The System.AccessViolationException is thrown if one tries to marshal unamanaged memory from an invalid location, e.g. using a bad pointer. Prior to .NET 4.0, this exception was catch-able from within the CLR.
With .NET 4.0, System.AccessViolationException exception is no longer catched within the CLR, meaning that the application now crashes without necessarily logging the information in log4net. The stack trace can however be seen in windows’ event viewer.
It is possible to mark the method in which the exception is thrown using HandleProcessCorruptedStateExceptionsAttribute, thus making the method throw the exception into the CLR and making it catchable.