Drop down menu 无法在页面加载时检索dropdownlist.selectedvalue？

我有一个下拉框，它有一个从datasqlsource检索的值，但是我想在页面加载时获取计数值，但它似乎不计数

'Dim adapter As New SqlDataAdapter
    'Dim ds As New DataSet


    'Dim connectionString = ConfigurationManager.ConnectionStrings("myProject").ConnectionString
    'Dim myConn As New SqlConnection(connectionString)

    'Dim cmd = "SELECT * FROM Group Where groupID='" & DropDownList1.SelectedValue & "' AND customerID='" & Session("customerID") & "'"






    'Try
    '    myConn5.Open()
    '    Dim myCmd5 As New SqlCommand(cmd5, myConn5)
    '    adapter.SelectCommand = myCmd5
    '    adapter.Fill(ds, "Group")
    '    ' txtQuan.Text = adapter.SelectCommand.ExecuteScalar().ToString()
    '    adapter.Dispose()
    '    myCmd5.Dispose()
    '    ' txtQuan.Text = ds.Tables(0).Rows.Count
    '    If ds.Tables(0).Rows.Count >= 10 Then
    '        lblNoPpl.Text = "The  group is full"
    '    Else
    '        Dim numLefts As Integer = 10 - ds.Tables(0).Rows.Count
    '        lblNoPpl.Text = numLefts.ToString() + "space left"



    'Catch ex As Exception
    '    MsgBox("Can not open connection ! ")
    'End Try

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如果使用cmd.ExcuteScalar方法检索计数，则sql应为

从组中选择COUNT*，其中groupID='&DropDownList1.SelectedValue&'和customerID='&SessioncustomerID&'