When SynchronizingObject is null, methods handling the EntryWritten event are called on a thread from the system thread pool. For more information on system thread pools, see ThreadPool.
When the EntryWritten event is handled by a visual Windows Forms component, such as a button, accessing the component through the system thread pool might not work, or might result in an exception. Avoid this by setting SynchronizingObject to a Windows Forms component, which causes the methods handling the EntryWritten event to be called on the same thread on which the component was created.
If the EventLog is used inside Visual Studio 2005 in a Windows Forms designer, SynchronizingObject is automatically set to the control containing the EventLog. For example, if you place an EventLog on a designer for Form1 (which inherits from Form) the SynchronizingObject property of EventLog is set to the instance of Form1.
Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)