JSON.NET:如何使DefaultValueHandling仅适用于某些类型

Question

我有一个看起来像这样的自定义类：

public class PartnerLoginOptions
{
    public string Username { get; set; }
    public string Password { get; set; }
    public string DeviceModel { get; set; }
    public string Version { get; set; }
    public bool? IncludeUrls { get; set; }
    public bool? ReturnDeviceType { get; set; }
    public bool? ReturnUpdatePromptVersions { get; set; }
}

我想忽略序列化时具有默认值的任何bool?成员，但要保持带有空值的字符串。例如，如果我有这样的对象

var options = new PartnerLoginOptions
{
    Username = null,
    Password = "123",
    IncludeUrls = null,
    ReturnDeviceType = true
};

然后序列化将导致这一点：

{
    "username": null,
    "password": "123",
    "deviceModel": null,
    "version": null,
    "returnDeviceType": true
}

这是我到目前为止的代码：

var settings = new JsonSerializerSettings
{
    ContractResolver = new CamelCasePropertyNamesContractResolver(),
    DefaultValueHandling = DefaultValueHandling.Ignore // this applies to strings too, not just bool?
};
return JsonConvert.SerializeObject(options, settings);

有什么方法可以在不单独使用[JsonProperty(DefaultValueHandling = DefaultValueHandling.Ignore)]标记每个OptionalBool属性的情况下进行此操作？谢谢。

Answer 1

您可以创建一个从DefaultContractResolver继承的自定义合同解析器，该解析器将DefaultValueHandling.Ignore应用于所需类型的所有属性。但是，您的特定问题有一些约束：

1. 您想将默认值处理应用于系统类型，即bool?。因此，不能通过将某些自定义属性应用于该类型，然后在CreateProperty()中检查合同解决方案。相反，有必要将一系列覆盖物传递给合同解析器的构造函数。

2. 您正在使用CamelCasePropertyNamesContractResolver。如果您将其子类并将不同的替代列表传递给不同的实例，那么您将遇到这个问题的错误，即在每个子类型的所有实例中都强行共享合同，即使不同的实例否则会返回不同的合同。因此，有必要从没有此错误的DefaultContractResolver继承，然后将DefaultContractResolver.NamingStrategy设置为CamelCaseNamingStrategy。

因此，您的合同解析器应该看起来像：

public class DefaultValueContractResolver : DefaultContractResolver
{
    readonly Dictionary<Type, DefaultValueHandling> overrides;
    public DefaultValueContractResolver(IEnumerable<KeyValuePair<Type, DefaultValueHandling>> overrides) : base()
    {
        if (overrides == null)
            throw new ArgumentNullException("overrides");
        this.overrides = overrides.ToDictionary(p => p.Key, p => p.Value);
    }
    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        var property = base.CreateProperty(member, memberSerialization);
        if (property.DefaultValueHandling == null)
        {
            DefaultValueHandling handling;
            if (overrides.TryGetValue(property.PropertyType, out handling))
                property.DefaultValueHandling = handling;
        }
        return property;
    }
}

然后使用它：

var options = new PartnerLoginOptions
{
    Username = null,
    Password = "123",
    IncludeUrls = null,
    ReturnDeviceType = true
};
var resolver = new DefaultValueContractResolver(
    new Dictionary<Type, DefaultValueHandling>
    { 
        { typeof(bool?), DefaultValueHandling.Ignore } 
    })
    {
        NamingStrategy = new CamelCaseNamingStrategy()
    };
var settings = new JsonSerializerSettings { ContractResolver = resolver };
var json = JsonConvert.SerializeObject(options, Formatting.Indented, settings);

注意：

• 如果您有一个标准的类型列表，您始终应用默认值处理覆盖，则应缓存合同解析器的实例以获得最佳性能。

• JSON.NET 9.0.1中引入了合同解析器命名策略。如果您使用的是较早版本并且需要骆驼套，则需要子类DefaultValueContractResolver并自己添加，例如如下：

  public class DefaultValueCamelCaseContractResolver : DefaultValueContractResolver
  {
      public DefaultValueCamelCaseContractResolver(IEnumerable<KeyValuePair<Type, DefaultValueHandling>> overrides) : base(overrides) { }
      protected override string ResolvePropertyName(string propertyName)
      {
          return propertyName.ToCamelCase();
      }
  }
  public static class CamelCaseNameExtensions
  {
      class CamelCaseNameResolver : CamelCasePropertyNamesContractResolver
      {
          // Explicit static constructor to tell C# compiler not to mark type as beforefieldinit
          static CamelCaseNameResolver() { }
          internal static readonly CamelCaseNameResolver Instance = new CamelCaseNameResolver();
          // Purely to make the protected method public.
          public string ToCamelCase(string propertyName)
          {
              return ResolvePropertyName(propertyName);
          }
      }
      public static string ToCamelCase(this string propertyName)
      {
          if (propertyName == null)
              return null;
          return CamelCaseNameResolver.Instance.ToCamelCase(propertyName);
      }
  }
  

演示小提琴。