Question

I’m writing a multi threaded application, where one part of the code must be thread safe to keep track of task numbers.

I have this method:

private void IncrementTaskNumber() {
   Interlocked.Increment(ref _TaskNumber);
}

_TaskNumber is a private int in the same class. The problem is, this throws me a “A property, indexer or dynamic member access may not be passed as an out or ref parameter” exception.
To get around this, I do:

private void IncrementTaskNumber() {
   int _taskNum = _TaskNumber;
   Interlocked.Increment(ref _taskNum);
   _TaskNumber = _taskNum;
}

Is this still thread safe?

Answer 1

_TaskNumber is a private int in the same class.

_TaskNumber has to be a private field for this to work. You likely have it as a private property.

Define it as:

private int _TaskNumber;

And it will work.

Also note that your current workaround introduces a race condition – you’re effectively getting rid of the atomic increment by doing it using a temporary variable, which defeats the purpose of using Interlocked in the first place. You need to increment the field directly.