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

Question

我有一个如下所示的自定义类：

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; }
}

我想在序列化时忽略任何具有默认值的bool? 成员，但保留具有空值的字符串。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：

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

这是我目前的代码：

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

return JsonConvert.SerializeObject(options, settings);

有没有办法做到这一点而不用[JsonProperty(DefaultValueHandling = DefaultValueHandling.Ignore)] 单独标记每个OptionalBool 属性？谢谢。

Answer 1

您可以创建一个从DefaultContractResolver 继承的custom contract resolver，它将DefaultValueHandling.Ignore 应用于所需类型的所有属性。但是，您的具体问题有一些限制：

1. 您希望将默认值处理应用于系统类型，即bool?。因此，不能通过将一些自定义属性应用于类型，然后在CreateProperty() 中检查它来实现合同解析器。相反，有必要将一组覆盖传递给合约解析器的构造函数。

2. 您正在使用CamelCasePropertyNamesContractResolver。如果您将其子类化并将不同的覆盖列表传递给不同的实例，您将遇到来自this question 的错误，即强制在每个子类型的所有实例之间共享合约，即使不同的实例会返回不同的合约。因此有必要从没有此错误的DefaultContractResolver 继承，并将DefaultContractResolver.NamingStrategy 设置为CamelCaseNamingStrategy。

因此，您的合同解析器应如下所示：

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;
    }
}

然后像这样使用它：

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);

注意事项：

• 如果您有一个标准类型列表，您始终对其应用默认值处理覆盖，您应该cache an instance of the contract resolver 以获得最佳性能。

• Json.NET 9.0.1 中引入了合约解析器命名策略。如果您使用的是早期版本并需要驼色外壳，则需要将DefaultValueContractResolver 子类化并自己添加，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here.