字段值为 null 时，序列化或反序列化成其他值

using Newtonsoft.Json;
using Newtonsoft.Json.Serialization;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;

namespace Json.Useage
{
    /// <summary>
    /// serialize null to empty
    /// </summary>
    class Program
    {
        static void Main(string[] args)
        {
            var test = new Test();
            var settings = new JsonSerializerSettings() { ContractResolver = new NullToEmptyStringResolver() };
            var obj = JsonConvert.SerializeObject(test, settings);
            var obj2 = JsonConvert.SerializeObject(test);

            var json = "{'Field': null }";
            var t = JsonConvert.DeserializeObject<Test>(json, settings);
            Console.WriteLine(t.Field);
            Console.WriteLine(obj2);
            Console.WriteLine(obj);

            Console.ReadKey();
        }
    }

    public class Test
    {
        [DefaultValue("")]
        public string Field { get; set; }
    }

    public class NullToEmptyStringResolver : Newtonsoft.Json.Serialization.DefaultContractResolver
    {
        protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
        {
            return type.GetProperties()
                    .Select(p =>
                    {
                        var jp = base.CreateProperty(p, memberSerialization);
                        jp.ValueProvider = new NullToEmptyStringValueProvider(p);
                        return jp;
                    }).ToList();
        }
    }

    public class NullToEmptyStringValueProvider : IValueProvider
    {
        PropertyInfo _MemberInfo;
        public NullToEmptyStringValueProvider(PropertyInfo memberInfo)
        {
            _MemberInfo = memberInfo;
        }

        public object GetValue(object target)
        {
            object result = _MemberInfo.GetValue(target);

            if (_MemberInfo.PropertyType == typeof(string) && result == null) result = " ";

            return result;

        }

        public void SetValue(object target, object value)
        {
            if (_MemberInfo.PropertyType == typeof(string) && value == null) value = "";

            _MemberInfo.SetValue(target, value);
        }
    }

}

这里只写了 string 类型的，其他类型需要转换的再加 if 条件就好了