Reliably stop System.Threading.Timer?

An easier solution might to be to set the Timer never to resume; the method Timer.Change can take values for dueTime and period that instruct the timer never to restart:

this.Timer.Change(Timeout.Infinite, Timeout.Infinite);

Whilst changing to use System.Timers.Timer might be a "better" solution, there are always going to be times when that's not practical; just using Timeout.Infinite should suffice.

like Conrad Frix suggested you should use the System.Timers.Timer class instead, like:

private System.Timers.Timer _timer = new System.Timers.Timer();
private volatile bool _requestStop = false;

public constructor()
{
    _timer.Interval = 100;
    _timer.Elapsed += OnTimerElapsed;
    _timer.AutoReset = false;
    _timer.Start();
}

private void OnTimerElapsed(object sender, System.Timers.ElapsedEventArgs e)
{
    // do work....
    if (!_requestStop)
    {
        _timer.Start();//restart the timer
    }
}

private void Stop()
{
    _requestStop = true;
    _timer.Stop();
}

private void Start()
{
    _requestStop = false;
    _timer.Start();
}