反序列化委托属性时出现NullReferenceException

Question

我正在使用类似于以下的类反序列化存储在mongodb中的数据。在MongoDB中，SmoothingFn存储为字符串，使用switch语句反序列化。使用此构造函数，反序列化过程失败。使用MongoDB的C＃驱动程序设置委托的最佳方法是什么？有办法吗？

编辑：抛出的异常是NullReferenceException：对象引用未设置为对象的实例。

public class MacroModelCfg{
        [DataMember]
        public string Name;
        // More variables
        //
        [BsonIgnore]
        [DataMember]
        public Func<TimeSeries, TimeSeries> SmoothingFn = new Func<TimeSeries, TimeSeries>(x => x);

    [BsonConstructor]
    public MacroModelCfg(string Name, string SmoothingFn)
    {
        switch (SmoothingFn)
        {
            case ("MAV2"):
                this.SmoothingFn = new Func<TimeSeries, TimeSeries>(x => Fn.SMA(x, 1));
                break;
            case ("MAV3"):
                this.SmoothingFn = new Func<TimeSeries, TimeSeries>(x => Fn.SMA(x, 2));
                break;
            case ("MAV6"):
                this.SmoothingFn = new Func<TimeSeries, TimeSeries>(x => Fn.SMA(x, 5));
                break;
            case ("EMA2"):
                this.SmoothingFn = new Func<TimeSeries, TimeSeries>(x => Fn.EMA(x, 1));
                break;
            case ("EMA3"):
                this.SmoothingFn = new Func<TimeSeries, TimeSeries>(x => Fn.EMA(x, 2));
                break;
            case ("EMA6"):
                this.SmoothingFn = new Func<TimeSeries, TimeSeries>(x => Fn.EMA(x, 5));
                break;
            default:
                throw new Exception("Unrecognized PredictionSmoothingFunction " + SmoothingFn);
        }
    }


}