C# Newtonsoft.Json.Linq.JValue 总是返回 Int64

Question

我正在使用 Newtonsoft.Json 程序集将 Json 字符串反序列化为动态对象 (ExpandoObject)。我遇到的问题是 int 值总是作为 Int64 返回，而我期待的是 Int32。代码如下。

namespace Serialization
{
    using System;
    using System.Collections.Generic;
    using System.Dynamic;
    using System.Linq;

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

    public static class JsonSerializer
    {
        #region Public Methods

        public static string Serialize(dynamic obj)
        {
            return JsonConvert.SerializeObject(obj);
        }

        public static dynamic Deserialize(string s)
        {
            var obj = JsonConvert.DeserializeObject(s);
            return obj is string ? obj as string : Deserialize((JToken)obj);
        }

        #endregion

        #region Methods

        private static dynamic Deserialize(JToken token)
        {
            // FROM : http://blog.petegoo.com/archive/2009/10/27/using-json.net-to-eval-json-into-a-dynamic-variable-in.aspx
            // Ideally in the future Json.Net will support dynamic and this can be eliminated.
            if (token is JValue) return ((JValue)token).Value;
            if (token is JObject)
            {
                var expando = new ExpandoObject();
                (from childToken in token
                 where childToken is JProperty
                 select childToken as JProperty).ToList().
                    ForEach(property => ((IDictionary<string, object>)expando).Add(property.Name, Deserialize(property.Value)));
                return expando;
            }
            if (token is JArray)
            {
                var items = new List<object>();
                foreach (var arrayItem in ((JArray)token)) items.Add(Deserialize(arrayItem));
                return items;
            }
            throw new ArgumentException(string.Format("Unknown token type '{0}'", token.GetType()), "token");
        }

        #endregion
    }
}

通常我不会注意到这一点，但是这个特殊的 int 正被用于反射以进行某些类型检查，但它失败了。任何想法为什么会发生这种情况将不胜感激。

Answer 1

交叉链接回答https://stackoverflow.com/a/9444519/1037948

来自How do I change the default Type for Numeric deserialization?

释义：

• 作者故意选择所有 int 都返回为 Int64 以避免溢出错误，并且更容易检查（对于 Json.NET 内部，而不是您）
• 您可以使用自定义转换器（如链接答案中发布的转换器）来解决此问题。

这是一个非常通用的转换器；不完全确定CanConvert 检查，但对我有用的重要部分是允许typeof(object)：

/// <summary>
/// To address issues with automatic Int64 deserialization -- see https://stackoverflow.com/a/9444519/1037948
/// </summary>
public class JsonInt32Converter : JsonConverter
{
    #region Overrides of JsonConverter

    /// <summary>
    /// Only want to deserialize
    /// </summary>
    public override bool CanWrite { get { return false; } }

    /// <summary>
    /// Placeholder for inheritance -- not called because <see cref="CanWrite"/> returns false
    /// </summary>
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        // since CanWrite returns false, we don't need to implement this
        throw new NotImplementedException();
    }

    /// <summary>
    /// Reads the JSON representation of the object.
    /// </summary>
    /// <param name="reader">The <see cref="T:Newtonsoft.Json.JsonReader"/> to read from.</param><param name="objectType">Type of the object.</param><param name="existingValue">The existing value of object being read.</param><param name="serializer">The calling serializer.</param>
    /// <returns>
    /// The object value.
    /// </returns>
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        return (reader.TokenType == JsonToken.Integer)
            ? Convert.ToInt32(reader.Value)     // convert to Int32 instead of Int64
            : serializer.Deserialize(reader);   // default to regular deserialization
    }

    /// <summary>
    /// Determines whether this instance can convert the specified object type.
    /// </summary>
    /// <param name="objectType">Type of the object.</param>
    /// <returns>
    /// <c>true</c> if this instance can convert the specified object type; otherwise, <c>false</c>.
    /// </returns>
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(Int32) ||
                objectType == typeof(Int64) ||
                // need this last one in case we "weren't given" the type
                // and this will be accounted for by `ReadJson` checking tokentype
                objectType == typeof(object)
            ;
    }

    #endregion
}

Answer 2

我遇到了一些类似的问题，但继续回答您的问题 - 如果可能，请选择Int32，然后如果可能，请选择Int16。我也包括了测试。对于未来的读者来说，对其他值类型也这样做是有意义的，但我在这里只实现了有符号整数。

namespace Serialization
{
    using System;
    using System.Collections.Generic;
    using System.Dynamic;
    using System.Linq;

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

    public static class JsonSerializer
    {
        #region Public Methods

        public static string Serialize(dynamic obj)
        {
            return JsonConvert.SerializeObject(obj);
        }

        public static dynamic Deserialize(string s)
        {
            var obj = JsonConvert.DeserializeObject(s);
            return obj is string ? obj as string : Deserialize((JToken)obj);
        }

        #endregion

        #region Methods

        private static dynamic Deserialize(JToken token)
        {
            // FROM : http://blog.petegoo.com/archive/2009/10/27/using-json.net-to-eval-json-into-a-dynamic-variable-in.aspx
            // Ideally in the future Json.Net will support dynamic and this can be eliminated.
            if (token is JValue)
            {
                var value = ((JValue)token).Value;
                if (value is Int64)
                {
                    var lValue = (Int64)value;
                    if (Int32.MinValue <= lValue && lValue <= 0 || 0 < lValue && lValue <= Int32.MaxValue)
                    {
                        var iValue = (Int32)lValue;
                        value = iValue;
                        // Take out this if you don't want to cast down to Int16.
                        if (Int16.MinValue <= iValue && iValue <= 0 || 0 < iValue && iValue <= Int16.MaxValue)
                        {
                            value = (Int16)iValue;
                        }
                    }
                }
                return value;
            }
            if (token is JObject)
            {
                var expando = new ExpandoObject();
                (from childToken in token
                 where childToken is JProperty
                 select childToken as JProperty).ToList().
                    ForEach(property => ((IDictionary<string, object>)expando).Add(property.Name, Deserialize(property.Value)));
                return expando;
            }
            if (token is JArray)
            {
                var items = new List<object>();
                foreach (var arrayItem in ((JArray)token)) items.Add(Deserialize(arrayItem));
                return items;
            }
            throw new ArgumentException(string.Format("Unknown token type '{0}'", token.GetType()), "token");
        }

        #endregion
    }
}

namespace Serialization.Tests
{
    public class JsonSerializerTests
    {
        [Test]
        public void ShouldDeserializeAsInt16([Values(0, Int16.MaxValue, Int16.MinValue)] Int16 x)
        {
            var json = string.Format("{{ x: {0} }}", x);
            var dynamic = JsonSerializer.Deserialize(json);

            Assert.That(dynamic.x.GetType(), Is.EqualTo(typeof(Int16)));
        }

        [Test]
        public void ShouldDeserializeAsInt32([Values(Int16.MaxValue + 1, Int16.MinValue - 1)] Int32 x)
        {
            var json = string.Format("{{ x: {0} }}", x);
            var dynamic = JsonSerializer.Deserialize(json);

            Assert.That(dynamic.x.GetType(), Is.EqualTo(typeof(Int32)));
        }

        [Test]
        public void ShouldDeserializeAsInt64([Values(Int32.MaxValue + 1L, Int32.MinValue - 1L)] Int64 x)
        {
            var json = string.Format("{{ x: {0} }}", x);
            var dynamic = JsonSerializer.Deserialize(json);

            Assert.That(dynamic.x.GetType(), Is.EqualTo(typeof(Int64)));
        }
    }
}