I have an application that runs as a windows service and spins up multiple AppDomains, each with their own loggers. I will often run into an issue where log4net appears to be causing one or more of the spun up AppDomains to not function properly. The relevant stack trace looks as follows:
System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
at log4net.Util.ReadOnlyPropertiesDictionary.GetObjectData(Serialization info, StreamingContext context)
at System.Runtime.Serialization.ObjectCloneHelper.GetObjectData(Object serObj, String& typeName, String& assemName, String& fieldNames, Object& fieldValues)
I have not been able to find the specific conditions to reproduce this but it happens often enough to be a huge problem. Looking at the log4net source, the ReadOnlyPropertiesDictionary.GetObjectData() method does a foreach over the InnerHashTable collection but there is nothing to stop that collection from being modified outside of that loop which is leading to the exception. I am not sure where in my code I would be doing anything that modified that collection indirectly but I do know that I push several things on and off of the LogicalThreadContext.