ASP.NET MVC: Register action filter without modifying controller

I think global filters is what you need.

Once you created the filter register it in the global.asax:

protected void Application_Start() {

    AreaRegistration.RegisterAllAreas();

    // Register global filter
    GlobalFilters.Filters.Add(new MyActionFilterAttribute());

    RegisterGlobalFilters(GlobalFilters.Filters);
    RegisterRoutes(RouteTable.Routes); 
}

Add custom validation logic to filter if you want to apply it not to all actions.

If you want your filter to be registered for every action (or it is otherwise OK to do so), then MVC 3 allows you to apply Global action filters. Of course this requires that nopCommerce is built on MVC 3, which I believe the newest version is?

In NopCommerce 3.5 (the latest as of this answer, and newer than the question date), the best way I've found to add a global action filter is by creating a plugin with an IStartupTask implementation in it. This method completely avoids altering any NopCommerce core files.

The NopCommerce Application_Start event initializes the EngineContext, which creates the NopEngine instance. The NopEngine initialization finds all IStartupTask implementations, and executes them in their specified order. So an IStartupTask is the place to do anything that needs to happen on application start.

Sample code below:

public class Plugin : BasePlugin
{
    public Plugin()
    {
    }

    /// <summary>
    /// Check to see if this plugin is installed
    /// </summary>
    public static bool IsInstalled(ITypeFinder typeFinder)
    {
        IEnumerable<Type> types = typeFinder.FindClassesOfType<IPluginFinder>(true);

        if (types.Count() == 1)
        {
            IPluginFinder plugins = Activator.CreateInstance(types.First()) as IPluginFinder;
            PluginDescriptor descriptor = plugins.GetPluginDescriptorBySystemName("MyPluginName");

            if (descriptor != null && descriptor.Installed)
            {
                return true;
            }
        }

        return false;
    }
}

/// <summary>
/// Redirects to the 404 page if criteria not met
/// </summary>
public class FluffyTextureRequiredAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        if (Kitten.Texture != Textures.Fluffy)
        {
            var routeValues = new RouteValueDictionary();
            routeValues.Add("controller", "Common");
            routeValues.Add("action", "PageNotFound");

            filterContext.Result = new RedirectToRouteResult(routeValues);
        }
    }
}

/// <summary>
/// Does application start event stuff for the plugin, e.g. registering
/// global action filters
/// </summary>
public class StartupTask : IStartupTask
{
    private ITypeFinder _typeFinder;

    public StartupTask()
    {
        //IStartupTask objects are created via Activator.CreateInstance with a parameterless constructor call, so dependencies must be manually resolved.
        _typeFinder = EngineContext.Current.Resolve<ITypeFinder>();
    }

    public void Execute()
    {
        // only execute if plugin is installed
        if (Plugin.IsInstalled(_typeFinder))
        {
            // GlobalFilters is in System.Web.Mvc
            GlobalFilters.Filters.Add(new FluffyTextureRequiredAttribute());
        }
    }

    public int Order
    {
        get { return int.MaxValue; }
    }
}