CustomizableJsonStringEnumConverter.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.ComponentModel;
using System.Diagnostics.CodeAnalysis;
#if !NET9_0_OR_GREATER
using System.Reflection;
using System.Text.Json;
#endif
using System.Text.Json.Serialization;
#if !NET9_0_OR_GREATER
using ModelContextProtocol;
#endif

// NOTE:
// This is a workaround for lack of System.Text.Json's JsonStringEnumConverter<T>
// 9.x support for JsonStringEnumMemberNameAttribute. Once all builds use the System.Text.Json 9.x
// version, this whole file can be removed. Note that the type is public so that external source
// generators can use it, so removing it is a potential breaking change.

namespace ModelContextProtocol
{
    /// <summary>
    /// A JSON converter for enums that allows customizing the serialized string value of enum members
    /// using the <see cref="JsonStringEnumMemberNameAttribute"/>.
    /// </summary>
    /// <typeparam name="TEnum">The enum type to convert.</typeparam>
    /// <remarks>
    /// This is a temporary workaround for lack of System.Text.Json's JsonStringEnumConverter&lt;T&gt;
    /// 9.x support for custom enum member naming. It will be replaced by the built-in functionality
    /// once .NET 9 is fully adopted.
    /// </remarks>
    [EditorBrowsable(EditorBrowsableState.Never)]
    public sealed class CustomizableJsonStringEnumConverter<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicFields)] TEnum> :
        JsonStringEnumConverter<TEnum> where TEnum : struct, Enum
    {
#if !NET9_0_OR_GREATER
        /// <summary>
        /// Initializes a new instance of the <see cref="CustomizableJsonStringEnumConverter{TEnum}"/> class.
        /// </summary>
        /// <remarks>
        /// The converter automatically detects any enum members decorated with <see cref="JsonStringEnumMemberNameAttribute"/>
        /// and uses those values during serialization and deserialization.
        /// </remarks>
        public CustomizableJsonStringEnumConverter() :
            base(namingPolicy: ResolveNamingPolicy())
        {
        }

        private static JsonNamingPolicy? ResolveNamingPolicy()
        {
            var map = typeof(TEnum).GetFields(BindingFlags.Public | BindingFlags.Static)
                .Select(f => (f.Name, AttributeName: f.GetCustomAttribute<JsonStringEnumMemberNameAttribute>()?.Name))
                .Where(pair => pair.AttributeName != null)
                .ToDictionary(pair => pair.Name, pair => pair.AttributeName);

            return map.Count > 0 ? new EnumMemberNamingPolicy(map!) : null;
        }

        private sealed class EnumMemberNamingPolicy(Dictionary<string, string> map) : JsonNamingPolicy
        {
            public override string ConvertName(string name) =>
                map.TryGetValue(name, out string? newName) ?
                    newName :
                    name;
        }
#endif
    }
}

#if !NET9_0_OR_GREATER
namespace System.Text.Json.Serialization
{
    /// <summary>
    /// Determines the custom string value that should be used when serializing an enum member using JSON.
    /// </summary>
    /// <remarks>
    /// This attribute is a temporary workaround for lack of System.Text.Json's support for custom enum member naming
    /// in versions prior to .NET 9. It works together with <see cref="CustomizableJsonStringEnumConverter{TEnum}"/>
    /// to provide customized string representations of enum values during JSON serialization and deserialization.
    /// </remarks>
    [AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
    internal sealed class JsonStringEnumMemberNameAttribute : Attribute
    {
        /// <summary>
        /// Creates new attribute instance with a specified enum member name.
        /// </summary>
        /// <param name="name">The name to apply to the current enum member when serialized to JSON.</param>
        public JsonStringEnumMemberNameAttribute(string name)
        {
            Name = name;
        }

        /// <summary>
        /// Gets the custom JSON name of the enum member.
        /// </summary>
        public string Name { get; }
    }
}
#endif