JsonConverter 属性：使用自定义构造函数和 Autofac 反序列化

Question

我正在使用自定义 JsonConverter 来转换我的 JSON 对象。这是通过下面的 IQuery 对象的 JsonConverter 属性实现的

[JsonConverter(typeof(CustomConverter<IQuery>))]
public interface IQuery
{
}

下面是自定义泛型 class（为简洁起见，删除了一些位）

public class CustomConverter<T> : JsonConverter
{
    // This should be created via AutoFac
    public ICustomObjectCreator<T> ObjectCreator { get; set; }

    // This default constructr always gets called
    public CustomConverter() {}

    // I want to call this constructor
    [JsonConstructor]
    public CustomConverter(ICustomObjectCreator<T> objectCreator)
    {
        Context = context;
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
            return null;

        // Load JObject from stream 
        var jObject = JObject.Load(reader);

        // Create target object based on JObject 
        var target = Create(objectType, jObject);

        // Populate the object properties 
        serializer.Populate(jObject.CreateReader(), target);

        return target;
    }

    protected T Create(Type objectType, JObject jObject)
    {
        var type = jObject.GetValue("type", StringComparison.OrdinalIgnoreCase)?.Value<string>();
        return ObjectCreator.Create(type);
    }
}

ICustomObjectConverter 接口很简单

public interface ICustomObjectCreator<out T>
{
    T Create(string type);
}

及其实现之一

public class QueryObjectCreator : ICustomObjectCreator<IQuery>
{
    public IQuery Create(string type)
    {
        // ... some logic to create a concrete object
        return (IQuery)concreteObject;
    }
}

最后，Autofac 被连线以遵守上述内容

builder.RegisterType<QueryObjectCreator>()
       .As<ICustomObjectCreator<IQuery>>()
       .InstancePerLifetimeScope();

问题：

调用CustomJsonConverter时，只调用其默认构造函数。永远不会调用 JsonConstructor。
如果我删除默认构造函数，则永远不会调用整个 JsonConverter！

我知道调用 JsonConverter 时从未调用 AutoFac。我什至尝试属性注入来显式构造 QueryObjectConstruct，但即使这样也从未调用过。我怎样才能让它工作，以便通过 DI 注入我的 QueryObjectCretor？

我找到了 this 关于依赖注入和 JSON.net 反序列化的文章。但是，这是使用 DeserializeObject<>() 调用的手动解析，如果它有效，我如何才能使其与 JsonConverter 属性一起使用？

谢谢

Answer 1

您可以执行以下步骤来实现您的目标：

为您的 ICustomObjectCreator 接口创建一个非通用接口，以便更方便地创建对象。
引入一个通用的 ObjectCreatorBase<T> 基础 class，它调用您的通用 Create 方法。
创建和设置 JsonConvert 使用的默认设置。
将 AutofacContractResolver 设置为 ContractResolver。

请参阅以下示例以帮助您入门：

void Main()
{
    var builder = new ContainerBuilder();

    builder.RegisterType<QueryObjectCreator>()
        .As<ICustomObjectCreator<IQuery>>()
        .InstancePerLifetimeScope();

    var container = builder.Build();

    Func<JsonSerializerSettings> settingsFactory = () =>
    {
        var settings = new JsonSerializerSettings();
        settings.ContractResolver = new AutofacContractResolver(container);

        return settings;
    };

    JsonConvert.DefaultSettings = settingsFactory;

    var myObject = new MyObject { Query = new Query(42) };

    var json = JsonConvert.SerializeObject(myObject);

    myObject = JsonConvert.DeserializeObject<MyObject>(json);
    Console.WriteLine(myObject.Query.MyProperty);
}

// Define other methods and classes here
public class AutofacContractResolver : DefaultContractResolver
{
    private readonly IContainer _container;

    public AutofacContractResolver(IContainer container)
    {
        _container = container;
    }

    protected override JsonObjectContract CreateObjectContract(Type objectType)
    {
        JsonObjectContract contract = base.CreateObjectContract(objectType);

        var customObjectCreatorType = typeof(ICustomObjectCreator<>).MakeGenericType(objectType);

        if (!_container.IsRegistered(customObjectCreatorType))
            return contract;

        var customObjectCreator = (ICustomObjectCreator) _container.Resolve(customObjectCreatorType);

        // I don't know how you want to obtain the string which shall be passed to CreateObject
        contract.DefaultCreator = () => customObjectCreator.CreateObject("XYZ");
        return contract;
    }
}

public interface ICustomObjectCreator
{
    object CreateObject(string type);
}

public interface ICustomObjectCreator<out T> : ICustomObjectCreator
{
    T Create(string type);
}

public abstract class ObjectCreatorBase<T> : ICustomObjectCreator<T>
{
    public object CreateObject(string type)
    {
        return Create(type);
    }

    public abstract T Create(string type);
}

public class QueryObjectCreator : ObjectCreatorBase<IQuery>
{
    public override IQuery Create(string type)
    {
        Console.WriteLine("Create called");

        // ... some logic to create a concrete object
        var concreteObject = new Query();
        return (IQuery)concreteObject;
    }
}

public interface IQuery
{
    int MyProperty { get; set; }
}

public class Query : IQuery
{
    public int MyProperty { get; set; }

    public Query()
    {
    }

    public Query(int myProperty)
    {
        MyProperty = myProperty;
    }
}

public class MyObject
{
    public IQuery Query { get; set; }
}

输出应该是

Create called
42

也许您可以通过简单地使用 Autofac 直接创建对象来删除所有 ICustomObjectCreator 实例来简化代码。

更新

第一种方法可行，但它没有考虑到您需要获取一个字符串来决定您正在创建哪种对象 (type)。

要使其正常工作，您可以执行以下操作：

将 CustomConverter 注册为通用。
将 ResolveContractConverter 方法覆盖为 return 转换器实例，以防任何 ICustomObjectCreator 类型已注册。
更改 DefaultSettings 以便使用 AutofacContractResolver。

