Umbraco-CMS/src/Umbraco.Infrastructure/Serialization/AutoInterningStringConverter.cs

using System;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;

namespace Umbraco.Cms.Infrastructure.Serialization
{

    /// <summary>
    /// When applied to a string or string collection field will ensure the deserialized strings are interned
    /// </summary>
    /// <remarks>
    /// Borrowed from https://stackoverflow.com/a/34906004/694494
    /// On the same page an interesting approach of using a local intern pool https://stackoverflow.com/a/39605620/694494 which re-uses .NET System.Xml.NameTable
    /// </remarks>
    public class AutoInterningStringConverter : JsonConverter
    {
        public override bool CanConvert(Type objectType)
        {
            // CanConvert is not called when a converter is applied directly to a property.
            throw new NotImplementedException($"{nameof(AutoInterningStringConverter)} should not be used globally");
        }

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            if (reader.TokenType == JsonToken.Null)
                return null;
            // Check is in case the value is a non-string literal such as an integer.
            var s = reader.TokenType == JsonToken.String
                ? string.Intern((string)reader.Value)
                : string.Intern((string)JToken.Load(reader));
            return s;
        }

        public override bool CanWrite => false;
        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) => throw new NotImplementedException();
    }
}