web.config xpath transform - find by parent element - asp.net-mvc

Question

I am trying to change a log file name after deploying, so transform this:
<log4net>
...
<appender name="GeneralAppender" type="log4net.Appender.RollingFileAppender, log4net">
<file value="c:\logs\Co.App.log" />
...
</appender>
</log4net>
to this:
<log4net>
...
<appender name="GeneralAppender" type="log4net.Appender.RollingFileAppender, log4net">
<file value="c:\logs\Co.App.localhost.log" />
...
</appender>
</log4net>
the actual file node doesn't have any attributes, so I am trying to locate it by parent node
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<log4net>
<appender >
<file value="c:\logs\Co.App.localhost.log" xdt:Transform="Replace" xdt:Locator="XPath(../appender[#name='GeneralAppender'])" />
</appender>
</log4net>
</assemblyBinding>
</runtime>
i've also tried all permutations of absolute and relative xpath's but i don't see it having any effect in transform preview.
i tried:
xdt:Locator="XPath(//appender[#name='GeneralAppender']/file)"
and even:
xdt:Transform="Remove" xdt:Locator="XPath(//file)"

Answer 1

found it!
<file value="c:\logs\Co.App.local.log" xdt:Transform="Replace" xdt:Locator="Condition(../#name='GeneralAppender')" />

Answer 2

Slight extension:
If the parent node has TWO (or more children), then the above solution is not enough.
This is the case in log4net, when the EventLogAppend is used, which has:
<appender name="EventLogAppender" type="log4net.Appender.EventLogAppender" >
...
<param name="LogName" value="MyLog"/>
<param name="ApplicationName" value="MyApplication"/>
...
Then you need to use an 'and' + an atribute more to hit the right node:,
like xdt:Locator="Condition(../#name='EventLogAppender' and #name='LogName')
Example for the EventLogAppender, where both the params are replaced:
<param name="LogName" value="MyNewLog" xdt:Transform="Replace" xdt:Locator="Condition(../#name='EventLogAppender' and #name='LogName')" />
<param name="ApplicationName" value="MyNewApplication" xdt:Transform="Replace" xdt:Locator="Condition(../#name='EventLogAppender' and #name='ApplicationName')" />

Answer 3

The MSDN documentation mentions that if you use XPath, then it's going to append the expression that you pass to the current context in the transform file. So XPath is good for cases where you want to modify the current element or one of it's predecessors.
Where as if you want to traverse to parents in a relative manner, there is no XPath expression that does that. XPath starts with a parent and allows you to traverse the descendants and not ascendants. That's when condition works.
I tried to use xdt:Locator="XPath(.)" and it worked perfectly to replace the current element, and if needed it's descendants. But it doesn't for ascendants.