asp.net - LINQ:不支持查询运算符 'ElementAtOrDefault'

Question

为什么下面的代码会产生错误？

The query operator 'ElementAtOrDefault' is not supported

Dim Im = (From view In Db.Views Where _
               view.Pass = txtCode.Text _
          Select New With {.Id = view.UniqueID.ToString}_
          ).Distinct

Response.Redirect("~/test.aspx?x=" & Im(0).Id)

有没有办法在不使用 FirstOrDefault 选项的情况下修复它？

更新:这是 StackTrace

   at System.Data.Linq.SqlClient.QueryConverter.VisitSequenceOperatorCall(MethodCallExpression mc)
   at System.Data.Linq.SqlClient.QueryConverter.VisitMethodCall(MethodCallExpression mc)
   at System.Data.Linq.SqlClient.QueryConverter.VisitInner(Expression node)
   at System.Data.Linq.SqlClient.QueryConverter.ConvertOuter(Expression node)
   at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(Expression query, SqlNodeAnnotations annotations)
   at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query)
   at System.Data.Linq.DataQuery`1.System.Linq.IQueryProvider.Execute[S](Expression expression)
   at System.Linq.Queryable.ElementAtOrDefault[TSource](IQueryable`1 source, Int32 index)
   at Login.btnLogin_Click(Object sender, EventArgs e) in D:\Projects\Memoria\Login.aspx.vb:line 14
   at System.Web.UI.WebControls.Button.OnClick(EventArgs e)
   at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
   at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Answer 1

您需要做的是添加.ToList()到您的查询结束。这应该有效:

Dim Im = (From view In Db.Views Where _
           view.Pass = txtCode.Text _
      Select New With {.Id = view.UniqueID.ToString}_
      ).Distinct.ToList()

Response.Redirect("~/test.aspx?x=" & Im(0).Id)

无 .ToList() ，查询只返回 DataQuery(Of T) 而不是 List(Of T)。添加 ToList 调用有两件事:

强制查询立即执行，

返回支持 ElementAtOrDefault() 的集合类型

希望有帮助!