What's the best way to trap a HttpRequestValidationException and add it to the ModelState?

Question

Regarding the decorator [AllowHtml] on a property or even the [ValidateInput(false)] on a method, what is the best way to catch the HttpRequestValidationException and append it to the ModelState to show as a friendly error on the user side and not show an error page (either throwing a new page under Application_Error or make the use of custom error pages.

Inside the global.asax I have a trap:

protected void Application_Error()
{
    // http://romsteady.blogspot.dk/2007/06/how-to-catch-httprequestvalidationexcep.html
    // Code that runs when an unhandled error occurs

    System.Exception ex = Server.GetLastError();

    if (ex is System.Web.HttpRequestValidationException)
    { 
        // I got the exception here, I can do plenty now!

        Server.ClearError(); // no need to continue, I know the error
    }
}

how do I get from here to the model state, without using any Session/Application variables (thinking about cloud here and all those different server hosting the user request)?

I was thinking add to the route, or TempData but such is not available here... maybe a Cookie but seams to hacky...

Any ideas?

Answer 1

I once handled this situation via a custom ModelBinder and throwing a try/catch around the base.BindModel call. It's ugly, but it gets the job done.

I repeat, it's ugly.

Here's an example:

public class FooModelBinder : DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        Foo model;
        try
        {
            model = (Foo)base.BindModel(controllerContext, bindingContext);
        }
        catch (HttpRequestValidationException)
        {
            // handle here
        }
    }
}

Now, in a valiant effort to populate the ModelState with errors, I have a helper class that does its best to maintain state. Its use (and implementation) leave a lot to be desired (a lot of plumbing, use of magic strings, type-specific, regex of exception message text, etc), so any suggestions are welcome. This is the ugliest part, imo.

Usage:

// from above code snippet
catch (HttpRequestValidationException)
{
    // handle any potentially dangerous form values here.  Don't want an exception bubbling up to the user
    // so handle the HttpRequestValidationException by hand here
    // manually populate the model here so that the original values are presented back to the user
    model = new Foo()
    {
        Bar = HandleHttpRequestValidationExceptionHelper.TryAssignment(bindingContext.ModelState, () => bindingContext.ValueProvider.GetValue("Bar").AttemptedValue),
        Baz = HandleHttpRequestValidationExceptionHelper.TryAssignment(bindingContext.ModelState, () => bindingContext.ValueProvider.GetValue("Baz").AttemptedValue)
    };
}

return model;

The helper does its best to mine out pertinent error information for the user, but it's really crappy. (Notice a theme?)

Implementation:

public static class HandleHttpRequestValidationExceptionHelper
{
    /// <summary>
    /// Use TryAssignment in anticipation of a HttpRequestValidationException; it's used to help return error information to the user
    /// </summary>
    /// <param name="modelStateDictionary">The ModelStateDictionary to add the errors to</param>
    /// <param name="action">The attempted value to assign</param>
    /// <returns>Either the proper value or the errored value read from the HttpRequestValidationException Message property</returns>
    public static string TryAssignment(ModelStateDictionary modelStateDictionary, Func<string> action)
    {
        try
        {
            return action();
        }
        catch (HttpRequestValidationException ex)
        {
            // in effort to better inform the user, try to fish out the offending form field
            var parenthesesMatch = Regex.Match(ex.Message, @"\(([^)]*)\)");
            if (parenthesesMatch.Success)
            {
                var badFormInput = parenthesesMatch.Groups[1].Value.Split('=');
                modelStateDictionary.AddModelError(badFormInput[0], badFormInput[1] + " is not valid.");
                return badFormInput[1].TrimStart('"').TrimEnd('"');
            }
            else
            {
                // if attempt to find the offending field fails, just give a general error
                modelStateDictionary.AddModelError("", "Please enter valid information.");
                return string.Empty;
            }
        }
    }

    /// <summary>
    /// Use TryAssignment in anticipation of a HttpRequestValidationException; it's used to help return error information to the user
    /// </summary>
    /// <typeparam name="T">Type of the value</typeparam>
    /// <param name="modelStateDictionary">The ModelStateDictionary to add the errors to</param>
    /// <param name="action">The attempted value to assign</param>
    /// <returns>Either the proper value or default(T)</returns>
    public static T TryAssignment<T>(ModelStateDictionary modelState, Func<T> action)
    {
        try
        {
            return action();
        }
        catch (HttpRequestValidationException ex)
        {
            // in effort to better inform the user, try to fish out the offending form field
            var parenthesesMatch = Regex.Match(ex.Message, @"\(([^)]*)\)");
            if (parenthesesMatch.Success)
            {
                var badFormInput = parenthesesMatch.Groups[1].Value.Split('=');
                modelState.AddModelError(badFormInput[0], badFormInput[1] + " is not valid.");
                // can't really cast a string to an unknown type T.  safer to just return default(T)
            }
            else
            {
                // if attempt to find the offending field fails, just give a general error
                modelState.AddModelError("", "Please enter valid information.");
            }
            return default(T);
        }
    }
}

Basically, upon catching an exception, try rebinding the model manually, ready to catch a potential HttpRequestValidationException error for each property. If one is caught, populate the ModelStateDictionary accordingly with as specific of a message as I can get.

I really wish the framework made it easier to 1) catch this exception and 2) gracefully handle it instead of crapping the entire bed.

Answer 2

Error handling in ASP.NET MVC is a controversial subject. You have different choices to handle errors. Read:

• How can I properly handle 404 in ASP.NET MVC?
• ASP.NET MVC HandleError