这里的基本困难是 Json.NET 是一个基于契约的序列化器,它为每个要序列化的类型创建一个契约,然后根据契约进行序列化。无论类型出现在对象图中的哪个位置,都适用相同的约定。但是您希望根据给定类型在对象图中的深度有选择地包含其属性,这与基本的“一种类型一种契约”设计相冲突,因此需要一些工作。
完成你想要的事情的一种方法是创建一个JsonConverter对每个对象执行默认序列化,然后按照以下方式修剪不需要的属性在返回给客户端之前修改 JSON 的通用方法。请注意,这对于树等递归结构有问题,因为转换器必须针对子节点禁用自身以避免无限递归。
另一种可能性是创建一个custom IContractResolver根据序列化深度,为每种类型返回不同的合约。这必须需要利用序列化回调跟踪对象序列化何时开始和结束,因为合约解析器不知道序列化深度:
[System.AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = true, Inherited = true)]
public class JsonIncludeAtDepthAttribute : System.Attribute
{
public JsonIncludeAtDepthAttribute()
{
}
}
public class DepthPruningContractResolver : IContractResolver
{
readonly int depth;
public DepthPruningContractResolver()
: this(0)
{
}
public DepthPruningContractResolver(int depth)
{
if (depth < 0)
throw new ArgumentOutOfRangeException("depth");
this.depth = depth;
}
[ThreadStatic]
static DepthTracker currentTracker;
static DepthTracker CurrentTracker { get { return currentTracker; } set { currentTracker = value; } }
class DepthTracker : IDisposable
{
int isDisposed;
DepthTracker oldTracker;
public DepthTracker()
{
isDisposed = 0;
oldTracker = CurrentTracker;
currentTracker = this;
}
#region IDisposable Members
public void Dispose()
{
if (0 == Interlocked.Exchange(ref isDisposed, 1))
{
CurrentTracker = oldTracker;
oldTracker = null;
}
}
#endregion
public int Depth { get; set; }
}
abstract class DepthTrackingContractResolver : DefaultContractResolver
{
static DepthTrackingContractResolver() { } // Mark type with beforefieldinit.
static SerializationCallback OnSerializing = (o, context) =>
{
if (CurrentTracker != null)
CurrentTracker.Depth++;
};
static SerializationCallback OnSerialized = (o, context) =>
{
if (CurrentTracker != null)
CurrentTracker.Depth--;
};
protected override JsonObjectContract CreateObjectContract(Type objectType)
{
var contract = base.CreateObjectContract(objectType);
contract.OnSerializingCallbacks.Add(OnSerializing);
contract.OnSerializedCallbacks.Add(OnSerialized);
return contract;
}
}
sealed class RootContractResolver : DepthTrackingContractResolver
{
// As of 7.0.1, Json.NET suggests using a static instance for "stateless" contract resolvers, for performance reasons.
// http://www.newtonsoft.com/json/help/html/ContractResolver.htm
// http://www.newtonsoft.com/json/help/html/M_Newtonsoft_Json_Serialization_DefaultContractResolver__ctor_1.htm
// "Use the parameterless constructor and cache instances of the contract resolver within your application for optimal performance."
static RootContractResolver instance;
static RootContractResolver() { instance = new RootContractResolver(); }
public static RootContractResolver Instance { get { return instance; } }
}
sealed class NestedContractResolver : DepthTrackingContractResolver
{
static NestedContractResolver instance;
static NestedContractResolver() { instance = new NestedContractResolver(); }
public static NestedContractResolver Instance { get { return instance; } }
protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
{
var property = base.CreateProperty(member, memberSerialization);
if (property.AttributeProvider.GetAttributes(typeof(JsonIncludeAtDepthAttribute), true).Count == 0)
{
property.Ignored = true;
}
return property;
}
}
public static IDisposable CreateTracker()
{
return new DepthTracker();
}
#region IContractResolver Members
public JsonContract ResolveContract(Type type)
{
if (CurrentTracker != null && CurrentTracker.Depth > depth)
return NestedContractResolver.Instance.ResolveContract(type);
else
return RootContractResolver.Instance.ResolveContract(type);
}
#endregion
}
然后按如下方式标记您的课程:
class FooA
{
[JsonIncludeAtDepthAttribute]
public int SomeValueA { get; set; }
public int SomeValueB { get; set; }
public int SomeValueC { get; set; }
}
class FooB
{
public FooA FooA { get; set; }
}
并序列化如下:
var settings = new JsonSerializerSettings { ContractResolver = new DepthPruningContractResolver(depth), Formatting = Formatting.Indented };
using (DepthPruningContractResolver.CreateTracker())
{
var jsonB = JsonConvert.SerializeObject(foob, settings);
Console.WriteLine(jsonB);
var jsonA = JsonConvert.SerializeObject(foob.FooA, settings);
Console.WriteLine(jsonA);
}
稍显尴尬的CreateTracker()
需要确保,如果在序列化过程中抛出异常,当前对象深度会被重置并且不会影响将来的调用JsonConvert.SerializeObject()
.