Don’t use async callbacks with System.Threading.Timer
You might be tempted to do this in cs/.net, which compiles and seems to run just fine:
// do something every 10 seconds
System.Threading.Timer timer = new (async _ =>
{
try
{
// do work
await Task.Delay(TimeSpan.FromSeconds(5));
}
}, null, TimeSpan.Zero, TimeSpan.FromSeconds(10));
While it will work on the happy path, there are two problems with this approach:
- Exceptions will not bubble up anywhere
- Shutdown will not block for an active task—it will be abandoned
Attempts to improve it by manually disposing the timer with a WaitHandle on shutdown do not work:
// this does not help anything!
AutoResetEvent reset = new(false);
timer.Dispose(reset);
reset.WaitOne();
The problem is that the timer’s callback is not awaited anywhere—the callback finishes as soon as it
constructs the Task to hold it (i.e. instantly) so there’s nothing for .WaitOne to wait for.
Here’s a better option, which works better with async semantics and has fewer surprises:
// do something every 10 seconds
using PeriodicTimer timer = new(TimeSpan.FromSeconds(10));
while (await timer.WaitForNextTickAsync(cancellationToken))
{
// do work
await Task.Delay(TimeSpan.FromSeconds(5));
}
The PeriodicTimer has a lot going for it:
- It won’t lead to overlapping executions if the body takes longer than the timer period
.WaitForNextTickAsyncreturns false when theCancellationTokenis fired, which is very convenient- All the fraught threading stuff is limited to the existing
asyncparadigm - Shutdown is blocked until the “do work” task is finished, and you can handle cancellation appropriately for your situation
The only lingering annoyance is that you can’t initialize a PeriodicTimer with a due time different from the period,
i.e. you can’t make it fire immediately, and then again every period, like you can with a Timer. Accounting for that
is not so bad, though:
// do it now
await DoWork();
// and every 10 seconds
using PeriodicTimer timer = new(TimeSpan.FromSeconds(10));
while (await timer.WaitForNextTickAsync(cancellationToken))
{
// do work
await DoWork();
}
// ...
private async Task DoWork() { /* ... */ }