How to Deploy changes to the web.config file as a feature for a SharePoint web application

I am currently working for a SharePoint hosting provider and as part of the general guidence, direct web.config changes are not allowed. Any change should be incorporated as a SharePoint feature, if you think about it, this is a good practice to get into.

Pros for deploying as features:

• A solution repository can be maintained, which will make migration easy
   
• Any future upgrades can lead to missing manual config changes
• Fuctionality can be targeted to a web application or a sitec collection etc

Anyways, the post is about how to deploy changes to a web config via a feature.

*** Warning - Programatically modifying web.config changes are not very clean, so

Web config changes are specific to a web app, hence we will write and deploy a feature targetting a web app.

1. Open visual studio and create a empty SharePoint project
2. Add a new feature and set the Target to " Web Application"  (Make sure you name your Feature with an appropriate name)
3. Double click on the Feature and in the Properties window, set the "Activate on Default" to false - We want to manually activate the feature per web application
    
4. Add a Feature Receiver
5. in the feature activated event, add the following code:
   
   
    public override void FeatureActivated(SPFeatureReceiverProperties properties)
           {
               try
               {
                  SPWebApplication webApp = SPWebApplication.Lookup(new Uri("http://mywebapp"));
   
                   SPWebConfigModification moduleModification = new SPWebConfigModification();
                   moduleModification.Path = "configuration/system.webServer/modules runAllManagedModulesForAllRequests=\"true\"";
                   moduleModification.Name = "add[@name=\"myHttpModule\"][@type=\"myHttpModule.myHttpModuleClassLib, myHttpModule\"]";
                   moduleModification.Sequence = 0;
                   moduleModification.Owner = "admin";
                   moduleModification.Type = SPWebConfigModification.SPWebConfigModificationType.EnsureChildNode;
                   moduleModification.Value = "<add name=\"myHttpModule\" type=\"myHttpModule.myHttpModule, myHttpModule\" />";
                   webApp.WebConfigModifications.Add(moduleModification);
   
                   webApp.Update();
                   //service.ApplyWebConfigModifications();
                   webApp.Farm.Services.GetValue<SPWebService>().ApplyWebConfigModifications();
               }
               catch (Exception ex)
               {
                   //Log exceptions in the Event Log
                   using (EventLog eventLog = new EventLog("Application"))
                   {
                       eventLog.Source = "myHttpModule";
                       eventLog.WriteEntry(string.Format("{0}: - {1} Error: \"{2}\" ", EventLogEntryType.Error, "myHttpModule- Feature Activating", ex.ToString() + Environment.NewLine + ex.StackTrace.ToString()), EventLogEntryType.Error);
                   }
                  
                   //Throw the exception for the admin
                   throw ex;
               }
   
           }
6.   Add the following code to feature deactivating
   
   
    public override void FeatureDeactivating(SPFeatureReceiverProperties properties)
           {
               try
               {
                   SPWebConfigModification configModFound = null;
                   SPWebApplication webApp = SPWebApplication.Lookup(new Uri("http://mywebapp"));
                   Collection<SPWebConfigModification> modsCollection = webApp.WebConfigModifications;
                   for (int i = 0; i < modsCollection.Count; i++)
                   {
                       if (modsCollection[i].Owner == "fu1adm" && modsCollection[i].Name == "add[@name=\"myHttpModule\"]")
                           configModFound = modsCollection[i];
                   }
                   if (configModFound != null)
                   {
                       modsCollection.Remove(configModFound);
                       webApp.Update();
                       webApp.Farm.Services.GetValue<SPWebService>().ApplyWebConfigModifications();
                   }
               }
               catch (Exception ex)
               {
                   //Log exceptions in the Event Log
                   EventLog eventLog = new EventLog("Application");
                   eventLog.WriteEntry(string.Format("{0}: - {1} Error: \"{2}\" ", EventLogEntryType.Error, "myHttpModule- Feature Deactivate", ex.ToString() + Environment.NewLine + ex.StackTrace.ToString()), EventLogEntryType.Error);
                   throw ex;
               }
   
   
           }
7.  Build the solution and add it to the solution store
   
8. Deploy the solution to the web app or globally
9. In Central Admin > Click on the Application management > Manage web applications > Select the appropriate web application and click on Manage Features > Click on activate.
10.  Make sure the web.config has been modified.

There are numerous reasons why web.config changes should be avoided via code. Microsoft has limited documentation on this

References:
http://msdn.microsoft.com/en-au/library/bb861909%28v=office.14%29.aspx