自定义JsonConverter属性:获取属性名称

Question

我有这个示例转换器的guid属性:

public class CustomGuidConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof (Guid?) || objectType == typeof (Guid);
    }
    public override void WriteJson(JsonWriter writer, object oldValue, JsonSerializer serializer)
    {
        if (value != null)
        {
            var newValue = convert(oldValue); // do some conversion
            writer.WriteValue(newValue);
        }
    }
}

像这样使用

public class Outer {
    public int Id { get; set; }
    [JsonConverter(typeof(InterfaceLabelConverter))]
    public Guid? ProductFamilyId { get; set; }
}

如何访问WriteJson方法中当前属性的名称?我想用另一个属性名将旧值写入writer，像这样:

{ Id: 1234, ProductFamilyId: 'newValue', ProductFamilyIdOld: 'oldValue' }

Answer 1

我建议简单地将转换后的GUID作为包含类的私有get-only属性。如果你用[JsonProperty]标记它，它将被序列化:

public class Outer
{
    public int Id { get; set; }
    public Guid? ProductFamilyId { get; set; }
    [JsonProperty(NullValueHandling=NullValueHandling.Ignore)]
    Guid? OldProductFamilyId
    {
        get
        {
            return Convert(ProductFamilyId);
        }
    }
    private Guid? Convert(Guid? guid)
    {
        if (guid != null)
        {
            var bytes = guid.Value.ToByteArray();
            bytes[0] = unchecked((byte)~bytes[0]); // For example
            guid = new Guid(bytes);
        }
        return guid;
    }
}

也就是说，你可以从JsonWriter.Path属性中选择当前属性名:

public class InterfaceLabelConverter : JsonConverter
{
    private Guid? Convert(Guid? guid)
    {
        if (guid != null)
        {
            var bytes = guid.Value.ToByteArray();
            bytes[0] = unchecked((byte)~bytes[0]); // For example
            guid = new Guid(bytes);
        }
        return guid;
    }
    public override bool CanConvert(Type objectType)
    {
        throw new InvalidOperationException(); // This converter should only be applied directly to a property.
    }
    public override bool CanRead { get { return false; } }
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var path = writer.Path;
        var propertyName = path.Split(new[] { '.' }, StringSplitOptions.RemoveEmptyEntries).Last(); // Throw an exception if not found.
        if (propertyName.StartsWith("[") && propertyName.EndsWith("]"))
            throw new InvalidOperationException(); // Trying to use this converter for an array element.
        var guid = (Guid?)value;
        writer.WriteValue(guid);
        if (guid != null)
        {
            // Note -- actually the converter isn't called for null values, see
            // https://stackoverflow.com/questions/8833961/serializing-null-in-json-net
            var nextGuid = Convert(guid);
            var nextName = "Old" + propertyName;
            writer.WritePropertyName(nextName);
            writer.WriteValue(nextGuid);
        }
    }
}

并像这样使用:

public class Outer
{
    public int Id { get; set; }
    [JsonConverter(typeof(InterfaceLabelConverter))]
    public Guid? ProductFamilyId { get; set; }
}