(可选)根据其运行时值序列化属性

Question

从根本上说,我想根据序列化时的值,在生成的Json中包含或省略属性.

更具体地说,我有一个知道是否已为其分配值的类型,并且我只想序列化该类型的属性(如果已经为其分配了某些内容(因此我需要在运行时检查该值).我试图让我的API轻松检测"有默认值"和"根本没有指定"之间的区别.

自定义JsonConverter似乎不够用; 我试过了,我相信在调用转换器之前,属性名已经序列化了.在我的情况下,我想省略属性名称.

我已经看过扩展DefaultContractResolver,但CreateProperty和CreateProperties(返回JsonProperty序列化元数据)只接受被序列化的Type,所以我无法检查实例本身.在一般情况下,我没有看到在DefaultContractResolver让我来控制任何东西,如果一个实例被序列化; 也许我错过了它.

我也想过我可能需要创建一个ContractResolver,为我的类型返回一个自定义的JsonObjectContract.但是,我再也没有在JsonObjectContract上看到任何基于实例做出决策的内容.

有没有一个很好的方法来实现我的目标？我只是缺少一些简单的东西吗？非常感谢您提供的任何帮助.由于Json.NET是如此可扩展,我认为这不会太难.但是我开始认为我已经离开这里的杂草了.:)

Answer 1

好吧,在Json.NET源码中挖掘了一段时间之后,我终于得到了这个工作,它甚至会尊重Json.NET支持的ShouldSerialize*和*Specified成员.警告:这肯定会进入杂草.

所以我意识到,DefaultContractResolver.CreateProperty返回JsonProperty类有ShouldSerialize和转换器性能,这让我来指定,如果属性实例实际上应该被序列化,如果是这样,怎么办呢.

然而,反序列化需要一些不同的东西.默认情况下,对于自定义类型,DefaultContractResolver.ResolveContract将返回具有null Converter属性的JsonObjectContract.为了正确反序列化我的类型,我需要在合同适用于我的类型时设置Converter属性.

这是代码(删除错误处理/等以保持尽可能小的代码).

首先,需要特殊处理的类型:

public struct Optional<T>
{
    public readonly bool ValueProvided;
    public readonly T Value;

    private Optional( T value )
    {
        this.ValueProvided = true;
        this.Value = value;
    }

    public static implicit operator Optional<T>( T value )
    {
        return new Optional<T>( value );
    }
}

在我们知道它应该被序列化后,转换器将正确地序列化它:

public class OptionalJsonConverter<T> : JsonConverter
{
    public static OptionalJsonConverter<T> Instance = new OptionalJsonConverter<T>();

    public override void WriteJson( JsonWriter writer, object value, JsonSerializer serializer )
    {
        var optional = (Optional<T>)value; // Cast so we can access the Optional<T> members
        serializer.Serialize( writer, optional.Value );
    }

    public override object ReadJson( JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer )
    {
        var valueType = objectType.GetGenericArguments()[ 0 ];
        var innerValue = (T)serializer.Deserialize( reader, valueType );
        return (Optional<T>)innerValue; // Explicitly invoke the conversion from T to Optional<T>
    }

    public override bool CanConvert( Type objectType )
    {
        return objectType == typeof( Optional<T> );
    }
}

最后,最冗长的是,这里是插入钩子的ContractResolver:

public class CustomContractResolver : DefaultContractResolver
{
    // For deserialization. Detect when the type is being deserialized and set the converter for it.
    public override JsonContract ResolveContract( Type type )
    {
        var contract = base.ResolveContract( type );
        if( contract.Converter == null && type.IsGenericType && type.GetGenericTypeDefinition() == typeof( Optional<> ) )
        {
            // This may look fancy but it's just calling GetOptionalJsonConverter<T> with the correct T
            var optionalValueType = type.GetGenericArguments()[ 0 ];
            var genericMethod = this.GetAndMakeGenericMethod( "GetOptionalJsonConverter", optionalValueType );
            var converter = (JsonConverter)genericMethod.Invoke( null, null );
            // Set the converter for the type
            contract.Converter = converter;
        }
        return contract;
    }

    public static OptionalJsonConverter<T> GetOptionalJsonConverter<T>()
    {
        return OptionalJsonConverter<T>.Instance;
    }

    // For serialization. Detect when we're creating a JsonProperty for an Optional<T> member and modify it accordingly.
    protected override JsonProperty CreateProperty( MemberInfo member, MemberSerialization memberSerialization )
    {
        var jsonProperty = base.CreateProperty( member, memberSerialization );
        var type = jsonProperty.PropertyType;
        if( type.IsGenericType && type.GetGenericTypeDefinition() == typeof( Optional<> ) )
        {
            // This may look fancy but it's just calling SetJsonPropertyValuesForOptionalMember<T> with the correct T
            var optionalValueType = type.GetGenericArguments()[ 0 ];
            var genericMethod = this.GetAndMakeGenericMethod( "SetJsonPropertyValuesForOptionalMember", optionalValueType );
            genericMethod.Invoke( null, new object[]{ member.Name, jsonProperty } );
        }
        return jsonProperty;
    }

    public static void SetJsonPropertyValuesForOptionalMember<T>( string memberName, JsonProperty jsonProperty )
    {
        if( jsonProperty.ShouldSerialize == null ) // Honor ShouldSerialize*
        {
            jsonProperty.ShouldSerialize =
                ( declaringObject ) =>
                {
                    if( jsonProperty.GetIsSpecified != null && jsonProperty.GetIsSpecified( declaringObject ) ) // Honor *Specified
                    {
                        return true;
                    }                    
                    object optionalValue;
                    if( !TryGetPropertyValue( declaringObject, memberName, out optionalValue ) &&
                        !TryGetFieldValue( declaringObject, memberName, out optionalValue ) )
                    {
                        throw new InvalidOperationException( "Better error message here" );
                    }
                    return ( (Optional<T>)optionalValue ).ValueProvided;
                };
        }
        if( jsonProperty.Converter == null )
        {
            jsonProperty.Converter = CustomContractResolver.GetOptionalJsonConverter<T>();
        }
    }

    // Utility methods used in this class
    private MethodInfo GetAndMakeGenericMethod( string methodName, params Type[] typeArguments )
    {
        var method = this.GetType().GetMethod( methodName, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static );
        return method.MakeGenericMethod( typeArguments );
    }

    private static bool TryGetPropertyValue( object declaringObject, string propertyName, out object value )
    {
        var propertyInfo = declaringObject.GetType().GetProperty( propertyName, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance );
        if( propertyInfo == null )
        {
            value = null;
            return false;
        }
        value = propertyInfo.GetValue( declaringObject, BindingFlags.GetProperty, null, null, null );
        return true;
    }

    private static bool TryGetFieldValue( object declaringObject, string fieldName, out object value )
    {
        var fieldInfo = declaringObject.GetType().GetField( fieldName, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance );
        if( fieldInfo == null )
        {
            value = null;
            return false;
        }
        value = fieldInfo.GetValue( declaringObject );
        return true;
    }
}

我希望能帮助别人.如果有什么不清楚或者看起来我错过了什么,请随意提问.