Using C# external Windows Forms based library Control in ASP.NET

Can you try the attached sample ?
panel1.Controls.Add(this.captureImageBox).  --  Here you can add webcontrols only, you can't add windows forms controls.
This is the reason you are getting cast error.
Instead of adding the control in the runtime, try to add the control in aspx design time itself.
<object id="myName" classid="http:WindowsFormsControlLibrary1.dll#WindowsFormsControlLibrary1.UserControl1"
height="500" width="500" />
Hope this helps you.

I was able to build the application with an imported dll but couldn't make use of it. Originally, in the Form implementation, I have attached ImageBox (derived from Forms Control PictureBox) to Forms.Panel Control through

panel1.Controls.Add(this.captureImageBox).

This resulted in the image being continuously rendered on Application form.

Now, when I try to do the same in Web Application (asp.net), I am not able to cast PictureBox to a recognizable Web Control.

I have seen the articles. But, I would like to programmatically use them in the Default page code.
Anyway, reg what is accomplished in the articles - Can it be easily done by just adding a new reference in Project->References->Add Reference->System.Windows.Forms dll?

I have added the third party library to the Visual Studio tool box on a new tab. But, none of the items show up in the Web Project. They show up in a Forms project.

http://www.beansoftware.com/ASP.NET-Tutorials/Place-Windows-Control-To-Web-Form.aspx
http://steveorr.net/articles/WinformControls.aspx
 
Hope these links will help you.