通过选择组合框来筛选列表视图数据

Question

我正在学习ASP.Net开发&试图创建Web应用程序，其中ListView数据应该在ComboBox中完成筛选器选择。我已经做的ListView &绑定它与我的数据库表&试图把组合框过滤器，但得到这个错误

（System.Web.HttpException：请求不可用在这种情况下）的输出。

VB代码

Imports System.Data 
Imports System.Data.SqlClient 
Imports System.Configuration 
Partial Class VB 
    Inherits System.Web.UI.Page 
    Dim query As String 
    Dim strArea As String = Request.QueryString("Country") 

    Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load 
     If Not Me.IsPostBack Then 
      Me.BindListView() 
     End If 
    End Sub 

    Private Sub BindListView() 
     Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString 
     Using con As New SqlConnection(constr) 
      Using cmd As New SqlCommand() 
       cmd.CommandText = "SELECT CustomerId, ContactName, Country FROM Customers" 
       cmd.Connection = con 
       Using sda As New SqlDataAdapter(cmd) 
        Dim dt As New DataTable() 
        sda.Fill(dt) 
        lvCustomers.DataSource = dt 
        lvCustomers.DataBind() 
       End Using 
      End Using 
     End Using 
    End Sub 

    Protected Sub OnPagePropertiesChanging(sender As Object, e As PagePropertiesChangingEventArgs) 
     TryCast(lvCustomers.FindControl("DataPager1"), DataPager).SetPageProperties(e.StartRowIndex, e.MaximumRows, False) 
     Me.BindListView() 
    End Sub 

    Protected Sub country_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles country.SelectedIndexChanged 
     Try 
      If country.SelectedValue <> "All" Then 
       query = "Select CutomerId, Contact Name, Country from Customers where Country like '%" + strArea + "%' order by rank;" 
      End If 
      Dim cmd As New SqlCommand(query) 
      Dim da As New SqlDataAdapter(cmd) 
      Dim table As New DataTable 
     Catch ex As Exception 

     End Try 

    End Sub 
End Class 

Answer 1

Request只读属性属于Page类继承的类成员只有它继承它（在你的情况VB）类的成员中是可用的。改变你的代码是这样的： -

Partial Class VB 
    Inherits System.Web.UI.Page 
    Dim query As String 
    Dim strArea As String = String.Empty 

    Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load 
     If Not Me.IsPostBack Then 
      Me.BindListView() 
     End If 
     strArea = Request.QueryString("Country") 
    End Sub 

而且，请注意，这将是更好的检查，如果分配给它之前，你的查询字符串变量中存在的任何值。