Hello Luke,

I will try to explain in detail what is happening and how you could
reproduce it:

1.
Create a web.sitemap file with the following content:

<siteMap xmlns="http://schemas.microsoft.com/AspNet/SiteMap-File-1.0" >
   <siteMapNode url="" title="Virtual topic (not displayed)"  description="">
     <siteMapNode url="" title="Topic 1"  description="" >
       <siteMapNode url="" title="Topic 1.1"  description="" />
       <siteMapNode url="" title="Topic 1.2"  description="" />
     </siteMapNode>
     <siteMapNode url="" title="Topic 2"  description="" >
       <siteMapNode url="" title="Topic 2.1"  description="" />
       <siteMapNode url="" title="Topic 2.2"  description="" />
     </siteMapNode>
     <siteMapNode url="" title="Topic 3"  description="" >
       <siteMapNode url="" title="Topic 3.1"  description="" />
       <siteMapNode url="" title="Topic 3.2"  description="" />
     </siteMapNode>
   </siteMapNode>

</siteMap>

2.
Make a new webform, put the following stuff in between your <form></form> tag:

<asp:Menu ID="Menu1" runat="server" DataSourceID="SiteMapDataSource1"
Orientation="Horizontal">
</asp:Menu>
<asp:SiteMapDataSource ID="SiteMapDataSource1" runat="server"
ShowStartingNode="False" />

3.
View that page on a downlevel browser. If you don't have one accessible, you
may also view the page in Pocket PC 2003 device emulator.
You will see the following output:
Topic 1   Topic 2   Topic 3

4.
Now, click on Topic 1. You will see the following output:
Topic 1   Topic 1.1   Topic 1.2   ^up one level

5.
Now click on "^up one level".
You should see again output as described in 3.

==> If caching is enabled with my VaryByCustom (combining some querystring
parameters), you won't see output from  but output from 4.
==> If caching is enabled with VaryByParams="*", I get the expected result
(output from 3.). However, with VaryByParams="*" always new pages are
returned when navigation through the menu. No cached pages are used.

What I need is some kind of identifier describing the current state of the
menu to be able to concatenate it to my current implementation of
GetVaryByCustom.
I cannot use __EVENTTARGET and __EVENTARGUMENT, since the first is always
the (same) id of the menu control and the latter always contains "u" when
hitting the "one level up" link, no matter in which depth of the menu
hierarchy I am.
Í thought the sitemap path would be the string I could use for that purpose:
combine the SiteMap.CurrentNode with the __EVENTARGUMENT "u". But CurrentNode
is always null.

Any suggestions what "kind of identifier describing the current state" I
could use for my GetVaryByCustom implementation?
Or: what value do I have to use for VaryByParams (I don't want "'*") to take
care about the different menu states?

TIA,
Dieter