当“主”下拉列表更改时，如何重新填充“依赖”下拉列表？

Question

我有三个下拉列表，其中第二个的内容取决于第一个中选择的内容，第三个的内容取决于第二个中选择的内容。这在页面首次显示时有效：

Protected Sub Page_Init(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    If PageAlreadyInitted = True Then Exit Sub
    If Page.IsPostBack = True Then Exit Sub
    PageAlreadyInitted = True

    Dim sqlDAL As New SQLServer(ConfigurationSettings.AppSettings("ConnectionString"))
    'Populate the Units dropdown
    Dim unitsDT As DataTable
    Dim sql As String = "Select distinct mu.unit from masterunits mu left join MasterUnitsProjSales mps on mps.Unit = mu.Unit where abs(active) = 1 and mps.NewBiz != 0 order by mu.unit"
    Dim retDS As DataSet = sqlDAL.runSQLDataSet(sql)
    unitsDT = retDS.Tables(0)
    DropDownListUnits.DataSource = unitsDT
    DropDownListUnits.DataTextField = "unit"
    DropDownListUnits.DataValueField = "unit"
    DropDownListUnits.DataBind()

    'Populate the Members dropdown - TODO: Call this when Unit changes, too
    Dim selectedUnit As String = DropDownListUnits.SelectedItem.Text
    Dim membersDT As DataTable
    sql = "select distinct shortname, M.memberno from members M left join memberunitproducts mup on M.MemberNo = mup.MemberNo where unit = '" + selectedUnit + "' order by shortname"
    retDS = sqlDAL.runSQLDataSet(sql)
    membersDT = retDS.Tables(0)
    DropDownListMembers.DataSource = membersDT
    DropDownListMembers.DataTextField = "shortname"
    DropDownListMembers.DataValueField = "memberno"
    DropDownListMembers.DataBind()

    'Populate the Customers dropdown - TODO: Call this when Member changes, too
    Dim selectedMember As String = DropDownListMembers.SelectedItem.Value
    Dim customersDT As DataTable
    sql = "select distinct companyname, custno from customers C left join members M on M.MemberNo = C.MemberNo where M.MemberNo = '" + selectedMember + "' order by companyname"
    retDS = sqlDAL.runSQLDataSet(sql)
    customersDT = retDS.Tables(0)
    DropDownListCustomers.DataSource = customersDT
    DropDownListCustomers.DataTextField = "companyname"
    DropDownListCustomers.DataValueField = "custno"
    DropDownListCustomers.DataBind()

    sqlDAL.Dispose()
End Sub

...但是当第一个的选择发生变化时，我需要第二个下拉列表来更新，而当第二个的选择发生变化时，第三个下拉列表需要更新。我有这个代码来尝试实现这一点：

Protected Sub DropDownListUnits_SelectedIndexChanged(sender As Object, e As EventArgs) Handles DropDownListUnits.SelectedIndexChanged
    DropDownListMembers.Items.Clear()
    DropDownListCustomers.Items.Clear()

    'Dim selectedUnit As String = DropDownListUnits.SelectedItem.Text
    Dim selectedUnit As String = DropDownListUnits.SelectedItem.ToString()
    Label2.Text = selectedUnit

    Dim sqlDAL As New SQLServer(ConfigurationSettings.AppSettings("ConnectionString"))
    Dim membersDT As DataTable
    Dim retDS As DataSet
    Dim sql As String = "select distinct shortname, M.memberno from members M left join memberunitproducts mup on M.MemberNo = mup.MemberNo where unit = '" + selectedUnit + "' order by shortname"
    retDS = sqlDAL.runSQLDataSet(sql)
    membersDT = retDS.Tables(0)

    DropDownListMembers.DataSource = membersDT
    DropDownListMembers.DataTextField = "shortname"
    DropDownListMembers.DataValueField = "memberno"
    DropDownListMembers.DataBind()
End Sub

...但它不值一堆豆子 - 可能是因为事件可能没有触发。无论如何，它确实似乎不是 - 这很明显，因为 DropDownListMembers 和 DropDownListCustomers 都没有被清除，也没有用不同的项目集重新填充 DropDownListMembers。

所以我的问题是，当一个下拉列表发生变化时，如何重新填充“依赖”下拉列表，但问题可能真的应该是，“我怎样才能触发 SelectedIndexChanged() 事件？

Answer 1

事件未触发，因为 AutoPostBack 属性未设置为 true。将其设置为 true 将确保每当用户更改所选项目时，它将触发 SelectedIndexChanged 处理程序。