If you want your log file to be place at a specified location which will be decided at run time may be your project output directory then you can configure your .config file entry in that way

<file type="log4net.Util.PatternString" value="%property{LogFileName}.txt" />

and then in the code before calling log4net configure, set the new path like below

log4net.GlobalContext.Properties["LogFileName"] = @"E:\\file1"; //log file path

log4net.Config.XmlConfigurator.Configure();