System.Timers.Timer How to get the time remaining until Elapse

I am aware, that the topic is more than 3 years old. However I came across it while tackling exactly the same problem.

Inspired by Samuel Neff, I came up with a WinForms-less solution by extending the standard System.Timers.Timer class:

public class TimerPlus : System.Timers.Timer
{
    private DateTime m_dueTime;

    public TimerPlus() : base() => this.Elapsed += this.ElapsedAction;

    protected new void Dispose()
    {
        this.Elapsed -= this.ElapsedAction;
        base.Dispose();
    }

    public double TimeLeft => (this.m_dueTime - DateTime.Now).TotalMilliseconds;
    public new void Start()
    {
        this.m_dueTime = DateTime.Now.AddMilliseconds(this.Interval);
        base.Start();
    }

    private void ElapsedAction(object sender, System.Timers.ElapsedEventArgs e)
    {
        if (this.AutoReset)
            this.m_dueTime = DateTime.Now.AddMilliseconds(this.Interval);
    }
}

I hope it helps.

The built-in timer doesn't provide the time remaining until elapse. You'll need to create your own class which wraps a timer and exposes this info.

Something like this should work.

public class TimerPlus : IDisposable
{
    private readonly TimerCallback _realCallback;
    private readonly Timer _timer;
    private TimeSpan _period;
    private DateTime _next;

    public TimerPlus(TimerCallback callback, object state, TimeSpan dueTime, TimeSpan period)
    {
        _timer = new Timer(Callback, state, dueTime, period);
        _realCallback = callback;
        _period = period;
        _next = DateTime.Now.Add(dueTime);
    }

    private void Callback(object state)
    {
        _next = DateTime.Now.Add(_period);
        _realCallback(state);
    }

    public TimeSpan Period => _period;
    public DateTime Next => _next;
    public TimeSpan DueTime => _next - DateTime.Now;

    public bool Change(TimeSpan dueTime, TimeSpan period)
    {
        _period = period;
        _next = DateTime.Now.Add(dueTime);
        return _timer.Change(dueTime, period);
    }

    public void Dispose() => _timer.Dispose();
}

I guess the best method is to hold the start time in a variable and then calculate the elapsed time as

TimeSpan t = DateTime.Now - StartTime;