Example showing infinite loop deserialising polymorphic types with a custom JsonConverter.

CustomJsonConverter.cs

namespace JsonTests
{
    public class CustomJsonConverter : JsonConverter
    {
        public override bool CanConvert(Type objectType)
        {
            return typeof(BaseType).IsAssignableFrom(objectType);
        }

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            JObject item = JObject.Load(reader);

#if USE_TO_OBJECT
            var @object = item.ToObject<DerivedType>();
            return @object;
#else // USE_POPULATE
            var @object = new DerivedType();
            serializer.Populate(item.CreateReader(), @object);
            return @object;
#endif
        }

        public override bool CanWrite => false;

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
            throw new NotImplementedException();
        }
    }
}

Model.cs

namespace JsonTests
{
    [JsonConverter(typeof(CustomJsonConverter))]
    public abstract class BaseType
    {
        public string Value { get; set; }
    }

    public class DerivedType : BaseType
    {
        public string DerivedValue { get; set; }
    }

    public class Container
    {
        public List<BaseType> Objects { get; set; }
    }
}

Test.cs

namespace JsonTests
{
    [TestFixture]
    public class Tests
    {
        [Test]
        public void Test()
        {
            const string json =
                "{ 'Objects': [" +
                " { 'Value': 'test value' } " +
                "] }";

#if USE_TO_OBJECT
            Assert.That(() => JsonConvert.DeserializeObject<Container>(json),
                        Throws.InstanceOf<StackOverflowException>());
#else // USE_POPULATE
            Assert.That(() => JsonConvert.DeserializeObject<Container>(json),
                        Throws.Nothing);
#endif
        }
    }
}

Thanks Chris, this was very helpful. I had an example where I was using the toObject method in JsonSerializerSettings and it worked so I was surprised when my new code used that method and was causing the stack overflow exception.