应用 onPage 过滤器时未将对象引用设置为对象的实例

Question

这似乎是非常基本的问题，但无法解决。我有列表视图控件和一些过滤器作为单选按钮列表。现在，当页面加载时，它会正确显示数据。但是当我尝试对其应用过滤器时，它会显示异常“对象引用未设置为对象的实例”。并且还请建议是否可以在过滤器上应用任何简化的方法来减少我当前的编码长度。

vb codePreRender 事件

Private Sub chemists_PreRender(sender As Object, e As EventArgs) Handles Me.PreRender
    Try
        Dim citySelector As DropDownList = Page.Master.FindControl("locationSelector")
        If Session("masterLocation") Is Nothing Then
            citySelector.Text = "Pune"
        Else
            citySelector.Text = Session("masterLocation").ToString()
        End If
    Catch ex As Exception
        Response.Write(ex)
    End Try

    Me.PopulateAreas()

    Try
        If Not Session("Data") Is Nothing Then
            chemists.DataSource = Session("Data")
            chemists.DataBind()
        Else
            Dim citySelector As DropDownList = Page.Master.FindControl("locationSelector")
            query = "SELECT chemistID, name, address, thumbnail, slot1From, slot1To, products FROM chemists where city like '" + citySelector.SelectedItem.ToString + "%' and status = 'active'"
            Dim cmd As New MySqlCommand(query, con)
            cmd.CommandTimeout = 120
            Dim da As New MySqlDataAdapter(cmd)
            Dim table As New DataTable
            da.Fill(table)
            ViewState("Data") = table
            chemists.DataSource = table
            chemists.DataBind()
        End If

        'mainCount.Text = table.Rows(0)("countRows").ToString
    Catch ex As Exception
        Response.Write(ex)
    End Try
End Sub

这里我使用了预渲染事件，因为我在母版页中有下拉列表，并且我从母版页获得了它的控制权。直到这里一切正常。

当我尝试在其中应用页面过滤器时出现问题。以下是代码

Private Sub areasList_SelectedIndexChanged(sender As Object, e As EventArgs) Handles areasList.SelectedIndexChanged
    Dim citySelector As DropDownList = Page.Master.FindControl("locationSelector")
    locationFilter.Text = areasList.SelectedItem.ToString
    city.Text = citySelector.SelectedItem.ToString
    area.Text = areasList.SelectedIndex.ToString
    Try
        Session("Data") = Nothing
        If areasList.SelectedValue <> "All" And productList.SelectedValue <> "All" And paymentsList.SelectedValue <> "All" Then
            query = "SELECT * FROM chemists where address Like '%" + areasList.SelectedItem.ToString + "%' and products Like '%" + productList.SelectedItem.ToString + "%' and payment Like '%" + paymentsList.SelectedItem.ToString + "%' and city = '" + citySelector.SelectedItem.ToString + "' and status = 'active' "

        ElseIf areasList.SelectedValue = "All" And productList.SelectedValue <> "All" And paymentsList.SelectedValue.ToString <> "All" Then
            query = "SELECT * FROM chemists where products Like '%" + productList.SelectedItem.ToString + "%' and payment Like '%" + paymentsList.SelectedItem.ToString + "%' and city = '" + citySelector.SelectedItem.ToString + "' and status = 'active'"

        ElseIf areasList.SelectedValue = "All" And productList.SelectedValue = "All" And paymentsList.SelectedValue.ToString <> "All" Then
            query = "SELECT * FROM chemists WHERE payment Like '%" + paymentsList.SelectedItem.ToString + "%' and city = '" + citySelector.SelectedItem.ToString + "' and status = 'active'"

        ElseIf areasList.SelectedValue <> "All" And productList.SelectedValue = "All" And paymentsList.SelectedValue.ToString = "All" Then
            query = "SELECT * FROM chemists WHERE address Like '%" + areasList.SelectedItem.ToString + "%' and city = '" + citySelector.SelectedItem.ToString + "' and status = 'active'"

        ElseIf areasList.SelectedValue <> "All" And productList.SelectedValue <> "All" And paymentsList.SelectedValue.ToString = "All" Then
            query = "SELECT * FROM chemists WHERE address Like '%" + areasList.SelectedItem.ToString + "%' and products Like '%" + productList.SelectedItem.ToString + "%' and city = '" + citySelector.SelectedItem.ToString + "' and status = 'active'"

        ElseIf areasList.SelectedValue = "All" And productList.SelectedValue <> "All" And paymentsList.SelectedValue.ToString = "All" Then
            query = "SELECT * FROM chemists WHERE products Like '%" + productList.SelectedItem.ToString + "%' and city = '" + citySelector.SelectedItem.ToString + "' and status = 'active'"

        ElseIf areasList.SelectedValue = "All" And productList.SelectedValue = "All" And paymentsList.SelectedValue = "All" Then
            query = "SELECT * FROM chemists where city = '" + citySelector.SelectedItem.ToString + "' and status = 'active'"
        End If

        Dim cmd As New MySqlCommand(query, con)
        Dim da As New MySqlDataAdapter(cmd)
        Dim table As New DataTable

        da.Fill(table)
        Session("Data") = table
        chemists.DataSource = table
        chemists.DataBind()
    Catch ex As Exception
        Response.Write(ex)
    End Try
End Sub

Answer 1

我检查了代码，发现 productList 下拉有界属性，但不知何故它没有维护选定的索引。

所以请尝试如下，

productList.SelectedIndex = 0

这将解决问题。