ASP.NET

<asp:ImageButton ImageUrl="~/images/ButtonCancel.png" style="float:right" OnClientClick="return RequestMessage();"
                            ID="btnCancel" CssClass="button" runat="server" OnClick="btnCancel_Click" 
                            CausesValidation="False" />

<script type="text/javascript">
      function RequestMessage()
      {
        if($get("ctl00_BodyPH_ConfirmationMsg").value=="true")
        {
            return confirm('Are you sure you want to continue? Your changes will be lost.')
        }
        else
            return true;
      }
    </script>

<asp:HiddenField runat="server" ID="ConfirmationMsg" Value="true" />

ScriptManager.RegisterHiddenField(this.Page, ConfirmationMsg.ClientID, "false");

<input type="hidden" name="ctl00_BodyPH_ConfirmationMsg" id="ctl00_BodyPH_ConfirmationMsg" value="false" />

protected void txtCustomerID_TextChanged(object sender, EventArgs e)
{
    ConfirmationMsg.Value = "true";
    loadCustomerInfo(false);
}

[ParseChildren(true), PersistChildren(false)]
[ToolboxData("<{0}:TabStrip runat=server></{0}:TabStrip>")]
public class TabStrip : CompositeControl, IPostBackEventHandler
{
    private Tab mySelectedTab;
    private TabCollection myTabs;

    public TabStrip()
    {
        this.myTabs = new TabCollection(this);
    }

    [MergableProperty(false)]
    public Tab SelectedTab
    {
        get
        {
            foreach (Tab tab in this.Tabs)
                if (tab.IsSelected)
                    return tab;
            return null;
        }
        set { value.IsSelected = true; }
    }

    [PersistenceMode(PersistenceMode.InnerProperty), NotifyParentProperty(true)]
    [MergableProperty(false)]
    public TabCollection Tabs
    {
        get { return myTabs; }
    }
}