Newtonsoft.Json将可空类型转换为空字符串（支持所有类型）

第一步：json自定义解析协议

/// <summary>
/// json自定义解析协议
/// </summary>
public class SpecialJsonContractResolver : DefaultContractResolver
{
    protected override IValueProvider CreateMemberValueProvider(MemberInfo member)
    {
        if (member.MemberType == MemberTypes.Property)
        {
            var pi = (PropertyInfo)member;
            if (pi.PropertyType == typeof(string) || pi.PropertyType == typeof(DateTime) || pi.PropertyType == typeof(DateTime?))
            {
                return new NullableJsonValueProvider(pi);
            }
        }
        return base.CreateMemberValueProvider(member);
    }
}    <br>    /// <summary>
///空值序列化提供程序
/// </summary>
public class NullableJsonValueProvider : IValueProvider
{
    private static readonly string _defaultValue = string.Empty;
 
    private readonly PropertyInfo _propertyInfo;
 
    public NullableJsonValueProvider(PropertyInfo propertyInfo)
    {
        _propertyInfo = propertyInfo;
    }
 
    public void SetValue(object target, object value)
    {
        _propertyInfo.SetValue(target, value);
    }
 
    public object GetValue(object target)
    {
        return _propertyInfo.GetValue(target) ?? _defaultValue;
    }
}

第二步：空值处理（以时间类型为例）

public class DateTimeFormatConverter : IsoDateTimeConverter
   {
       private static readonly string strEmpty = string.Empty;
       public DateTimeFormatConverter(string format)
       {
           DateTimeFormat = format;
       }
 
       /// <summary>
       /// 重载序列化方法
       /// </summary>
       /// <param name="writer"></param>
       /// <param name="value"></param>
       /// <param name="serializer"></param>
       public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
       {
           if (value == null || value.Equals("") || value.Equals(DateTime.MinValue))
           {
               writer.WriteValue(strEmpty);
           }
           else
           {
               base.WriteJson(writer, value, serializer);
           }
       }
   }

第三步：使用

public string TestJson()
{
    JsonSerializerSettings settings = new JsonSerializerSettings();
    settings.Converters.Add(new DecimalDigitsConvert());//只处理decimal类型的属性         
    settings.Converters.Add(new DateTimeFormatConverter("yyyy/MM/dd"));//默认时间格式处理        
    settings.ContractResolver = new SpecialJsonContractResolver(); //自定义解析协议  
 
    string json = JsonConvert.SerializeObject(new Models.StudentInfo.StudentInfo_ClassInfo() { ClassName = "班级名称" }, settings);
    return json;
}