Update authorization section of asp.net web.config file programatically

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C#

ASP.NET

web.config

Hi am trying to modify the web.config file on run time. My application uses windows authentication. The code which I have written is working fine on my local system. But when i host the application in the IIS on the server, application is throwing an error, when I try to modify web.config file on run time.
below is the code used to modify the the web.config file.

protected void btnAdd_Click(object sender, EventArgs e)
   {
       AuthorizationSection section;
       System.Configuration.Configuration config = WebConfigurationManager.OpenWebConfiguration(System.Web.Hosting.HostingEnvironment.ApplicationVirtualPath);

       section = (AuthorizationSection)config.GetSection("system.web/authorization");

       AuthorizationRule arule = new AuthorizationRule(AuthorizationRuleAction.Allow);

       string[] strAllowedUsers = GetUsers();


       section.Rules.Clear();

       foreach (string element in strAllowedUsers)
       {
           arule.Users.Add(element);
       }
       arule.Users.Add(txtAddUser.Text.ToString());
       section.Rules.Add(arule);



       config.Save();

   }

private string[] GetUsers()
   {
       string[] strUser = null;
       try
       {
           AuthorizationSection section;
           System.Configuration.Configuration config = WebConfigurationManager.OpenWebConfiguration(System.Web.Hosting.HostingEnvironment.ApplicationVirtualPath);
           section = (AuthorizationSection)config.GetSection("system.web/authorization");

           var rules = section.Rules;
           var allowedUsers = rules
             .OfType<authorizationrule>()
             .Where(r => r.Action == AuthorizationRuleAction.Allow)
             .Select(r => r.Users).First();
           if (allowedUsers.ToString() != null)
               strUser = allowedUsers.ToString().Split(',');
       }
       catch { }
       return strUser;
   }

It throws an error -->

Unable to save config to file 'c:\inetpub\wwwroot\myapplication\web.config

Note: I am hosting the application on IIS 5.1