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JSON Serializable Custom Types

docs/decisions/0021-json-serializable-custom-types.md

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JSON Serializable Custom Types

Context and Problem Statement

This ADR aims to simplify the usage of custom types by allowing developers to use any type that can be serialized using System.Text.Json.

Standardizing on a JSON-serializable type is necessary to allow functions to be described using a JSON Schema within a planner's function manual. Using a JSON Schema to describe a function's input and output types will allow the planner to validate that the function is being used correctly.

Today, use of custom types within Semantic Kernel requires developers to implement a custom TypeConverter to convert to/from the string representation of the type. This is demonstrated in [Functions/MethodFunctions_Advanced] as seen below:

csharp
    [TypeConverter(typeof(MyCustomTypeConverter))]
    private sealed class MyCustomType
    {
        public int Number { get; set; }

        public string? Text { get; set; }
    }

    private sealed class MyCustomTypeConverter : TypeConverter
    {
        public override bool CanConvertFrom(ITypeDescriptorContext? context, Type sourceType) => true;

        public override object? ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object value)
        {
            return JsonSerializer.Deserialize<MyCustomType>((string)value);
        }

        public override object? ConvertTo(ITypeDescriptorContext? context, CultureInfo? culture, object? value, Type destinationType)
        {
            return JsonSerializer.Serialize(value);
        }
    }

The above approach will now only be needed when a custom type cannot be serialized using System.Text.Json.

Considered Options

1. Fallback to serialization using System.Text.Json if a TypeConverter is not available for the given type

  • Primitive types will be handled using their native TypeConverters
    • We preserve the use of the native TypeConverter for primitive types to prevent any lossy conversions.
  • Complex types will be handled by their registered TypeConverter, if provided.
  • If no TypeConverter is registered for a complex type, our own JsonSerializationTypeConverter will be used to attempt JSON serialization/deserialization using System.Text.Json.
    • A detailed error message will be thrown if the type cannot be serialized/deserialized.

This will change the GetTypeConverter() method in NativeFunction.cs to look like the following, where before null was returned if no TypeConverter was found for the type:

csharp
private static TypeConverter GetTypeConverter(Type targetType)
    {
        if (targetType == typeof(byte)) { return new ByteConverter(); }
        if (targetType == typeof(sbyte)) { return new SByteConverter(); }
        if (targetType == typeof(bool)) { return new BooleanConverter(); }
        if (targetType == typeof(ushort)) { return new UInt16Converter(); }
        if (targetType == typeof(short)) { return new Int16Converter(); }
        if (targetType == typeof(char)) { return new CharConverter(); }
        if (targetType == typeof(uint)) { return new UInt32Converter(); }
        if (targetType == typeof(int)) { return new Int32Converter(); }
        if (targetType == typeof(ulong)) { return new UInt64Converter(); }
        if (targetType == typeof(long)) { return new Int64Converter(); }
        if (targetType == typeof(float)) { return new SingleConverter(); }
        if (targetType == typeof(double)) { return new DoubleConverter(); }
        if (targetType == typeof(decimal)) { return new DecimalConverter(); }
        if (targetType == typeof(TimeSpan)) { return new TimeSpanConverter(); }
        if (targetType == typeof(DateTime)) { return new DateTimeConverter(); }
        if (targetType == typeof(DateTimeOffset)) { return new DateTimeOffsetConverter(); }
        if (targetType == typeof(Uri)) { return new UriTypeConverter(); }
        if (targetType == typeof(Guid)) { return new GuidConverter(); }

        if (targetType.GetCustomAttribute<TypeConverterAttribute>() is TypeConverterAttribute tca &&
            Type.GetType(tca.ConverterTypeName, throwOnError: false) is Type converterType &&
            Activator.CreateInstance(converterType) is TypeConverter converter)
        {
            return converter;
        }

        // now returns a JSON-serializing TypeConverter by default, instead of returning null
        return new JsonSerializationTypeConverter();
    }

    private sealed class JsonSerializationTypeConverter : TypeConverter
    {
        public override bool CanConvertFrom(ITypeDescriptorContext? context, Type sourceType) => true;

        public override object? ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object value)
        {
            return JsonSerializer.Deserialize<object>((string)value);
        }

        public override object? ConvertTo(ITypeDescriptorContext? context, CultureInfo? culture, object? value, Type destinationType)
        {
            return JsonSerializer.Serialize(value);
        }
    }

When is serialization/deserialization required?

Required

  • Native to Semantic: Passing variables from Native to Semantic will require serialization of the output of the Native Function from complex type to string so that it can be passed to the LLM.
  • Semantic to Native: Passing variables from Semantic to Native will require de-serialization of the output of the Semantic Function between string to the complex type format that the Native Function is expecting.

Not required

  • Native to Native: Passing variables from Native to Native will not require any serialization or deserialization as the complex type can be passed as-is.
  • Semantic to Semantic: Passing variables from Semantic to Semantic will not require any serialization or deserialization as the the complex type will be passed around using its string representation.

2. Only use native serialization methods This option was originally considered, which would have effectively removed the use of the TypeConverters in favor of a simple JsonConverter, but it was pointed out that this may result in lossy conversion between primitive types. For example, when converting from a float to an int, the primitive may be truncated in a way by the native serialization methods that does not provide an accurate result.

Decision Outcome