Json.NET 自定义 ValueProvider 将对象转换为 Guid

Question

我正在尝试为 Json.NET 创建一个自定义 ValueProvider，它将跳过对所有对象的序列化，并且将只返回一个 Guid 类型的属性来表示它们的主键（作为参考）。

例子：

jsonData: {
    myObject: {
        id: "23e23-2gg5-6y666556-y6yg33",
        property2: ""
    }
}

应该变成：

jsonData: {
    myObjectId: "23e23-2gg5-6y666556-y6yg33"
}

这是我目前写的代码。我非常接近让它工作，但在我的CustomValueProvider 中，我似乎无法获得对象值。我该怎么做？

    private class CustomValueProvider : IValueProvider
    {
        private readonly MemberInfo _member;

        public CustomValueProvider(MemberInfo member)
        {
            _member = member;
        }
        public void SetValue(object target, object value)
        {
            throw new NotImplementedException();
        }

        public object GetValue(object target)
        {
            return // WHAT HERE??
        }
    }

    private class CustomResolver : CamelCasePropertyNamesContractResolver
    {
        protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
        {
            var jsonProperty = base.CreateProperty(member, memberSerialization);

            if (jsonProperty.PropertyType.IsClass && jsonProperty.PropertyType != typeof(string))
            {
                jsonProperty = new JsonProperty
                {
                    PropertyName = member.Name.ToFirstCharLower() + "Id",
                    Readable = true,
                    ShouldSerialize = value => true,
                    PropertyType = typeof(Guid),
                    ValueProvider = new CustomValueProvider(member)
                };
            }

            return jsonProperty;
        }
    }

    private static readonly JsonSerializerSettings JsonSettings = new JsonSerializerSettings
    {
        ContractResolver = new CustomResolver(),
        Formatting = Formatting.Indented
    };

Answer 1

您希望将嵌套对象内部的 "Id" 值提升到父对象。为此，您需要将两个价值提供者链接在一起：

• 获取成员值的外部值提供者。
• 获取成员Id值的内部值提供者。

以下是这样做的：

class NestedValueProvider : IValueProvider
{
    readonly IValueProvider outerProvider;
    readonly IValueProvider innerProvider;

    public NestedValueProvider(IValueProvider outerProvider, IValueProvider innerProvider)
    {
        if (outerProvider == null || innerProvider == null)
            throw new ArgumentNullException();
        this.outerProvider = outerProvider;
        this.innerProvider = innerProvider;
    }
    public void SetValue(object target, object value)
    {
        throw new NotImplementedException();
    }

    public object GetValue(object target)
    {
        var innerTarget = outerProvider.GetValue(target);
        if (innerTarget == null)
            return null;
        return innerProvider.GetValue(innerTarget);
    }
}

class CustomResolver : CamelCasePropertyNamesContractResolver
{
    // Using an inner resolver prevents difficulties with recursion.
    readonly CamelCasePropertyNamesContractResolver innerResolver = new CamelCasePropertyNamesContractResolver();

    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        var jsonProperty = base.CreateProperty(member, memberSerialization);

        if (!jsonProperty.PropertyType.IsPrimitive && jsonProperty.PropertyType != typeof(string) && jsonProperty.Readable)
        {
            var innerContract = innerResolver.ResolveContract(jsonProperty.PropertyType);
            if (innerContract is JsonObjectContract)
            {
                var objectContract = (JsonObjectContract)innerContract;
                var idProperty = objectContract.Properties.GetClosestMatchProperty(ResolvePropertyName("Id"));
                if (idProperty != null && idProperty.Readable && (innerResolver.ResolveContract(idProperty.PropertyType) is JsonPrimitiveContract))
                {
                    jsonProperty = new JsonProperty
                    {
                        PropertyName = ResolvePropertyName(member.Name + "Id"),
                        Readable = true,
                        PropertyType = idProperty.PropertyType,
                        ValueProvider = new NestedValueProvider(jsonProperty.ValueProvider, idProperty.ValueProvider),
                    };
                }
            }
            // Possibly handle innerContract is JsonArrayContract?
            // Possibly handle innerContract is JsonDictionaryConract?
        }

        return jsonProperty;
    }
}

注意内部合约解析器的使用。这可以防止递归类型的递归调用出现问题。

您可能还需要考虑处理具有 ID 的对象集合或具有 ID 的对象的字典。例如在下面的对象中，List<MyObject> 和 Dictionary<string, MyObject> 属性将暴露 MyObject 的内容：

public class RootObject
{
    // Correctly not remapped
    public string StringValue { get; set; }

    // Correctly remaps to a GUID.
    public MyObject MyObject { get; set; }

    // Remap to a List<Guid> ?
    public List<MyObject> MyObjectList { get; set; }

    // Remap to a Dictionary<string, Guid> ?
    public Dictionary<string, MyObject> MyObjectDictionary { get; set; }
}

public class MyObject
{
    public Guid Id { get; set; }
    public string Property2 { get; set; }
}