dropdownlist如何与slqdatasource绑定以检索正确的值

我想从下拉列表中检索要为我的.cs编写的值。sqldatasource根据他/她拥有的帐户类型返回Saving Account或Current Account。

<asp:DropDownList ID="ddlPayBy2" runat="server" AutoPostBack="True" 
                            DataSourceID="SqlDataSource2" DataTextField="accountType" 
                            DataValueField="accountType">
                        </asp:DropDownList>
                        <asp:SqlDataSource ID="SqlDataSource2" runat="server" 
                            ConnectionString="<%$ ConnectionStrings:Oakhorizons %>" 
                            SelectCommand="SELECT [accountType] FROM [SavingAccount] WHERE ([custID] = @custID)">
                            <SelectParameters>
                                <asp:SessionParameter DefaultValue="OH00002" Name="custID" 
                                    SessionField="Session[&quot;custID&quot;]" Type="String" />
                            </SelectParameters>
                        </asp:SqlDataSource>

它什么也没还给我。我试过了，但没有任何回报。我总是得到else语句，我试图调试它，它说SelectedValue="

 if (ddlPayBy.SelectedValue == "Savings"){
          account = "Savings";
                                    }
 else if (ddlPayBy.SelectedValue == "Current")
                                    {
          account = "Current";
                                    }
 else
                                    {
          Alert.Show("Please choose your account type properly");
                                    }