Question

In the default implementation of the JsonValueProviderFactory, JavaScriptSerializer.DeserializeObject() method is used. This method throws an exception if the json string is ill-formed. The server then throws a 500 yellow page to the browser. I want to suppress the exception and show a custom error message through my controller action.

This is what I tried. I removed the default JsonValueProviderFactory and introduced a custom JsonValueProviderFactory with essentially the same code. The only difference is in the GetDeserializedObject method where I have introduced a try-catch block.

private static object GetDeserializedObject(ControllerContext controllerContext)
    {
        if (!controllerContext.HttpContext.Request.ContentType.StartsWith("application/json", StringComparison.OrdinalIgnoreCase))
        {
            // not JSON request
            return null;
        }

        StreamReader reader = new StreamReader(controllerContext.HttpContext.Request.InputStream);
        string bodyText = reader.ReadToEnd();
        if (String.IsNullOrEmpty(bodyText))
        {
            // no JSON data
            return null;
        }

        JavaScriptSerializer serializer = new JavaScriptSerializer();
        object jsonData = null;
        try
        {
            jsonData = serializer.DeserializeObject(bodyText);
        }
        catch (Exception ex) {
            ex.Data["jsonString"] = bodyText;
            throw;
        }
        return jsonData;
    }

Instead of simply throwing the exception, I want to pass the exception error message to the modelbinder and eventually set it in the modelstate dictionary. Then I can read the modelstate dictionary in controller action and customize the error message sent to browser. Can this be done ?
If it can’t be done, is there an alternative way to achieve the same goal ?

Answer 1

I finally found the right way of doing it. I am documenting it for future reference by others.

controllerContext.Controller.ViewData.ModelState.AddModelError("improper json", ex)

This is exactly how the modelstate in modelbindingContext of modelbinder is set. I copying the relevant snippent of code from MVC3 sources. The method is present in ControllerActionInvoker.cs.

 protected virtual object GetParameterValue(ControllerContext controllerContext, ParameterDescriptor parameterDescriptor) {
        // collect all of the necessary binding properties
        Type parameterType = parameterDescriptor.ParameterType;
        IModelBinder binder = GetModelBinder(parameterDescriptor);
        IValueProvider valueProvider = controllerContext.Controller.ValueProvider;
        string parameterName = parameterDescriptor.BindingInfo.Prefix ?? parameterDescriptor.ParameterName;
        Predicate<string> propertyFilter = GetPropertyFilter(parameterDescriptor);

        // finally, call into the binder
        ModelBindingContext bindingContext = new ModelBindingContext() {
            FallbackToEmptyPrefix = (parameterDescriptor.BindingInfo.Prefix == null), // only fall back if prefix not specified
            ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, parameterType),
            ModelName = parameterName,
            ModelState = controllerContext.Controller.ViewData.ModelState,
            PropertyFilter = propertyFilter,
            ValueProvider = valueProvider
        };

        object result = binder.BindModel(controllerContext, bindingContext);
        return result ?? parameterDescriptor.DefaultValue;
    }