<asp:DropDownList ID="lstCls" runat ="server" AutoPostBack ="True" Width ="200px" 
        DataSourceID="dataProductCls" DataTextField="Class" 
        DataValueField="ID"/>
    
    <br />
   
    <asp:DropDownList ID="lstSubCls" runat ="server" Width ="200" AutoPostBack ="true" />

Protected Sub lstCls_SelectedIndexChanged(sender As Object, e As System.EventArgs) Handles lstCls.SelectedIndexChanged
        Dim strSelected As String = lstCls.SelectedItem.Text
        dataProductSubCls.SelectParameters.Clear()
        dataProductSubCls.SelectParameters.Add("Selected", TypeCode.String, strSelected)
        dataProductSubCls.SelectCommand = "SELECT ProductCls.Class, ProductSubCls.ID, ProductSubCls.SubClass FROM ProductSubCls " &
                                          "INNER JOIN ProductCls ON ProductSubCls.Class = ProductCls.ID " &
                                          "WHERE ProductCls.Class = @Selected"
        lstSubCls.Enabled = True
        lstSubCls.DataSourceID = "dataProductSubCls"
        lstSubCls.DataTextField = "SubClass"
        lstSubCls.DataValueField = "ID"
        lstSubCls.DataBind()
    End Sub

    Protected Sub lstSubCls_SelectedIndexChanged(sender As Object, e As System.EventArgs) Handles lstSubCls.SelectedIndexChanged
        Dim strSelected As String = lstSubCls.SelectedItem.Text
        sqlComm = New SqlCommand("SELECT Pic, Alt FROM ProductSubCls WHERE SubClass = @Selected", sqlConn)
        sqlComm.Parameters.Add("@Selected", Data.SqlDbType.NVarChar, 50)
        sqlComm.Parameters("@Selected").Value = strSelected
        Try
            sqlConn.Open()
            reader = sqlComm.ExecuteReader
            If reader.Read Then
                ...'将内容附给其他控件
            Else
                ...'执行其他代码
            End If
        Catch ex As Exception
            Response.Write (ex.Message)
        Finally
            sqlConn.Close()
        End Try
    End Sub