How to specify common application data folder for log4net?

I want log4net to write log files (using RollingFileAppender) to a subfolder of the common application data folder (e.g. C:\Documents and Settings\All Users\Application Data\Company\Product\Logs).
However, on Win XP, there is no environment variable that specifies this folder. We have %ALLUSERSPROFILE%, we have %APPDATA%, but there is nothing like %ALLUSERSAPPDATA%.
Programatically, I could use Environment.SpecialFolder.CommonApplicationData, but I need to put it in the log4net config, something like this:

<appender name="RollingFileAppender" type="log4net.Appender.RollingFileAppender">
   <file value="%ALLUSERSAPPDATA%\Company\Product\Logs\error.log" />
</appender>

OK, we could define this in our setup, but maybe someone comes up with a better idea?

We just use this:

<param name="File" value="${ALLUSERSPROFILE}/Company/Product/Logs/error.log"/>

It works great.

<appender name="RollingFileAppender" type="log4net.Appender.RollingFileAppender">
   <param name="File" value="${ALLUSERSPROFILE}/Company/Product/Logs/error.log"/>
</appender>

This posting on the log4net mailinglist explains how you can define your own path replacement variables.

Here's the full code from the log4net mailing list that pilif linked to:

Basically the method is to implement a custom pattern converter for the log4net config file.

First add this class to your project:

public class SpecialFolderPatternConverter : log4net.Util.PatternConverter
{
    override protected void Convert(System.IO.TextWriter writer, object state)
    {
        Environment.SpecialFolder specialFolder = (Environment.SpecialFolder)Enum.Parse(typeof(Environment.SpecialFolder), base.Option, true);
        writer.Write(Environment.GetFolderPath(specialFolder));
    }
}

Then set up the File parameter of your FileAppender as follows:

<file type="log4net.Util.PatternString">
    <converter>
      <name value="folder" />
      <type value="MyAppName.SpecialFolderPatternConverter,MyAppName" />
    </converter>
    <conversionPattern value="%folder{CommonApplicationData}\\SomeOtherFolder\\log.txt" />
</file>

Basically the %folder tells it to look at the converter called folder which points it to the SpecialFolderPatternConverter class. It then calls Convert on that class, passing in the CommonApplicationData (or whatever) enum value.

Apparently in the next release of log4net (1.2.11) there will be a simpler method, as described here.