Vb.net 为什么这个下拉列表没有填充？

我有三个下拉列表；第二个（成员）根据第一个（单位）中的内容填充，第三个（客户）应根据第二个（单位）中的内容填充；但事实并非如此

以下是在LINQPad中工作的查询（返回公司名称列表）：

…但在以下代码中不起作用：

'Populate the Members dropdown
Dim selectedUnit As String = DropDownListUnits.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
Dim selectedMember As String = DropDownListMembers.DataValueField
Dim customersDT As DataTable
sql = "select distinct companyname 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 = "companyname"
DropDownListCustomers.DataBind()

第三个（客户）下拉列表在运行时仍然不受欢迎-为什么

可能不需要，但下面是第一个（“单位”）下拉列表的代码：

'Populate the Units dropdown
Dim unitsDT As DataTable
sql = "Select distinct unit from masterunits where abs(active) = 1"
retDS = sqlDAL.runSQLDataSet(sql)
unitsDT = retDS.Tables(0)
DropDownListUnits.DataSource = unitsDT
DropDownListUnits.DataTextField = "unit"
DropDownListUnits.DataValueField = "unit"
DropDownListUnits.DataBind()

我遗漏了什么或做错了什么？

我必须改变这一点：

Dim selectedMember As String = DropDownListMembers.DataValueField

……为此：

Dim selectedMember As String = DropDownListMembers.SelectedItem.Value

Dim selectedMember As String = DropDownListMembers.SelectedItem.Value