Question

It was working in Beta

I have an object of type ChangeSet<T> deriving from ChangeSet that needs some custom deserialization in order for the rest of my web-api to work. It was working in asp.net mvc 4 beta, but in the rc it’s broken, and in a way that I’m finding very hard to debug.

My scenario

The ChangeSet can represent a change of a number of T objects described by an ID-list. I wanted to deserialize into an object-array or dictionary of some sort to describe which properties of the T objects should be changed, but then I had issues deserializing into the correct datatypes. So I introduced the Change property of type T. The new problem then is that not all properties of Change should be persisted. Only the ones that was included in the json. So I let there be a Properties property containing a list of strings.

My Problem

To allow me to populate Properties I need to access the original json, which is why I need a custom MediaTypeFormatter. (The xml-serializer has a SetSerializer(..) but there is no such method for a JsonSerializer. In the below ReadFromStreamAsync I do this, the problem is that it crashes, and I can’t step into the point where it crashes to get an inner exception. All I get is the following json response to my request:

{
    "ExceptionType": "System.InvalidOperationException",
    "Message":       "Method may only be called on a Type for which Type.IsGenericParameter is true.",
    "StackTrace":    "   at System.RuntimeType.get_DeclaringMethod()"
}

If I remove my ChangeSetJsonFormatter the code doesn’t break, but of course i have no Properties.

My questions

1. What could be the cause of this?
2. How can I debug this properly? I have Log4Net, but what should I watch? Can I step into the problem somehow?

The code

This is my FormatterConfig:

public class FormatterConfig
{
    public static void RegisterGlobalFormatters(MediaTypeFormatterCollection formatters)
    {
        var jsonSerializerSettings = formatters.JsonFormatter.SerializerSettings;
        jsonSerializerSettings.Converters.Add(new IsoDateTimeConverter());
        jsonSerializerSettings.NullValueHandling = NullValueHandling.Ignore;

        // At index 0 so that it will try this before the default handler.
        formatters.Insert(0, new ChangeSetJsonFormatter(jsonSerializerSettings));

        formatters.Remove(formatters.XmlFormatter);
    }
}

This is my custom media formatter (sorry about the length):

public class ChangeSetJsonFormatter : JsonMediaTypeFormatter
{
    private static readonly Type ChangeSetType = typeof (ChangeSet<Entity>);

    public ChangeSetJsonFormatter(JsonSerializerSettings jsonSerializerSettings)
    {
        SerializerSettings = jsonSerializerSettings;
    }

    public override bool CanReadType(Type type)
    {
        return type.IsGenericType && type.Namespace == ChangeSetType.Namespace && type.Name == ChangeSetType.Name;
    }

    public override bool CanWriteType(Type type)
    {
        return type.IsGenericType && type.Namespace == ChangeSetType.Namespace && type.Name == ChangeSetType.Name;
    }

    public override Task<object> ReadFromStreamAsync(Type type, Stream stream, HttpContentHeaders contentHeaders, IFormatterLogger formatterLogger)
    {
        var task = Task.Factory.StartNew(() =>
        {
            using (var streamReader = new StreamReader(stream))
            {
                var jsonSource = streamReader.ReadToEnd();
                var deserializedObject =
                    JsonConvert.DeserializeObject(jsonSource, type,
                                                SerializerSettings);
                var changeSet = deserializedObject as ChangeSet;
                if (changeSet != null &&
                    (changeSet.Properties == null ||
                    changeSet.Properties.Count == 0))
                {
                    var properties =
                        JObject.Parse(jsonSource)["Change"]
                               .Select(t => ((JProperty) t).Name)
                               .ToList();
                    changeSet.Properties = properties;
                }

                return deserializedObject;
            }
        });

        return task;
    }
}

And these are the ChangeSet classes:

[DataContract]
public abstract class ChangeSet
{
    [DataMember]
    public IList<string> Properties { get; set; }

