MsgPack序列化抛出错误

我想创建一个像这样的序列化器

var serializer = MsgPack.Serialization.MessagePackSerializer.Create<tickdata>();

它将序列化tickdata类。

namespace TickDataDefinition
{
public class tickdata
{
    public List<data> _data=new List<data>();        
    public int returncount()
    {
        return _data.Count;
    }
}
public class data
{
    enum type { trade, quote }
    long time;
    double bid1;
    double ask1;
    double bidsize;
    double asksize;
    double price;
    uint size;
    public data(long t,double b,double a,double bs, double ask)
    {
        time = t;
        bid1 = b;
        ask1 = a;
        bidsize = bs;
        asksize = ask;     
    }
    public data(long t,double p,uint s)
    {
        time = t;
        price=p;
        size=s;
    }
}

}

奇怪的是这段代码不工作。错误是"调用的目标抛出了异常"。