How can I use data placed into a ViewBag by a filter in my Error.cshtml view?

Question

I have an action filter that is responsible for placing some common information into the ViewBag for use by all views in the shared _Layout.cshtml file.

public class ProductInfoFilterAttribute : ActionFilterAttribute
{
    public override void
    OnActionExecuting(ActionExecutingContext filterContext)
    {
        //  build product info
        //  ... (code omitted)

        dynamic viewBag = filterContext.Controller.ViewBag;
        viewBag.ProductInfo = info;
    }
}

In the shared _Layout.cshtml file, I use the information that has been put into the ViewBag.

...
@ViewBag.ProductInfo.Name
...

If an exception occurs while processing a controller action, the standard HandleErrorAttribute should display my shared Error.cshtml view, and this worked before I introduced the action filter above and started using the new values from ViewBag in _Layout.cshtml. Now what I get is the standard ASP.Net runtime error page instead of my custom Error.cshtml view.

I have tracked this down to the fact that while rendering the error view, a RuntimeBinderException ("Cannot perform runtime binding on a null reference") is thrown on the use of ViewBag.ProductInfo.Name in _Layout.cshtml.

It appears that even though my action filter has successfully set the value in the ViewBag before the original exception was thrown, a new context with an empty ViewBag is used when rendering my Error.cshtml view.

Is there any way to get data created by an action filter to be available to a custom error view?

Solution

I have come up with my own solution through the addition of another filter.

public class PreserveViewDataOnExceptionFilter : IExceptionFilter
{
    public void
    OnException(ExceptionContext filterContext)
    {
        //  copy view data contents from controller to result view
        ViewResult viewResult = filterContext.Result as ViewResult;
        if ( viewResult != null )
        {
            foreach ( var value in filterContext.Controller.ViewData )
            {
                if ( ! viewResult.ViewData.ContainsKey(value.Key) )
                {
                    viewResult.ViewData[value.Key] = value.Value;
                }
            }
        }
    }

    public static void
    Register()
    {
        FilterProviders.Providers.Add(new FilterProvider());
    }

    private class FilterProvider : IFilterProvider
    {
        public IEnumerable<Filter>
        GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor)
        {
            //  attach filter as "first" for all controllers / actions; note: exception filters run in reverse order
            //  so this really causes the filter to be the last filter to execute
            yield return new Filter(new PreserveViewDataOnExceptionFilter(), FilterScope.First, null);
        }
    }
}

This filter needs to be hooked in globally in the Global.asax.cs Application_Start() method by calling PreserveViewDataOnExceptionFilter.Register().

What I've done here is to set up a new exception filter that runs last, after the HandleErrorAttribute filter runs, and copies the contents of the ViewData collection that was available to the controller that threw the exception into the result created by the HandleErrorAttribute filter.

Comments

[Nick Coad]

This works really well for ViewData and I like the code, but do you have an equivalent for ViewBag?

[Peter Kerr]

filterContext.Controller.ViewData contains both the Model and the ViewBag as far as I can see, so can can pass it all on, or only the bits you need