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
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<file type="log4net.Util.PatternString" value="%property{LogFileName}.txt" />
and then in the code before calling log4net configure, set the new path like below
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log4net.GlobalContext.Properties["LogFileName"] = @"E:\\file1"; //log file path
log4net.Config.XmlConfigurator.Configure();