C#OperationContract POST请求正在分析空正文

我有我的ServiceContract接口

[ServiceContract]
public interface IService
{
    [OperationContract]
    [WebInvoke(Method = "POST",
         ResponseFormat = WebMessageFormat.Json,
         BodyStyle = WebMessageBodyStyle.WrappedRequest,
        RequestFormat = WebMessageFormat.Json,
         UriTemplate = "print")]
    [return: MessageParameter(Name = "Data")]
    int Print(PrintOrder orden);
}

该服务:

public class Service : IService
    {
        public int Print(PrintOrder orden)
        {
            try
            {
                Console.WriteLine("received order",orden.Body,orden.PrinterName);
                return 0;
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message,ex.StackTrace);
                return 1;
            }
        }
    }

数据合同呢

[DataContract]
    public class PrintOrder
    {
        [DataMember]
        public string PrinterName{ get; set; }
        [DataMember]
        public string Body { get; set; }
    }

问题是我的服务方式 Print 总是接收空值 PrintOrder 作为参数,每次我发送带有JSON正文的POST请求时:

{
  "PrinterName":"EPSON 1200",
  "Body":"test body"
}