Web API: Failed to serialize the response body for content type

Question

I am working with ASP.NET MVC 5 Web Api.

I am having existing application with many api's. Recently I have implemented custom JsonConverter which will convert Date as per timezone.

public class CustomInfoConverter : JsonConverter
{

    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(CustomType);
    }
    public override bool CanRead
    {
        get
        {
            return false;
        }
    }
    public override bool CanWrite
    {
        get
        {
            return true;
        }
    }
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var customType = (CustomType)value;
        if (customType == null || null== customType.TimeZone) return;
        //DateTime currentDateTime = customType.Date??DateTime.Now;
        DateTime currentDateTime = DateTime.SpecifyKind(customType.Date ?? DateTime.Now, DateTimeKind.Unspecified);

        DateTime userDateTime = TimeZoneInfo.ConvertTimeFromUtc(currentDateTime, customType.TimeZone);
        customType.Date = userDateTime;
        JsonSerializer innerSerializer = new JsonSerializer();
        foreach (var converter in serializer.Converters.Where(c => !(c is CustomInfoConverter)))
        {
            innerSerializer.Converters.Add(converter);
        }
        innerSerializer.Serialize(writer, customType);


    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }


}

After implementing this custom JsonConverter all api are working except one api, which is throwing below exception

{"Message":"An error has occurred.","ExceptionMessage":"The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'.","ExceptionType":"System.InvalidOperationException","StackTrace":null,"InnerException":{"Message":"An error has occurred.","ExceptionMessage":"Token PropertyName in state Property would result in an invalid JSON object. Path 'Data.Forms[0]'.","ExceptionType":"Newtonsoft.Json.JsonWriterException","StackTrace":" at Newtonsoft.Json.JsonWriter.AutoComplete(JsonToken tokenBeingWritten)\r\n at Newtonsoft.Json.JsonWriter.InternalWritePropertyName(String name)\r\n at Newtonsoft.Json.JsonTextWriter.WritePropertyName(String name, Boolean escape)\r\n at Newtonsoft.Json.Serialization.JsonProperty.WritePropertyName(JsonWriter writer)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value, Type objectType)\r\n at Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value, Type objectType)\r\n at Newtonsoft.Json.JsonSerializer.Serialize(JsonWriter jsonWriter, Object value)\r\n at System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, Encoding effectiveEncoding)\r\n at System.Net.Http.Formatting.JsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, Encoding effectiveEncoding)\r\n at System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, HttpContent content)\r\n at System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStreamAsync(Type type, Object value, Stream writeStream, HttpContent content, TransportContext transportContext, CancellationToken cancellationToken)\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Runtime.CompilerServices.TaskAwaiter.GetResult()\r\n at System.Web.Http.WebHost.HttpControllerHandler.d__1b.MoveNext()"}}

You can refer to this link for more details.

Answer 1

The problem looks to be that, in certain situations, you are returning from WriteJson() without writing anything, specifically when customType.TimeZone == null:

    var customType = (CustomType)value;
    if (customType == null || null== customType.TimeZone) return;

Doing so will result in an invalid JSON object, because the property name will already have been written by the caller, resulting in:

{
    "customType":
}

Trying to do this results in the exception you are seeing.

Instead, you need to prevent the property itself from being serialized. That's not possible in the converter however, it needs to be done in the containing type.

To avoid serializing a property with a null value, you should set NullValueHandling = NullValueHandling.Ignore in serializer settings, or on the property itself:

public class ContainerClass
{
    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
    public CustomType CustomType { get; set; }
}

To prevent serializing your property when its TimeZone property is null, you should use conditional property serialization by adding a ShouldSerializeXXX() method to the containing type, where XXX exactly matches your property name:

public class ContainerClass
{
    public CustomType CustomType { get; set; }

    public bool ShouldSerializeCustomType()
    {
        return CustomType != null && CustomType.TimeZone != null;
    }
}