps

1. - Create a new Class Library project in Visual Studio
2. - Create a new class that inherits from UserControl
3. - Create a new interface that exposes the controls methods and properties to COM interop
4. - Make the control class implement the new interface
5. - Mark the control as safe for scripting and initialization
6. - Create a .msi installer for the control
7. - Package the control in a .cab file for web deployment
8. - Initialize and test the control with JavaScript

1. Create a new Class Library project in Visual Studio

I'm using Visual Studio 2008, but other versions should work as well.

1. After starting Visual Studio click File -> New -> Project and select Class Library under C#.
2. Call the project 'AxControls' and click OK.

2. Create a new class that inherits from UserControl

1. Rename 'Class1.cs' to 'HelloWorld.cs', making sure to rename the class name as well.
2. Add a project reference to System.Windows.Forms.
3. Make the HelloWorld class inherit UserControl.

3. Create a new interface that exposes the controls methods and properties to COM interop

1. Right click the project in Visual Studio and click Add -> New Item.
2. Select 'Interface' from the list of components, name it 'IHelloWorld.cs' and click Add.
3. Edit the 'IHelloWorld.cs' file so it looks like this:

   using System;
   using System.Collections.Generic;
   using System.Text;
   using System.Runtime.InteropServices;
   
   namespace AxControls
   {
       [ComVisible(true)]
       [InterfaceType(ComInterfaceType.InterfaceIsDual)]
       [Guid("41E85D5D-C57A-4386-B722-4031D0B1E1B7")]
       public interface IHelloWorld
       {
           string GetText();
       }
   }
   

We now have a COM visible interface with a single method 'GetText()'.

[ComVisible(true)] makes the interface visible to COM.
[InterfaceType(ComInterfaceType.InterfaceIsDual)] sets the COM interface type to Dual, see InterfaceTypeAttribute Class on MSDN.
[Guid("41E85D5D-C57A-4386-B722-4031D0B1E1B7")] let's us manually assign a GUID to the interface. Use guidgen.exe to generate your own.

4. Make the control class implement the new interface

Make the HelloWorld class implement the IHelloWorld interface and have the GetText() method return a string of your choice. This is what the file might look like:

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace AxControls
{
    [ComVisible(true)]
    [ClassInterface(ClassInterfaceType.None)]
    [Guid("1FC0D50A-4803-4f97-94FB-2F41717F558D")]
    [ProgId("AxControls.HelloWorld")]
    [ComDefaultInterface(typeof(IHelloWorld))]
    public class HelloWorld : UserControl, IHelloWorld
    {
        #region IHelloWorld Members

        public string GetText()
        {
            return "Hello ActiveX World!";
        }

        #endregion
    }
}

We now have a COM visible control that implements the IHelloWorld interface.

[ComVisible(true)] makes the control visible to COM, see ComVisibleAttribute Class on MSDN.
[ClassInterface(ClassInterfaceType.None)] indicates that no class interface is generated for this class, see ClassInterfaceType Enumeration on MSDN.
[Guid("1FC0D50A-4803-4f97-94FB-2F41717F558D")] let's us manually assign a GUID to the control, see GuidAttribute Class on MSDN. Use guidgen.exe to generate your own.
[ProgId("AxControls.HelloWorld")] is a "user friendly" ID that we'll use later from JavaScript when initiating the control, see ProgIdAttribute Class on MSDN.
[ComDefaultInterface(typeof(IHelloWorld))] sets IHelloWorld as the default interface that will be exposed to COM, see ComDefaultInterfaceAttribute Class on MSDN.

5. Mark the control as safe for scripting and initialization

By default IE will not allow initializing and scripting an ActiveX control unless it is marked as safe. This means that we won't be able to create instances of our ActiveX class with JavaScript by default. We can get around this by modifying the browser security settings, but a more elegant way would be to mark the control as safe. Before you do this to a "real" control, be sure to understand the consequences. I found an ancient (1996) MSDN article that explains this here. We will mark the control as safe by implementing the IObjectSafety interface.

