如何反序列化Newtonsoft Json.. NET引用单独的、单独的实例

本文关键字：单独 实例 引用 NET 反序列化 Newtonsoft Json | 更新日期: 2023-09-27 18:02:42

我有一段JSON，看起来像这样:

[
  {
    "$id": "1",
    "Name": "James",
    "BirthDate": "1983-03-08T00:00Z",
    "LastModified": "2012-03-21T05:40Z"
  },
  {
    "$ref": "1"
  }
]

从$ref可以看出，这个JSON数组两次包含同一个Person (James)。第二次是第一次的引用。

我想知道是否有一种方法来反序列化这个JSON成一个对象，其中包含两个副本的詹姆斯人。

目前，我使用这个:

var jsonSerializerSettings = new JsonSerializerSettings()
{
     PreserveReferencesHandling = PreserveReferencesHandling.None,
     ReferenceLoopHandling = ReferenceLoopHandling.Ignore
};
var deserializedPersons = JsonConvert.DeserializeObject<List<Person>>(json, jsonSerializerSettings);

但是这只是给了我一个包含Person的相同实例的数组，两次:

object.ReferenceEquals(deserializedPersons[0], deserializedPersons[1]) // Evaluates to true

我发现了一个我不满意的解决方案，它只是反序列化JSON字符串，然后使用上面的jsonSerializerSettings序列化它，这将复制JSON中的人，然后再反序列化。这将导致我们正在使用的大型对象的严重减速。

注:我知道我可以改变API，我检索这个JSON从中复制数据，但保留引用节省大量空间时，通过网络发送响应JSON

如何反序列化Newtonsoft Json.. NET引用单独的、单独的实例

。

您可以使用自定义引用解析器。例如，假设Name是对象的"主键"，这应该可以工作。当然，你可能想让它更通用。

public class PersonReferenceResolver : IReferenceResolver
{
    private readonly IDictionary<string, Person> _objects = 
        new Dictionary<string, Person>();
    public object ResolveReference(object context, string reference)
    {
        Person p;
        if (_objects.TryGetValue(reference, out p))
        {
            //This is the "clever" bit. Instead of returning the found object
            //we just return a copy of it.
            //May be better to clone your class here...
            return new Person
            {
                Name = p.Name,
                BirthDate = p.BirthDate,
                LastModified = p.LastModified
            };
        }
        return null;
    }
    public string GetReference(object context, object value)
    {
        Person p = (Person)value;
        _objects[p.Name] = p;
        return p.Name;
    }
    public bool IsReferenced(object context, object value)
    {
        Person p = (Person)value;
        return _objects.ContainsKey(p.Name);
    }
    public void AddReference(object context, string reference, object value)
    {
        _objects[reference] = (Person)value;
    }
}

现在你像这样反序列化:

var jsonSerializerSettings = new JsonSerializerSettings()
{
    ReferenceResolver = new PersonReferenceResolver()
};
var deserializedPersons = JsonConvert.DeserializeObject<List<Person>>(
    json, jsonSerializerSettings);

编辑:我很无聊，所以我做了一个通用版本:

public class GenericResolver<TEntity> : IReferenceResolver
    where TEntity : ICloneable, new()
{
    private readonly IDictionary<string, TEntity> _objects = new Dictionary<string, TEntity>();
    private readonly Func<TEntity, string> _keyReader;
    public GenericResolver(Func<TEntity, string> keyReader)
    {
        _keyReader = keyReader;
    }
    public object ResolveReference(object context, string reference)
    {
        TEntity o;
        if (_objects.TryGetValue(reference, out o))
        {
            return o.Clone();
        }
        return null;
    }
    public string GetReference(object context, object value)
    {
        var o = (TEntity)value;
        var key = _keyReader(o);
        _objects[key] = o;
        return key;
    }
    public bool IsReferenced(object context, object value)
    {
        var o = (TEntity)value;
        return _objects.ContainsKey(_keyReader(o));
    }
    public void AddReference(object context, string reference, object value)
    {
        if(value is TEntity)
            _objects[reference] = (TEntity)value;
    }
}

有一点新用法:

var jsonSerializerSettings = new JsonSerializerSettings()
{
    //Now we need to specify the type and how to get the object's key
    ReferenceResolver = new GenericResolver<Person>(p => p.Name)
};
var deserializedPersons = JsonConvert.DeserializeObject<List<Person>>(
    json, jsonSerializerSettings);

$ref反序列化有问题。只有当元数据位于请求/响应的开头时，PreserveReferencesHandling才有帮助。否则我建议使用:

var settings = new JsonSerializerSettings();
settings.PreserveReferencesHandling = PreserveReferencesHandling.Objects; 
settings.MetadataPropertyHandling = MetadataPropertyHandling.Ignore; //ign
var deserializedObjects = Newtonsoft.Json.JsonConvert.DeserializeObject<T>(requestResult.Content, settings);

关于https://www.newtonsoft.com/json/help/html/T_Newtonsoft_Json_MetadataPropertyHandling.htm

Default 0   Read metadata properties located at the start of a JSON object.
ReadAhead   1   Read metadata properties located anywhere in a JSON object. Note that this setting will impact performance.
Ignore  2   Do not try to read metadata properties.
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