Do C# Timers elapse on a separate thread?

It depends. The System.Timers.Timer has two modes of operation.

If SynchronizingObject is set to an ISynchronizeInvoke instance then the Elapsed event will execute on the thread hosting the synchronizing object. Usually these ISynchronizeInvoke instances are none other than plain old Control and Form instances that we are all familiar with. So in that case the Elapsed event is invoked on the UI thread and it behaves similar to the System.Windows.Forms.Timer. Otherwise, it really depends on the specific ISynchronizeInvoke instance that was used.

If SynchronizingObject is null then the Elapsed event is invoked on a ThreadPool thread and it behaves similar to the System.Threading.Timer. In fact, it actually uses a System.Threading.Timer behind the scenes and does the marshaling operation after it receives the timer callback if needed.

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