1. Right click the project in Visual Studio and click Add -> New Item.
2. Select 'Interface' from the list of components, name it 'IObjectSafety.cs' and click Add.
3. Edit the 'IObjectSafety.cs' file so it looks like this:

   using System;
   using System.Collections.Generic;
   using System.Text;
   using System.Runtime.InteropServices;
   
   namespace AxControls
   {
       [ComImport()]
       [Guid("CB5BDC81-93C1-11CF-8F20-00805F2CD064")]
       [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
       interface IObjectSafety
       {
           [PreserveSig()]
           int GetInterfaceSafetyOptions(ref Guid riid, out int pdwSupportedOptions, out int pdwEnabledOptions);
   
           [PreserveSig()]
           int SetInterfaceSafetyOptions(ref Guid riid, int dwOptionSetMask, int dwEnabledOptions);
       }
   }
   

4. Make the HelloWorld class implement the IObjectSafety interface. The end result should look something like this:

   using System;
   using System.Collections.Generic;
   using System.Text;
   using System.Windows.Forms;
   using System.Runtime.InteropServices;
   
   namespace AxControls
   {
       [ComVisible(true)]
       [ClassInterface(ClassInterfaceType.None)]
       [Guid("1FC0D50A-4803-4f97-94FB-2F41717F558D")]
       [ProgId("AxControls.HelloWorld")]
       [ComDefaultInterface(typeof(IHelloWorld))]
       public class HelloWorld : UserControl, IHelloWorld, IObjectSafety
       {
           #region IHelloWorld Members
   
           public string GetText()
           {
               return "Hello ActiveX World!";
           }
   
           #endregion
   
           #region IObjectSafety Members
   
           public enum ObjectSafetyOptions
           {
               INTERFACESAFE_FOR_UNTRUSTED_CALLER = 0x00000001,
               INTERFACESAFE_FOR_UNTRUSTED_DATA = 0x00000002,
               INTERFACE_USES_DISPEX = 0x00000004,
               INTERFACE_USES_SECURITY_MANAGER = 0x00000008
           };
   
           public int GetInterfaceSafetyOptions(ref Guid riid, out int pdwSupportedOptions, out int pdwEnabledOptions)
           {
               ObjectSafetyOptions m_options = ObjectSafetyOptions.INTERFACESAFE_FOR_UNTRUSTED_CALLER | ObjectSafetyOptions.INTERFACESAFE_FOR_UNTRUSTED_DATA;
               pdwSupportedOptions = (int) m_options;
               pdwEnabledOptions = (int) m_options;
               return 0;
           }
   
           public int SetInterfaceSafetyOptions(ref Guid riid, int dwOptionSetMask, int dwEnabledOptions)
           {
               return 0;
           }
   
           #endregion
       }
   }
   

[ComImport()] IObjectSafety is a native interface so we have to redefine it for managed .Net use. This is done with the ComImport() attribute, see ComImportAttribute Class on MSDN.
[Guid("CB5BDC81-93C1-11CF-8F20-00805F2CD064")] This is the GUID of the original IObjectSafety interface. Do not change it.
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)] sets the COM interface type to Unknown, see InterfaceTypeAttribute Class on MSDN.

This is just a simple implemetation of the IObjectSafety interface that will mark the control as safe. In "real life" there would probably be some sort of logic to determine if the control is safe or not.

6. Create a .msi installer for the control

Before an ActiveX control can be used it must be installed and registered on the client. This can be done in a number of ways, from manually editing the registry to using regasm.exe, but we're going to create a Vistual Studio setup project to handle the installation for us.

1. Right click the Visual Studio solution, select Add -> New Project and select Setup Project under Other Project Types.
2. Call the project 'AxControlsInstaller' and click OK.
3. Right click the 'AxControlsInstaller' project, select Add -> Project Output, select 'Primary output' from the 'AxControls' project and click OK.
4. Right click 'Primary output from AxControls (Active)' and select Properties.
5. Change the Register property from 'vsdrpDoNotRegister' to 'vsdrpCOM'.
6. Right click the 'AxControlsInstaller' project and select Build.