参见以下示例：

void Main()
{
    var builder = new ContainerBuilder();

    builder.RegisterType<QueryObjectCreator>()
        .As<ICustomObjectCreator<IQuery>>()
        .InstancePerLifetimeScope();

    builder.RegisterGeneric(typeof(CustomConverter<>)).AsSelf().InstancePerLifetimeScope();

    var container = builder.Build();

    Func<JsonSerializerSettings> settingsFactory = () =>
    {
        var settings = new JsonSerializerSettings();
        settings.ContractResolver = new AutofacContractResolver(container);

        return settings;
    };

    JsonConvert.DefaultSettings = settingsFactory;

    var myObject = new MyObject { Query = new Query(42) };

    var json = JsonConvert.SerializeObject(myObject);

    myObject = JsonConvert.DeserializeObject<MyObject>(json);
    Console.WriteLine(myObject.Query.MyProperty);
}

// Define other methods and classes here
public class AutofacContractResolver : DefaultContractResolver
{
    private readonly IContainer _container;

    public AutofacContractResolver(IContainer container)
    {
        _container = container;
    }

    protected override JsonConverter ResolveContractConverter(Type objectType)
    {
        var customObjectCreatorType = typeof(ICustomObjectCreator<>).MakeGenericType(objectType);
        if (!_container.IsRegistered(customObjectCreatorType))
            return base.ResolveContractConverter(objectType);

        var customConverterType = typeof(CustomConverter<>).MakeGenericType(objectType);
        return (JsonConverter) _container.Resolve(customConverterType);
    }
}

public class CustomConverter<T> : JsonConverter
{
    // This should be created via AutoFac
    public ICustomObjectCreator<T> ObjectCreator { get; }

    // This default constructr always gets called
    public CustomConverter() { }

    // I want to call this constructor
    public CustomConverter(ICustomObjectCreator<T> objectCreator)
    {
        Console.WriteLine("Constructor called");
        ObjectCreator = objectCreator;
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
            return null;

        // Load JObject from stream 
        var jObject = JObject.Load(reader);

        // Create target object based on JObject 
        var target = Create(objectType, jObject);

        // Populate the object properties 
        serializer.Populate(jObject.CreateReader(), target);

        return target;
    }

    protected T Create(Type objectType, JObject jObject)
    {
        var type = jObject.GetValue("type", StringComparison.OrdinalIgnoreCase)?.Value<string>();
        return ObjectCreator.Create(type);
    }
}

public interface ICustomObjectCreator<out T> 
{
    T Create(string type);
}

public class QueryObjectCreator : ICustomObjectCreator<IQuery>
{
    public IQuery Create(string type)
    {
        Console.WriteLine("Create called");

        // ... some logic to create a concrete object
        var concreteObject = new Query();
        return (IQuery)concreteObject;
    }
}

public interface IQuery
{
    int MyProperty { get; set; }
}

public class Query : IQuery
{
    public int MyProperty { get; set; }

    public Query()
    {
    }

    public Query(int myProperty)
    {
        MyProperty = myProperty;
    }
}

public class MyObject
{
    public IQuery Query { get; set; }
}

输出应该是

Constructor called
Create called
42

这是示例的 .NET Fiddle link。

Answer 2

假设 autofac 设置如下：

public class AutofacContractResolver : DefaultContractResolver
{
    private readonly IContainer _container;

    public AutofacContractResolver(IContainer container)
    {
        _container = container;
    }

    protected override JsonObjectContract CreateObjectContract(Type objectType)
    {
        JsonObjectContract contract = base.CreateObjectContract(objectType);

        // use Autofac to create types that have been registered with it
        if (_container.IsRegistered(objectType))
        {
           contract.DefaultCreator = () => _container.Resolve(objectType);
        }  

        return contract;
    }
}

那么，假设你的class是这样的：

public class TaskController
{
    private readonly ITaskRepository _repository;
    private readonly ILogger _logger;

    public TaskController(ITaskRepository repository, ILogger logger)
    {
        _repository = repository;
        _logger = logger;
    }

    public ITaskRepository Repository
    {
        get { return _repository; }
    }

    public ILogger Logger
    {
        get { return _logger; }
    }
}

因此，解析器在反序列化中的用法可能是这样的：

ContainerBuilder builder = new ContainerBuilder();
builder.RegisterType<TaskRepository>().As<ITaskRepository>();
builder.RegisterType<TaskController>();
builder.Register(c => new LogService(new DateTime(2000, 12, 12))).As<ILogger>();

IContainer container = builder.Build();

AutofacContractResolver contractResolver = new AutofacContractResolver(container);

string json = @"{
      'Logger': {
        'Level':'Debug'
      }
}";

// ITaskRespository and ILogger constructor parameters are injected by Autofac 
TaskController controller = JsonConvert.DeserializeObject<TaskController>(json, new JsonSerializerSettings
{
    ContractResolver = contractResolver
});

Console.WriteLine(controller.Repository.GetType().Name);

您可以在http://www.newtonsoft.com/json/help/html/DeserializeWithDependencyInjection.htm

中查看更多详细信息

JsonConverter 属性：使用自定义构造函数和 Autofac 反序列化

JsonConverter Attribute : deserialize using custom constructor and Autofac

c#

dependency-injection

autofac

json.net

更新