c# - 我在反序列化枚举时遇到问题

我正在使用NLog，并且正在通过导线发送LogEventInfo对象，我不拥有此对象，因此无法使用[JsonConverter(typeof(StringEnumConverter))]装饰它，或者也许可以，但我不知道如何

我尝试了以下代码，但没有帮助

var strategy = new Newtonsoft.Json.Serialization.CamelCaseNamingStrategy();
var sec = new StringEnumConverter(strategy, false);

//TEMP Hard code serialization and message routing for just logs
LogEventInfo info = Newtonsoft.Json.JsonConvert.DeserializeObject<LogEventInfo>(msg.Body, sec);

没有人有运气让枚举反序列化并且枚举属于您不拥有的类吗？

这是消息：

{
   "date":"2019-06-04 21:48:24.0753",
   "level":"Error",
   "message":"{\"ApplicationId\":1390760,\"AppStatus\":\"#PG2\",\"Status\":400,\"ErrorCode\": 1053 }",
   "properties":"ResponseBody={\"ApplicationId\":1390760,\"AppStatus\":\"#PG2\",\"Status\":400,\"ErrorCode\": 1053 }|GroupId=392934|ApplicationId=1390760|Status=400",
   "callsite":"HFD.Enterprise.Logging.Tests.LogTest.LoadTest",
   "logger":"WebApiLog",
   "machinename":"BRANDONHOSTVS"
}

这是一个例外：

Newtonsoft.Json.JsonSerializationException: Error converting value "Error" to type 'NLog.LogLevel'. Path 'level', line 1, position 72. ---> System.InvalidCastException: Invalid cast from 'System.String' to 'NLog.LogLevel'.
   at System.Convert.DefaultToType(IConvertible value, Type targetType, IFormatProvider provider)
   at System.String.System.IConvertible.ToType(Type type, IFormatProvider provider)
   at System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.EnsureType(JsonReader reader, Object value, CultureInfo culture, JsonContract contract, Type targetType)
   --- End of inner exception stack trace ---

最佳答案

您的基本问题是LogEventInfo.Level不是枚举，它是LogLevel类，恰好实现了IConvertible并具有一组全局静态标准值：

public sealed class LogLevel : IComparable, IEquatable<LogLevel>, IConvertible
{
    /// <summary>
    /// Gets all the available log levels (Trace, Debug, Info, Warn, Error, Fatal, Off).
    /// </summary>
    public static IEnumerable<LogLevel> AllLevels => allLevels;

添加StringEnumConverter不会帮助您（反）序列化此类属性。

而且，NLog似乎没有为TypeConverter提供自定义的LogLevel。如果我调用TypeDescriptor.GetConverter(typeof(LogLevel))，则返回的值是默认转换器System.ComponentModel.TypeConverter的实例，这意味着Json.NET无法将序列化的LogLevel字符串值转换回LogLevel实例。

但是，您的JSON示例确实将Level表示为"level": "Error"，因此发送系统必须一直在对JsonConverter使用某种custom LogLevel来对LogLevel.Name进行序列化。我们可以很容易地重新创建这样的JsonConverter，如下所示：

public class LogLevelConverter : JsonConverter<LogLevel>
{
    public override LogLevel ReadJson(JsonReader reader, Type objectType, LogLevel existingValue, bool hasExistingValue, JsonSerializer serializer)
    {
        switch (reader.MoveToContentAndAssert().TokenType)
        {
            case JsonToken.Null:
                return null;
            case JsonToken.String:
                return LogLevel.FromString((string)reader.Value);
            default:
                throw new JsonSerializationException(string.Format("Unknown token {0}", reader.TokenType));
        }
    }

    public override void WriteJson(JsonWriter writer, LogLevel value, JsonSerializer serializer)
    {
        var logLevel = (LogLevel)value;
        writer.WriteValue(logLevel.Name);
    }
}

public static partial class JsonExtensions
{
    public static JsonReader MoveToContentAndAssert(this JsonReader reader)
    {
        if (reader == null)
            throw new ArgumentNullException();
        if (reader.TokenType == JsonToken.None)       // Skip past beginning of stream.
            reader.ReadAndAssert();
        while (reader.TokenType == JsonToken.Comment) // Skip past comments.
            reader.ReadAndAssert();
        return reader;
    }

    public static JsonReader ReadAndAssert(this JsonReader reader)
    {
        if (reader == null)
            throw new ArgumentNullException();
        if (!reader.Read())
            throw new JsonReaderException("Unexpected end of JSON stream.");
        return reader;
    }
}

然后按如下方式使用它：

var seq = new LogLevelConverter();
var info = Newtonsoft.Json.JsonConvert.DeserializeObject<LogEventInfo>(msg.Body, seq);

演示小提琴here。