Creating ActiveX Controls using C# in VS2005

This will be a short tutorial on how to create a simple ActiveX control that opens a windows forms Message Box while living in an aspx page. Pretty useless, I know, but it will illustrate the basics of how to create and interact with an ActiveX control from a web page in Visual Studio 2005.
A co-worker and myself worked on this for a current project and found bits and pieces of this information and figured it would be a great resource to compile it all into one location. It can get a bit tricky (especially if you add in security issues when handling ActiveX control events) but its not too bad.
Step 1
This first step in this tutorial is to create a web project in Visual Studio. I called mine ActiveXTest.
Step 2
Next, add a new Windows Control Library project to your solution.
This project is where you will develop your ActiveX control. Rename the default user control cs file and double click on it to open up the designer. You can build your control just like you would any other Windows User Control by adding controls and code-behind as necessary. This example doesn't need any controls, but I'll add a picture box just so that we can see it on the aspx page.
Step 3
Now that our project is setup with a web project, a Default.aspx page and a Windows Control Library with our new User Control in it, we are ready to turn the User Control into an ActiveX control by exposing and implementing an interface.
Next, create in interface that will expose parts of your ActiveX control to the browser.

public interface IComInterface {
string Msg { set; } // Exposes the Msg property through the interface
} 

Implement this interface in your User Control.

public partial class MessageBoxControl : UserControl, IComInterface { // Implement IComInterface 

public string Msg { // Implement the Property
        set{ MessageBox.Show(value); } // When the property's set is called, show the message box
    }
public MessageBoxControl() {
InitializeComponent();
}
}

You have just created an ActiveX control that is ready to be embeded in a web page and exposes a property (Msg) to COM through an interface (IComInterface).

Step 4

The next step is to embed your newly created ActiveX control in your web page (Default.aspx.) To do this, you will use the <object> tag like this.

<object id="MessageBoxControl " name="MessageBoxControl "
classid="ActiveXControls.dll#ActiveXControls.MessageBoxControl ">
</object> 

The id and name should be self explanatory. The class id is comprised of the name of your dll file, a # sign, and the namespace.classname of your control. That's all it takes to embed the control into your page. There are a couple of thing you will need to do to allow this to happen, but the coding is done.

Other things to consider...

1. You must have a copy of your Windows Control library dll on the same level of your web project as the default.aspx page. The dll cannot contained in the bin folder.
2. Add your local computer as a trusted zone through Internet Explorer. This will help bypass some of the security issues inherent with ActiveX controls. Do this through the Internet Options of Internet Explorer.
3. You may need to increase the trust level of your assembly before it will load correctly in Internet Explorer. You can do this from the .Net Configuration Tool located in the Administrator Tools of the Control Panel. (Configure Code Access Security Policy -> Increase Assembly Trust)
4. Strongly name your ActiveXControl assembly. (Note: You can do this through the properties page of the project.) This must be done prior to registering the dll in the gac.
5. You may also need to register your assembly/dll in the gac before it will work. You can do this by using gacutil from the command line from the directory of your dll. (gacutil /i ActiveXControls.dll) (Note: You might have to do this after every build of the project.)
6. Modify your AssemblyInfo.cs file and change the ComVisible attribute to true. ([assembly: ComVisible(true)]) This is maily when you want to expose ActiveX control events to your web page.
7. Set ISS so that Execute Permissions on your Virtual Directory are set to "Scripts Only". This is the only option in ISS that would work for me when deploying the ActiveX control and accessing it remotely and was a pain to track down.
Step 5

You can then access properties of your control exposed through your interface by using JavaScript on that page. Something similar to this.

<script type="text/javascript">
function showMessageBox(){
var control = document.getElementById('MessageBoxControl');
control.Msg = document.getElementById('txtMsg').value;
}
</script> 

Tips/Tricks

You can add a build event to your Windows Control Library project that will automatically copy it's dll to the web project when you build it. Open the properties of the project by right clicking on it and add a line like this to the build events.

copy "$(TargetPath)" "C:\Projects\vs2005\ActiveXTest\ActiveXTest"