The logEntry.Data.Add I posted a while ago is not a method of the LogEntry class. It is an instance of a LogEntry. There are a lot of ways to create a LogEntry. The easiest way is to create it using an empty constructor:
LogEntry logEntry = new LogEntry();
If you explore a bit more, you can see other overloads of this constructor.
The ExtendedProperties is just simply a collection. You can put information there which you might want to add aside from those which are already provided in the LogEntry class as properties. If you look at the template of your tracelistener,
there is a token there for the Extended Properties. Whatever you add in that collection, it will written in that section.
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