Hi,
you can try this for doing such thing
<asp:label id="Label1" runat="server" tooltip="<%#Eval("compid").ToString() + " " + Eval("compname").ToString() %>" text="<%#Eval("compoffaddr") %>" xmlns:asp="#unknown"></asp:label>
you can not mention objects side by side.they must cast before they placed side by side.
All the Best