gistfile1.cs

class FileCleanerAppender : RollingFileAppender
{
	public FileCleanerAppender()
	{
		File = "Foo.log";
		AppendToFile = false;
		RollingStyle = RollingMode.Once;
		LockingModel = new MinimalLock();
		Name = "SomeAppender";
		Layout = new PatternLayout { ConversionPattern = "%m" };
		ActivateOptions();
	}
}

var logger = ((log4net.Repository.Hierarchy.Logger)LogManager.GetLogger("SomeLogger").Logger);

logger.AddAppender(new FileCleanerAppender());