Async System.Threading.Timer sometimes throwing null exception

Question

I am trying to execute a code asynchronously after a certain interval of time and want to check again only after I am done with the code execution to prevent overlapping. For that I am using the following variables and timer code.

private System.Threading.Timer _timer;
private int _intervalInMilliSec = 10000;

_timer = new System.Threading.Timer(async (o) =>
{
    // Stop the timer;
    _timer.Change(-1, -1);

    // awaited api call goes here

    // start timer again (BeginTime, Interval)
    _timer.Change(_intervalInMilliSec, _intervalInMilliSec);
}, null, 0, _intervalInMilliSec);

Most of the times it is working fine but sometimes rarely I started getting the following exception at the line _timer.Change(-1, -1);

Object reference not set to an instance of an object

At that point "o" is showing as null. Please let me know what I am doing wrong. Thanks

Answer 1

I don't know why you get the NullReferenceException in your code, but I suggest to scrap the Timer approach altogether and replace it with a simpler Task.Delay in a loop:

var apiCallCts = new CancellationTokenSource();
var apiCallTask = Task.Run(async () =>
{
    while (true)
    {
        var delayTask = Task.Delay(TimeSpan.FromSeconds(10), apiCallCts.Token);
        await SomeApiCallAsync();
        await delayTask;
    }
});

And at the app shutdown:

apiCallCts.Cancel();

The above solution has a different behavior in case of an API failure. Instead of crashing the app, will silently stop calling the API. This is something that you'll have to consider.