Merge pull request #8030 from josalem/dev/josalem/eventlistener-constructor

Author: John Salem

xml/System.Diagnostics.Tracing/EventListener.xml

@@ -66,6 +66,9 @@
  Each event source   keeps a list of event listeners that it sends events to. A set of filtering criteria is associated with each event source-listener pair. These criteria determine the events that listener will receive for the current event source. Listeners receive the events on their <xref:System.Diagnostics.Tracing.EventListener.OnEventWritten%2A> method. Therefore, subclasses of <xref:System.Diagnostics.Tracing.EventListener> must override this method to do something useful with the data.  
   
  When a new event source is created, the <xref:System.Diagnostics.Tracing.EventListener.OnEventSourceCreated%2A> method is called. Every current event listener gets exactly one <xref:System.Diagnostics.Tracing.EventListener.OnEventSourceCreated%2A> call for every current event source that can send it log messages. When an event listener is created, a series of <xref:System.Diagnostics.Tracing.EventListener.OnEventSourceCreated%2A> calls are made to notify the new listener of all the event sources that existed before the listener was created.  
+
+> [!IMPORTANT]
+> When instantiating an `EventListener`, the callbacks to <xref:System.Diagnostics.Tracing.EventListener.OnEventSourceCreated%2A> and <xref:System.Diagnostics.Tracing.EventListener.OnEventWritten%2A> can happen _before_ the constructor has completed. Take care when you initialize instance members used in those callbacks.
   
  ]]></format>
     </remarks>