DropDown and Textbox items can not be recognized in code behind.

Hi There;
I have an ASP.NET web project and three C sharp class libraries. There are two files in the web project: default.aspx and default.aspx.cs.  Here are the two definition in the default.aspx:
<asp:TextBox ID="txtAmount" runat="server" Width="60px"/> <asp:DropDownList AutoPostBack="true" ID="ddlBankAccounts" runat="server" onselectedindexchanged="ddlBankAccounts_SelectedIndexChanged" /> When I try to use the textbox and dropdownlist txtAmount and ddlBankAccounts at the code behind, namely default.aspx.cs, I receive the error below:
- The name 'ddlBankAccounts' does not exist in the current context    filepath\ASPPatterns_Chap4_DomainModel\ASPPatterns_Chap4_DomainModel_UI_Web\default.aspx.cs    19    13    ASPPatterns_Chap4_DomainModel_UI_Web ...