JsonSerializer.Serialize throws StackOverflowException serializing dictionary with string key and object-converter that handles string

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This issue has been moved from a ticket on Developer Community.

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[severity:It's more difficult to complete my work]
Having a custom "generic" serializer that handles multiple types (including string) and then serializing a dictionary with string key makes the Serialize call to get stuck in a forever loop (with stack overflow exception as result). Perhaps a bit odd use case but I stumbled on it after converting from Newtonsoft to .NET so I just let you know in any case. My real use case is a bit more complicated as usual but this is what I could boil it down to:

using System.Text.Json;
using System.Text.Json.Serialization;

namespace JsonSerializationTest
{
    // example minimal "generic" converter
    public class GenericJsonConverter : JsonConverter<object> // using string here works but that is not an option for my real usecase
    {
        public override bool CanConvert(Type typeToConvert) => typeToConvert == typeof(string);
        public override object Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) => reader.GetString();
        public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options) => writer.WriteStringValue(value.ToString());
    }

    public class Tests
    {
        [Test]
        public void DoesNotWork()
        {
            var options = new JsonSerializerOptions();
            options.Converters.Add(new GenericJsonConverter());
            var value = new Dictionary<string, int> { { "key", 123 } };
            Assert.That(() => JsonSerializer.Serialize(value, options), Throws.InstanceOf<StackOverflowException>());
        }

        [Test]
        public void Works()
        {
            var options = new JsonSerializerOptions();
            var value = new Dictionary<string, int> { { "key", 123 } };
            Assert.DoesNotThrow(() => JsonSerializer.Serialize(value, options));
        }
    }
}

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Original Comments

Feedback Bot on 14/07/2025, 09:07 AM:

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[eiriktsarpalis]

I can reproduce, it appears to be an issue with how dictionary key serialization interacts with converters whose generic type doesn't match that of the serialized type. You could work around the issue either by making the converter be of type string, adopt the JsonConverterFactory pattern for generic converters, or manually override the WriteAsPropertyName method:

public class GenericJsonConverter : JsonConverter<object> // using string here works but that is not an option for my real usecase
{
    public override bool CanConvert(Type typeToConvert) => typeToConvert == typeof(string);
    public override object Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) => reader.GetString();
    public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options) => writer.WriteStringValue(value.ToString());
    public override void WriteAsPropertyName(Utf8JsonWriter writer, [DisallowNull] object value, JsonSerializerOptions options) => writer.WritePropertyName(value.ToString());
}