在C＃中生成WSDL时获取xsi：nil =“true”

Question

我创建了一个类

       public class SearchProgressResponse
       {
          public SearchProgressResponse()
          {
             Progress = new List<SearchProgress>();

          }
          public List<SearchProgress> Progress { get; set; }

       }


      public class SearchProgress
      {
       public SearchProgress()
        {
          SearchError = new Error();
         }
       public string handle { get; set; }
       public string code { get; set; }
       public string message { get; set; }
       public string name { get; set; }
       public string state { get; set; }
       public string image_data { get; set; }
       public string challenge { get; set; }
       public string image_type { get; set; }
       public Error SearchError { get; set; }
       }


      public class Error
      {

        public string ErrorMessage { get; set; }

        public string ErrorDetails { get; set; }
        }

这是我的班级，它正在生成以下wsdl。

    <soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"            xmlns:xsd="http://www.w3.org/2001/XMLSchema"xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
     <soap:Body>
<GetSearchHandleResponse xmlns="http://jobboards.com/">
  <GetSearchHandleResult>
    <Progress>
      <SearchProgress>
        <handle>string</handle>
        <code>string</code>
        <message>string</message>
        <name>string</name>
        <state>string</state>
        <image_data>string</image_data>
        <challenge>string</challenge>
        <image_type>string</image_type>
        <SearchError xsi:nil="true" />
      </SearchProgress>
      <SearchProgress>
        <handle>string</handle>
        <code>string</code>
        <message>string</message>
        <name>string</name>
        <state>string</state>
        <image_data>string</image_data>
        <challenge>string</challenge>
        <image_type>string</image_type>
        <SearchError xsi:nil="true" />
      </SearchProgress>
    </Progress>
  </GetSearchHandleResult>
</GetSearchHandleResponse>

我的问题是为什么它生成搜索元素如下

    <SearchError xsi:nil="true" />

我希望这个元素为

   <SearchError>
     <ErrorMessage>string</ErrorMessage>
     <ErrorDetails>string</ErrorDetails>
   </SearchError>

谁能告诉我在这里缺少什么？以及如何以上述格式生成？ 提前谢谢。