How to register automatically HTTP handler required by a Web server control

In ASP.NET when you building a server control that includes an HTTP handler you have this problem - the HTTP handler has to be registered in Web.config. That means it's not enough that your customer developer drops control on her Web form and sets up its properties. One more step is required - manual editing of the config, which is usability horror.

How do you make your customer aware she needs to perform this additional action? Documentation? Yes, but who reads documentation on controls? I know I never, I usually just drop it on the page and poke around its properties to figure out what I need to set up to make it working asap.

So here is nice trick how to avoid manual Web.config editing (found it in the ScriptAculoUs autocomplete web control).

1. Make sure your control has a designer.
2. In your control's designer class override ControlDesigner.GetDesignTimeHtml() method, which is called each time your control needs to be represented in design mode.
3. In the GetDesignTimeHtml() method check if your HTTP handler in already registered in Web.config and if it isn't - just register it.

Here is a sample code that worth hundred words: 

using System;
using System.Web.UI.Design;
using System.Security.Permissions;
using System.Configuration;
using System.Web.Configuration;
using System.Windows.Forms;

namespace XMLLab.WordXMLViewer
{
    [SecurityPermission(SecurityAction.Demand, 
        Flags = SecurityPermissionFlag.UnmanagedCode)]
    public class WordXMLViewerDesigner : ControlDesigner
    {
        private void RegisterImageHttpHandler()
        {
            IWebApplication webApplication = 
                (IWebApplication)this.GetService(typeof(IWebApplication));

            if (webApplication != null)
            {
                Configuration configuration = webApplication.OpenWebConfiguration(false);
                if (configuration != null)
                {
                    HttpHandlersSection section = 
                        (HttpHandlersSection)configuration.GetSection(
                        "system.web/httpHandlers");
                    if (section == null)
                    {
                        section = new HttpHandlersSection();
                        ConfigurationSectionGroup group = 
                            configuration.GetSectionGroup("system.web");
                        if (group == null)
                        {
                            configuration.SectionGroups.Add("system.web", 
                                new ConfigurationSectionGroup());
                        }
                        group.Sections.Add("httpHandlers", section);
                    }
                    section.Handlers.Add(Action);
                    configuration.Save(ConfigurationSaveMode.Minimal);
                }
            }
        }


        private bool IsHttpHandlerRegistered()
        {
            IWebApplication webApplication = 
                (IWebApplication)this.GetService(typeof(IWebApplication));

            if (webApplication != null)
            {
                Configuration configuration = 
                    webApplication.OpenWebConfiguration(true);

                if (configuration != null)
                {
                    HttpHandlersSection section = 
                        (HttpHandlersSection)configuration.GetSection(
                        "system.web/httpHandlers");

                    if ((section != null) && (section.Handlers.IndexOf(Action) >= 0))
                        return true;
                }
            }
            return false;
        }


        static HttpHandlerAction Action
        {
            get
            {
                return new HttpHandlerAction(
                    "image.ashx", 
                    "XMLLab.WordXMLViewer.ImageHandler, XMLLab.WordXMLViewer", 
                    "*"
                );
            }
        }

        public override string GetDesignTimeHtml(DesignerRegionCollection regions)
        {
            if (!IsHttpHandlerRegistered() && 
                (MessageBox.Show(
                "Do you want to automatically register the HttpHandler needed by this control in the web.config?", 
                "Confirmation", MessageBoxButtons.YesNo, 
                MessageBoxIcon.Exclamation) == DialogResult.Yes))
                RegisterImageHttpHandler();
            return base.CreatePlaceHolderDesignTimeHtml("Word 2003 XML Viewer");
        }
    }
}

Obviously it only works if your control gets rendered at least once in Design mode, which isn't always the case. Some freaks (including /me) prefer to work with Web forms in Source mode, so you still need to write in the documentation how to update Web.config to make your control working.