A filter (which should be used as a global filter) to check whether the application is configured as offline, and throw an exception if this is the case. This is an alternative to placing a file named App_Offline.htm in the application's root. Using App_Offline.htm affects all users, cannot be selectively bypassed as in this case, and does not a…

CheckApplicationOfflineAttribute.cs

using System;
using System.Net;
using System.Web.Mvc;
using System.Web.Routing;

namespace Web.Filters
{
    /// <summary>
    /// Check whether the application is marked as offline in the config file.
    /// If so, and the user does not have the override permission, throw an exception.
    /// The MVC error handling will deal with displaying the exception message to the user.
    /// </summary>
    public class CheckApplicationOfflineAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuting(ActionExecutingContext filterContext)
        {
            // Only need to carry out the check on parent actions, otherwise there is an unnecessary overhead
            if (filterContext.IsChildAction)
                return;

            if (ConfigurationHelper.ApplicationOffline && !SecurityHelper.UserHasApplicationOfflineOverridePermission)
                throw new OfflineException("The system is offline for maintenance.");
        }
    }
}

Global.asax.cs

// Apply a filter so that Elmah does not log any "system offline" exceptions. Otherwise the log will 
// be flooded with those messages as users try to access the system while it is down for maintenance.
void ErrorLog_Filtering(object sender, Elmah.ExceptionFilterEventArgs e)
{
    if (e.Exception is OfflineException)
        e.Dismiss();
}

OfflineException.cs

using System;

namespace Utility
{
    public class OfflineException : ApplicationException
    {
        public OACSOfflineException(string message)
            : base(message) 
        {
        }
    }
}