    [DataMember]
    public IList<int> IdList { get; set; }
}

[DataContract]
public class ChangeSet<T> : ChangeSet where T : Entity
{
    [DataMember]
    public T Change { get; set; }

    public Dictionary<PropertyInfo, object> Changes
    {
        get 
        {
            if (Properties == null || Properties.Count == 0)
                return new Dictionary<PropertyInfo, object>();

            return typeof (T)
                .GetProperties()
                .Where(pi => Properties.Contains(pi.Name))
                .ToDictionary(pi => pi, pi => pi.GetValue(Change, null));
        }
    }
}

Edit: I was able to produce the trace for the service stack, following this documentation. Now if only I was able to get an entry for my exception. Here’s the output from the default listener:

System.Web.Http.Request: ;;http://localhost:50500/MyService/User?_dc=1343396746443
System.Web.Http.Controllers: DefaultHttpControllerSelector;SelectController;Route='controller:User'
System.Web.Http.Controllers: DefaultHttpControllerSelector;SelectController;User
System.Web.Http.Controllers: HttpControllerDescriptor;CreateController;
System.Web.Http.Controllers: DefaultHttpControllerActivator;Create;
System.Web.Http.Controllers: DefaultHttpControllerActivator;Create;MyCompany.Admin.Service.Controllers.UserController
System.Web.Http.Controllers: HttpControllerDescriptor;CreateController;MyCompany.Admin.Service.Controllers.UserController
System.Web.Http.Controllers: UserController;ExecuteAsync;
System.Web.Http.Action: ApiControllerActionSelector;SelectAction;
System.Web.Http.Action: ApiControllerActionSelector;SelectAction;Selected action 'Put(ChangeSet`1 changeSet)'
System.Web.Http.ModelBinding: HttpActionBinding;ExecuteBindingAsync;
System.Web.Http.ModelBinding: FormatterParameterBinding;ExecuteBindingAsync;Binding parameter 'changeSet'
System.Net.Http.Formatting: ChangeSetJsonFormatter;ReadFromStreamAsync;Type='ChangeSet`1', content-type='application/json'
System.Net.Http.Formatting: ChangeSetJsonFormatter;ReadFromStreamAsync;Value read='MyCompany.Admin.Service.Data.ChangeSet`1[MyCompany.Data.Model.User]'
System.Web.Http.ModelBinding: FormatterParameterBinding;ExecuteBindingAsync;
System.Web.Http.ModelBinding: HttpActionBinding;ExecuteBindingAsync;
System.Web.Http.Controllers: UserController;ExecuteAsync;
System.Net.Http.Formatting: DefaultContentNegotiator;Negotiate;Type='HttpError', formatters=[JsonMediaTypeFormatterTracer, JsonMediaTypeFormatterTracer, FormUrlEncodedMediaTypeFormatterTracer, FormUrlEncodedMediaTypeFormatterTracer]
System.Net.Http.Formatting: JsonMediaTypeFormatter;GetPerRequestFormatterInstance;Obtaining formatter of type 'JsonMediaTypeFormatter' for type='HttpError', mediaType='application/json; charset=utf-8'
System.Net.Http.Formatting: JsonMediaTypeFormatter;GetPerRequestFormatterInstance;Will use same 'JsonMediaTypeFormatter' formatter
System.Net.Http.Formatting: DefaultContentNegotiator;Negotiate;Selected formatter='JsonMediaTypeFormatter', content-type='application/json; charset=utf-8'
System.Web.Http.Request: ;;Content-type='application/json; charset=utf-8', content-length=unknown
System.Net.Http.Formatting: JsonMediaTypeFormatter;WriteToStreamAsync;Value='System.Web.Http.HttpError', type='HttpError', content-type='application/json; charset=utf-8'
System.Net.Http.Formatting: JsonMediaTypeFormatter;WriteToStreamAsync;
System.Web.Http.Controllers: UserController;Dispose;
System.Web.Http.Controllers: UserController;Dispose;
The thread '<No Name>' (0x1ea0) has exited with code 0 (0x0).
The thread '<No Name>' (0x1d6c) has exited with code 0 (0x0).
The thread '<No Name>' (0x19b4) has exited with code 0 (0x0).

Answer 1

I finally figured out what was breaking. The ModelBinder validation breaks on getter using reflection. In that followup question I’m asking why it’s breaking.

In this question I was after what’s breaking, which I just answered. Thusly I’m closing this question.