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

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 属性来实现的

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

自定义泛型类如下（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 已满足上述要求

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

问题：

1. 调用 CustomJsonConverter 时，仅调用其默认构造函数。 JsonConstructor 永远不会被调用。
2. 如果我删除默认构造函数，那么整个 JsonConverter 永远不会被调用！

我有一个墨水链接，当调用 JsonConverter 时，AutoFac 永远不会被调用。我什至尝试使用属性注入来显式构造 QueryObjectConstruct，但即使如此也从未被调用。我怎样才能让它工作，以便通过 DI 注入我的 QueryObjectCretor？

I found this关于依赖注入和 JSON.net 反序列化的文章。但是，这是使用 DeserializeObject() 调用进行手动解析，如果有效，我如何才能使其与 JsonConverter 属性一起使用？

Thanks

您可以执行以下步骤来实现您的目标：

1. 为您创建一个非通用接口ICustomObjectCreator接口，使创建对象更加方便。
2. 引入一个通用的ObjectCreatorBase<T>调用您的泛型的基类Create method.
3. 创建并设置默认设置JsonConvert.
4. Set the AutofacContractResolver as 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

也许你可以通过删除所有来简化代码ICustomObjectCreator只需使用 Autofac 直接创建对象即可实例化。

Update

第一种方法有效，但它没有考虑到您需要获取一个字符串来决定您要创建哪种类型的对象（type).

要使其正常工作，您可以执行以下操作：

1. 注册CustomConverter作为通用的。
2. 覆盖ResolveContractConverter方法在任何情况下返回转换器的实例ICustomObjectCreator已注册该类型。
3. 改变DefaultSettings所以这样AutofacContractResolver将会被使用。

请参见以下示例：

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

这里是.NET Fiddle 链接对于样品。