In certain cases you want to offer the end-user a userfriendly interface to manage content, users or anything else. Thanks to Sitecore's backend flexibility this is possible in more ways than one. In this case I will use the technique which is described in this blogpost by Adam Conn.
In the example you can see that a new window is opened by the code below:
public override void Execute(CommandContext context)
{
Assert.ArgumentNotNull(context, "context");
UrlString url = new UrlString("/sitecore/shell/~/xaml/GridExample.GridExamplePage");
url["id"] = context.Items[0].ID.ToString();
SheerResponse.ShowModalDialog(url.ToString(), "650", "500");
}
As you can see this is done by calling a url with a /~xaml/ handler. The xaml handler calls the Sitecore.Web.UI.XamlSharp.Xaml.XamlPageHandlerFactory class which basicly searches for the control. In the example from Adam the control which is being searched for is placed in the "/sitecore modules" folder. By default Sitecore searches these controls in the "/sitecore modules" and "/sitecore/shell/Applications" folders.
But if you build custom applications within Sitecore the chance is that you do not want to place the custom code into the "/sitecore" or "/sitecore modules" folders. Of course you can place the code into your own location, but then the XAML handler cannot find the control.
The configuration which defines the folders for Sitecore to look for controls is inside the "/App_Config/XamlSharp.config" file. There are two watchers defined which tell Sitecore to look into the folder mentioned earlier. You can add a new watcher by adding it to the XamlSharp.config. Or better, use a Sitecore include config file.