How to fire timer.Elapsed event immediately

I know this answer is late but if you want your System.Timers.Timer to be fired within 100ms (default interval) then you could simply just initialize the Timer object without a specified interval, then set the interval within the called function to whatever you like. Here is an example of what I use in my Windows Service:

private static Timer _timer;

protected override void OnStart(string[] args)
{
    _timer = new Timer(); //This will set the default interval
    _timer.AutoReset = false;
    _timer.Elapsed = OnTimer;
    _timer.Start();
}

private void OnTimer(object sender, ElapsedEventArgs args)
{
    //Do some work here
    _timer.Stop();
    _timer.Interval = 50000; //Set your new interval here
    _timer.Start();
}

I just called the **ElapsedEventHandler** with null parameters.

Just call the Timer_Tick method yourself.

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If you don't want to deal with the Tick callback method's parameters, then just put the code that was in your Timer_Tick into another method, and call that from the Timer_Tick and from just after the Timer.Start() call

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As pointed out by @Yahia, you could also use the System.Threading.Timer timer, which you can set to have an initial delay to 0. Be aware though, that the callback will run on a different thread, as opposed to the callback on the Windows.Forms.Timer which runs on the UI thread. So if you update any UI controls using the System.Threading.Timer (without invoking correctly) it'll crash.