asp.net – LINQ：不支持查询运算符’ElementAtOrDefault’

发布时间：2020-12-15 20:36:56 所属栏目：asp.Net 来源：网络整理

导读：为什么以下代码会产生错误？ 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}_ ).DistinctResponse.Redirect("~/test.as

为什么以下代码会产生错误？

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:ProjectsMemoriaLogin.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)

解决方法

您需要做的是将.ToList()添加到查询的末尾.这应该工作：

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)

如果没有.ToList(),查询只返回一个DataQuery(Of T)而不是List(Of T).添加ToList调用有两个作用：

>强制查询立即执行,和
>返回支持ElementAtOrDefault()的集合类型

希望有所帮助！

（编辑：李大同）

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