JsonSerializer cannot serialize types that implement IDictionary<string, object>

[davidfowl]

I was trying to serialize the event counter payload (https://github.com/dotnet/coreclr/blob/25878db0c11bfb8d52088c75ccfcc13e4c8383f2/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/EventPayload.cs#L28) which is a type that implements IDictionary<string, object> and it failed with:

Unhandled exception. System.Diagnostics.Tracing.EventSourceException: An error occurred when writing to a listener. ---> System.InvalidCastException: Unable to cast object of type 'System.Diagnostics.Tracing.EventPayload' to type 'System.Collections.IDictionary'.
   at System.Text.Json.JsonSerializer.HandleDictionary(JsonClassInfo elementClassInfo, JsonSerializerOptions options, Utf8JsonWriter writer, WriteStack& state)
   at System.Text.Json.JsonSerializer.Write(Utf8JsonWriter writer, Int32 flushThreshold, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.JsonSerializer.WriteCore(PooledByteBufferWriter output, Object value, Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.WriteCoreString(Object value, Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.ToString(Object value, Type type, JsonSerializerOptions options)
   at WebApplication275.MyListener.OnEventWritten(EventWrittenEventArgs eventData) in C:\Users\david\source\repos\WebApplication275\WebApplication275\Startup.cs:line 61
   at System.Diagnostics.Tracing.EventSource.DispatchToAllListeners(Int32 eventId, Guid* childActivityID, EventWrittenEventArgs eventCallbackArgs)
   --- End of inner exception stack trace ---
   at System.Diagnostics.Tracing.EventSource.DispatchToAllListeners(Int32 eventId, Guid* childActivityID, EventWrittenEventArgs eventCallbackArgs)
   at System.Diagnostics.Tracing.EventSource.WriteToAllListeners(String eventName, EventDescriptor& eventDescriptor, EventTags tags, Guid* pActivityId, Guid* pChildActivityId, EventPayload payload)
   at System.Diagnostics.Tracing.EventSource.WriteImpl(String eventName, EventSourceOptions& options, Object data, Guid* pActivityId, Guid* pRelatedActivityId, TraceLoggingEventTypes eventTypes)
   at System.Diagnostics.Tracing.PollingCounter.WritePayload(Single intervalSec, Int32 pollingIntervalMillisec)
   at System.Diagnostics.Tracing.CounterGroup.OnTimer(Object state)
   at System.Diagnostics.Tracing.CounterGroup.<>c.<EnableTimer>b__15_0(Object s)
   at System.Threading.TimerQueueTimer.CallCallback(Boolean isThreadPool)
   at System.Threading.TimerQueueTimer.Fire(Boolean isThreadPool)
   at System.Threading.TimerQueue.FireNextTimers()
   at System.Threading.TimerQueue.AppDomainTimerCallback(Int32 id)
Unhandled exception. System.Diagnostics.Tracing.EventSourceException: An error occurred when writing to a listener. ---> System.InvalidCastException: Unable to cast object of type 'System.Diagnostics.Tracing.EventPayload' to type 'System.Collections.IDictionary'.
   at System.Text.Json.JsonSerializer.HandleDictionary(JsonClassInfo elementClassInfo, JsonSerializerOptions options, Utf8JsonWriter writer, WriteStack& state)
   at System.Text.Json.JsonSerializer.Write(Utf8JsonWriter writer, Int32 flushThreshold, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.JsonSerializer.WriteCore(PooledByteBufferWriter output, Object value, Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.WriteCoreString(Object value, Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.ToString(Object value, Type type, JsonSerializerOptions options)
   at WebApplication275.MyListener.OnEventWritten(EventWrittenEventArgs eventData) in C:\Users\david\source\repos\WebApplication275\WebApplication275\Startup.cs:line 61

I then tried do to this:

var value = JsonSerializer.ToString(eventData.Payload[0], typeof(IDictionary<string, object>));

But it still failed.

Seems like this should work, here's a full repro:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Text.Json;

namespace WebApplication275
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var values = new DictionaryWrapper(new Dictionary<string, object>
            {
                { "Name", "David" },
                { "Age", 32 }
            });

            var value = JsonSerializer.ToString(values, typeof(IDictionary<string, object>));

            Console.WriteLine(value);
        }
    }

    public class DictionaryWrapper : IDictionary<string, object>
    {
        private readonly IDictionary<string, object> _dict;

        public DictionaryWrapper(IDictionary<string,object> dict)
        {
            _dict = dict;
        }

        public object this[string key] { get => _dict[key]; set => _dict[key] = value; }

        public ICollection<string> Keys => _dict.Keys;

        public ICollection<object> Values => _dict.Values;

        public int Count => _dict.Count;

        public bool IsReadOnly => false;

        public void Add(string key, object value)
        {
            _dict.Add(key, value);
        }

        public void Add(KeyValuePair<string, object> item)
        {
            _dict.Add(item.Key, item.Value);
        }

        public void Clear()
        {
            _dict.Clear();
        }

        public bool Contains(KeyValuePair<string, object> item)
        {
            return _dict.Contains(item);
        }

        public bool ContainsKey(string key)
        {
            return _dict.ContainsKey(key);
        }

        public void CopyTo(KeyValuePair<string, object>[] array, int arrayIndex)
        {
            _dict.CopyTo(array, arrayIndex);
        }

        public IEnumerator<KeyValuePair<string, object>> GetEnumerator()
        {
            return _dict.GetEnumerator();
        }

        public bool Remove(string key)
        {
            return _dict.Remove(key);
        }

        public bool Remove(KeyValuePair<string, object> item)
        {
            return _dict.Remove(item);
        }

        public bool TryGetValue(string key, out object value)
        {
            return _dict.TryGetValue(key, out value);
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }
}

[ahsonkhan]

cc @layomia, @steveharter

[MarcoRossignoli]

I did some check, the problem is that we cast all dictionary to IDictionary I think to have access to Key,Vaulue object throught IDictionaryEnumerator returned by IDictionaryEnumerator GetEnumerator();
Unfortunatly IDictionary<TKey, TValue> doesn't implement IDictionary
In case of IDictionary<TKey, TValue> I think we need to go throught reflection to get key/value from Current, maybe two path IDictionaryEnumerator if IDictionary and reflection/emit for other.
I could try to PR a solution.

[ahsonkhan]

@layomia, can you please work with @MarcoRossignoli to get this bug resolved? Thanks.

[layomia]

I did some check, the problem is that we cast all dictionary to IDictionary

Thanks for your insight, Marco. This is indeed the problem. We need to first check if we can cast. I have a fix in dotnet/corefx#38512. Please let me know if you have further thoughts.