CS0117 error ... doesn't 'see' the code behind file?

 I'm getting the following error, after adding a control to an existing webform, adding the code in the code behind file and wiring the control with OnTextChanged.  There are other controls on the page that work and access the code behind. 
Compiler Error Message: CS0117: 'ASP.treatment_counselor_aspx' does not contain a definition for 'txtNetworkId_TextChanged'

Source Error:


Line 47: <td class="fieldlabel">Network ID</td> Line 48: <td> Line 49: <asp:TextBox ID="txtNetworkId" runat="server" AutoPostBack="true" OnTextChanged="txtNetworkId_TextChanged"></asp:TextBox> Line 50: </td> Line 51: <td class="fieldlabel">PIN</td>< ...