Question

我试图对推文列表（类：SimpleTweet）进行排序，每个推文都有与之关联的ID（x.ID，其中x是类SimpleTweet的对象）。我正在使用linq对此进行排序，使用＆＃34; OrderByDescending＆＃34;，但是在我设置List（Of SimpleTweet）类型的新对象等于排序列表的行上出现错误。我得到的错误是，＆＃34; System.InvalidCastException：无法转换类型为＆＃39; System.Linq.OrderedEnumerable 2[SimpleTweet,System.Int64]' to type 'System.Collections.Generic.List 1 [SimpleTweet]＆＃39;＆＃34;的对象。

代码：

 <WebMethod()> _
    Public Function GetTweetsByUserID(ByVal userID As Integer) As List(Of SimpleTweet)
        Dim result As New List(Of SimpleTweet)
        Dim urlTwitter As String = "https://api.twitter.com/1/statuses/user_timeline.xml?include_entities=true&include_rts=true&screen_name={0}&count=3"
        'Dim twitterfeed As String = utils.GetUserTwitterFeeds(userID, "docphin")
        Dim lq As New lqDFDataContext
        Dim var = lq.web_GetTweetsByUserID(userID).ToList()
        Dim sortedresult As New List(Of SimpleTweet)
        If Not var Is Nothing Then
            For Each twitterfeed In var

                Dim listURL As String = String.Format(urlTwitter, twitterFeed.TweeterFeed)

                Dim tweetXML As XmlDocument = utils.GetXMLForURL(listURL)

                Dim tweetnodelist As XmlNodeList = tweetXML.ChildNodes(1).ChildNodes

                For Each node As XmlNode In tweetnodelist
                    Dim tweet As New SimpleTweet
                    tweet.CreatedAt = node.SelectSingleNode("created_at").InnerText
                    tweet.HTMLText = utils.ReturnTextWithHRefLink(node.SelectSingleNode("text").InnerText)
                    tweet.ID = node.SelectSingleNode("id").InnerText
                    tweet.Name = node.SelectSingleNode("user/name").InnerText
                    tweet.ScreenName = node.SelectSingleNode("user/screen_name").InnerText
                    tweet.Text = node.SelectSingleNode("text").InnerText
                    tweet.UserID = node.SelectSingleNode("user/id").InnerText
                    tweet.ProfileImageURL = node.SelectSingleNode("user/profile_image_url_https").InnerText
                    result.Add(tweet)
                Next
            Next

        sortedresult = result.OrderByDescending(Function(tweet) tweet.ID)  
        End If
        Return sortedresult
    End Function

Answer 1

您需要通过调用result来实现.ToList()。将它添加到此行的末尾：

sortedresult = result.OrderByDescending(Function(tweet) tweet.ID)

sortedResult类型为List(Of SimpleTweet)，OrderByDescending返回IOrderedEnumerable(Of SimpleTweet)，无法自动转换为预期类型。

Answer 2

由于您要返回List(Of SimpleTweet)，因此需要致电ToList以从IEnumerable(Of SimpleTweet)创建新列表：

Return sortedresult.ToList()

ToList强制立即进行查询评估。

尝试使用linq命令对象列表时获取无效的强制转换异常

2 个答案: