Why does using the `[JsonConvert()]` annotation with custom JsonConverters sometimes result in a StackOverflowException, and how can I prevent it?

JSON.NET StackOverflowException and the [JsonConvert] Attribute

Applying the [JsonConvert] attribute to a custom JsonConverter can lead to StackOverflowException during serialization, particularly with complex objects. This is often due to infinite recursion within JSON.NET's internal serialization process. While directly using JsonConvert.SerializeObject() with your custom converter is the recommended solution, this section explores alternative approaches when the attribute is necessary.

1. Refining the WriteJson Method

The core of the problem frequently lies within the WriteJson method of your custom converter. Insufficient handling of edge cases can trigger the infinite loop. The following improved WriteJson method addresses several common scenarios:

public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) {
    if (ReferenceEquals(value, null)) {
        writer.WriteNull();
        return;
    }

    var contract = (JsonObjectContract)serializer.ContractResolver.ResolveContract(value.GetType());

    writer.WriteStartObject();

    foreach (var property in contract.Properties) {
        if (property.Ignored) continue;
        if (!ShouldSerialize(property, value)) continue;

        var propertyName = property.PropertyName;
        var propertyValue = property.ValueProvider.GetValue(value);

        writer.WritePropertyName(propertyName);
        if (property.Converter != null && property.Converter.CanWrite) {
            property.Converter.WriteJson(writer, propertyValue, serializer);
        } else {
            serializer.Serialize(writer, propertyValue);
        }
    }

    writer.WriteEndObject();
}

private static bool ShouldSerialize(JsonProperty property, object instance) {
    return property.ShouldSerialize == null || property.ShouldSerialize(instance);
}

This version explicitly handles null values and uses the contract resolver to iterate through properties, skipping ignored properties and those where ShouldSerialize returns false. It also correctly delegates serialization to the property's converter if one exists and can write, otherwise using the serializer directly.

2. Recursive Call with Stack Size Limit

Alternatively, you can add a stack depth check within your WriteJson method to prevent runaway recursion:

public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) {
    const int MaxStackSize = 65536; // Adjust as needed

    if (writer.CurrentDepth > MaxStackSize) {
        throw new JsonSerializationException("Stack overflow detected during serialization.");
    }

    // Your custom serialization logic here...
}

This approach throws a descriptive exception if the recursion depth exceeds a predefined limit, preventing the StackOverflowException. Adjust MaxStackSize based on your object complexity.

Conclusion

While the [JsonConvert] attribute offers convenience, using it with custom converters requires careful consideration. By enhancing your WriteJson method to handle edge cases and/or incorporating a stack size check, you can effectively mitigate the risk of StackOverflowException during serialization. Remember that manually calling JsonConvert.SerializeObject() remains the safest and most reliable approach.

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