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