C# - Capture Username With Log4net?

I currently write all log4net events to a database, and it seems to work just fine. To capture the logged in user account I use this piece of code:

HttpContext context = HttpContext.Current;
if (context != null && context.User != null && context.User.Identity.IsAuthenticated)
{
MDC.Set("user", HttpContext.Current.User.Identity.Name);
}

The code seems ok, except for events that have no user context associated with them (ie. a user on our public web page). In that case the log4net capture seems to sometime write the last logged in user account (bad) and sometime write a null (good). Anyone got this feature to work reliably in all cases? I believe I saw a note that MDC is no longer a recommended feature to use, but I wasn't able to find any alternatives that are recommended.

Note: I find it odd that MDC is set with an account name, but never cleared if no user is active. That could be part of the problem. However, I didn't find any MDC code extracts that also clear the username.