Question

我目前正在使用此页面http://www.silverlightshow.net/items/Silvester-A-Silverlight-Twitter-Widget.aspx创建一个Silverlight应用程序，该应用程序获取用户的Twitter提要并显示它。

我对代码做了一些更改但它应该仍然有效。但是，当我尝试转换xml：

时，我收到此错误消息

>System.NullReferenceException was unhandled by user code
  Message=Object reference not set to an instance of an object.
  Source=USElab.Web
  StackTrace:
       at USElab.Web.TwitterWebService.<GetUserTimeline>b__1(XElement status) in H:\USE\USE\USE\USElab\USElab.Web\TwitterWebService.asmx.cs:line 58
       at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
       at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
       at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
       at USElab.Web.TwitterWebService.GetUserTimeline(String twitterUser, String userName, String password) in H:\USE\USE\USE\USElab\USElab.Web\TwitterWebService.asmx.cs:line 57
  InnerException:

我无法弄清楚为什么会这样。这是代码：

   namespace USElab.Web
   {
/// <summary>
/// Summary description for TwitterWebService
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
[System.Web.Script.Services.ScriptService]

public class TwitterWebService : System.Web.Services.WebService
{
    private const string UserTimelineUri = "http://twitter.com/statuses/user_timeline/{0}.xml";
    private const string StatusElementName = "status";
    private const string CreatedAtElementName = "created_at";
    private const string TextElementName = "text";
    private const string ProfileImageUrlElementName = "profile_image_url";
    private const string NameElementName = "name";
    private const string UserElementName = "user";
    private const string UserUrlElementName = "url";
    public const int RequestRateLimit = 70;

    [WebMethod]
        public List<TwitterItem> GetUserTimeline( string twitterUser, string userName, string password )
        {
              if ( string.IsNullOrEmpty( twitterUser ) )
              {
                  throw new ArgumentNullException( "twitterUser", "twitterUser parameter is mandatory" );
              }

              WebRequest rq = HttpWebRequest.Create( string.Format( UserTimelineUri, twitterUser ) );

              if ( !string.IsNullOrEmpty( userName ) && !string.IsNullOrEmpty( password ) )
              {
                  rq.Credentials = new NetworkCredential( userName, password );
              }

              HttpWebResponse res = rq.GetResponse() as HttpWebResponse;

              // rate limit exceeded
              if ( res.StatusCode == HttpStatusCode.BadRequest )
              {
                  throw new ApplicationException( "Rate limit exceeded" );
              }

              XDocument xmlData = XDocument.Load(XmlReader.Create(res.GetResponseStream()));
              List<TwitterItem> data = (from status in xmlData.Descendants(StatusElementName)
                           select new TwitterItem
                           {
                               Message = status.Element(StatusElementName).Element(TextElementName).Value.Trim(),
                               ImageSource = status.Element(StatusElementName).Element(ProfileImageUrlElementName).Value.Trim(),
                               UserName = status.Element(StatusElementName).Element(UserElementName).Value.Trim()
                           }).ToList();



                return data;
          }

根据调试器，这就是开始选择新的TwitterItem的行。

非常感谢任何帮助！

Answer 1

当您尝试执行NullReferenceException

之类的操作时，通常会发生{p> null.Member

在调用.Element .Value或{之类的成员函数之前，您获得了非常高级的创建TwitterItems的代码，将其解压缩到另一个方法并对所有可能的null对象（在您的情况下都是元素）执行空检查{1}}

在你调用.Trim()之前，还要检查xmlData是否为null。

Silverlight Web项目中的Twitter Web服务

1 个答案: