c# - NewtonSoft Json Invalid Cast Exception

Question

This is related to my question HTTPClient Buffer Exceeded 2G; Cannot write more bytes to the buffer but is different enough that IMO it warrants a separate question.

In the other question, I'm trying to figure out how to deal with breaking the 2G request buffer. The idea was to use streaming, but I need to deserialize. In talking to Professor Google, I found that I have to use TextReader to stream/deserialize. so my code for that is:

    public async Task<API_Json_Special_Feeds.RootObject> walMart_Special_Feed_Lookup(string url)
    {
        special_Feed_Lookup_Working = true;
        HttpClientHandler handler = new HttpClientHandler()
        {
            AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate
        };
        using (HttpClient http = new HttpClient(handler))
        {

            http.DefaultRequestHeaders.AcceptEncoding.Add(new System.Net.Http.Headers.StringWithQualityHeaderValue("gzip"));
            http.Timeout = TimeSpan.FromMilliseconds(Timeout.Infinite);
            url = String.Format(url);
            using (var response = await http.GetAsync(url, HttpCompletionOption.ResponseHeadersRead))
            {
                Console.WriteLine(response);
                var serializer = new JsonSerializer();

                using (StreamReader sr = new StreamReader(await response.Content.ReadAsStreamAsync()))
                {
                    using (var jsonTextReader = new JsonTextReader(sr))
                    {
                        API_Json_Special_Feeds.RootObject root = (API_Json_Special_Feeds.RootObject)serializer.Deserialize(jsonTextReader);
                        return root;
                    }
                }
            }
        }
    }

Now, as you can see, the return type is strongly typed. The return type of the method matches. Now, I go to the calling line:

  API_Json_Special_Feeds.RootObject Items = await net.walMart_Special_Feed_Lookup(specialFeedsURLs[i].Replace("{apiKey}", Properties.Resources.API_Key_Walmart));

So, we have matching types API_Json_Special_Feeds.RootMethod all the way around.

When run, the calling line throws an InvalidCastException:

Undesired Result:

Unable to cast object of type 'Newtonsoft.Json.Linq.JObject' to type 'RootObject'

I've checked at the end of the method before return, and the result is indeed cast from an object to API_Json_Special_Feeds.RootMethod before being returned.

Question: somewhere between the return statement and the calling line, the object being returned is being converted from an API_Json_Special_Feeds.RootMethod to a Newtonsoft.Json.Linq.JObject. I can't debug it since there's no code in between. If I cast again in the calling line, I get a "Cannot cast" error. How can I prevent the degradation/changing of this object type?

Many, many thinks for your time, consideration, and for any thoughts or suggestions you can provide!

Answer 1

You need to use the generic overload JsonSerializer.Deserialize<T>()

var root = serializer.Deserialize<API_Json_Special_Feeds.RootObject>(jsonTextReader);

Unlike files generated by BinaryFormatter, JSON files generally do not include c# type information, so it's necessary for the receiving system to specify the expected type.

(There are extensions to the JSON standard in which c# type information can be included in a JSON file -- e.g. Json.NET's TypeNameHandling -- but it is still necessary to deserialize the JSON to an appropriate explicit base class.)

See Deserialize JSON from a file for another example of deserializing a strongly typed c# object from a stream.