The installer should now be located in the AxControlsInstaller's output folder (bin\Debug or bin\Release). In the corporate domain this .msi file can de run manually on the client, or automatically with a Group Policy.

7. Package the installer in a .cab file for web deployment

For public web sites we obviously can't deploy our ActiveX control to the client with a Group Policy. In this case we're gonna have to use Internet Explores built-in ability to download and install controls that are packaged in .cab files.

1. Download the Microsoft Cabinet Software Development Kit.
2. Unpack the kit to a local folder and copy Cabarc.exe to the 'AxControlsInstaller' folder.
3. Create a new file named 'AxControls.inf' in the 'AxControlsInstaller' folder and add the following content:

   [version]
   signature="$CHICAGO$"
   AdvancedINF=2.0
   
   [Add.Code]
   AxControlsInstaller.msi=AxControlsInstaller.msi
   
   [AxControlsInstaller.msi]
   file-win32-x86=thiscab
   clsid={1FC0D50A-4803-4f97-94FB-2F41717F558D}
   FileVersion=1,0,0,0
   
   [Setup Hooks]
   RunSetup=RunSetup
   
   [RunSetup]
   run="""msiexec.exe""" /i """%EXTRACT_DIR%\AxControlsInstaller.msi""" /qn
   

4. Click the AxControlsInstaller project and then click the Properties window (View -> Properties Window if it's not visible).
5. Click the '...' button next to the PostBuildEvent property and add the following content:

   "$(ProjectDir)\CABARC.EXE" N "$(ProjectDir)AxControls.cab" "$(ProjectDir)AxControls.inf" "$(ProjectDir)$(Configuration)\AxControlsInstaller.msi"
   

6. Right click the 'AxControlsInstaller' project and select Build.
7. There should now be a 'AxControls.cab' file in the 'AxControlsInstaller' folder.

NB! Make sure you use ANSI encoding for the 'AxControls.inf' file or you will be unable to install the control.

8. Initialize and test the control with JavaScript

1. Right click the AxControls solution, select Add -> New Project and select 'ASP.Net Web Application' under 'Web'.
2. Call the project 'WebAppTest' and click OK.
3. Right click the 'WebAppTest' project, select Add -> New Item and select 'HTML Page'.
4. Call it 'index.html' and click OK.
5. Add the following content to index.html:

   <html>
       <head>
   
           <object name="axHello" style='display:none' id='axHello' classid='CLSID:1FC0D50A-4803-4f97-94FB-2F41717F558D' codebase='AxControls.cab#version=1,0,0,0'></object>
   
         <script language="javascript">
   
           <!-- Load the ActiveX object  -->
           var x = new ActiveXObject("AxControls.HelloWorld");
   
           <!-- Display the String in a messagebox -->
           alert(x.GetText());
   
         </script>
       </head>
       <body>
       </body>
   </html>
   

   Note that 'classid' matches the GUID of the HelloWorld control.

6. Right click 'index.html' and select 'Set as start page'.
7. Right click the 'WebAppTest' project and select 'Set as startup project'.
8. Copy 'AxControls.cab' from the 'AxControlsInstaller' folder to the same folder as index.html.
9. Uninstall the control from the client by going to Control Panel -> Programs and Features, selecting 'AxControlsInstaller' on the list and clicking Uninstall. This forces Internet Explorer to download and install the .cab file and is an important step in case you've already installed the control.
10. Run the application (F5). This will open 'index.html' in Internet Explorer.
11. Internet Explorer will display a security warning, asking if you want to install 'AxControls.cab'. Click Install.
12. When the page loads it should display a message box with the string you defined in HelloWorld's GetText() method.

If the message box displayed without any more warnings or errors we've implemented everyting correctly.

UPDATE

I forgot to write that you have to register the assembly containing the ActiveX control for COM interop. Right-click the project, select Properties, go to Build and check the "Register for COM interop" checkbox. This should solve the error some of you are seeing about "Automation Server can’t create this object" and similar error messages. You might need to add the [ComVisible(true)] attribute to the methods and properties you are exposing as well, but I haven't had time to test this.