Method Not calling?

Question

<asp:Repeater ID="rptAlbumImages" runat="server" DataSourceID="XmlDataSource1">
  <ItemTemplate>
  <%--<asp:Label runat="server" ID="lblNAME" Text='<%#XPath("Name")%>'></asp:Label>--%>
  <asp:Image ID="Image1" runat="server" ImageUrl='<%#XPath("Images/Imagepath/ThumImagepath","~/Albums/"+XPath("Name")+"/Images/ThumnailImages/{0}")%>' />
  </ItemTemplate>
  </asp:Repeater>
  <asp:XmlDataSource ID="XmlDataSource1" runat="server" DataFile="~/App_Data/XML_Albums.xml" XPath="/Upload/Category[@ID='<%=GetCategoryID() %>']">
  </asp:XmlDataSource> 






public string GetCategoryID()
  {
  string id = categoryDropDowmList.SelectedValue.ToString();
  return (id);
  }

Why not calling???