Parameters in asynchronous lambdas

Question

I am trying to run several tasks at the same time and I came across an issue I can\'t seem to be able to understand nor solve.

I used to have a function like this :<

Answer 1

Make a local copy of your index variable:

for(int index = 0; index < MAX; index++) {
  var localIndex = index;
  Task.Run(async () => {
    await SomeAsynchronousTasks();
    var item = items[index];
    item.DoSomeProcessing();
    if(b)
        AVolatileList[index] = item;
    else
        AnotherVolatileList[index] = item;
  }
}

This is due to the way C# does a for loop: there is only one index variable that is updated, and all your lambdas are capturing that same variable (with lambdas, variables are captured, not values).

As a side note, I recommend that you:

1. Avoid async void. You can never know when an async void method completes, and they have difficult error handling semantics.
2. await all of your asynchronous operations. I.e., don't ignore the task returned from Task.Run. Use Task.WhenAll or the like to await for them. This allows exceptions to propagate.

For example, here's one way to use WhenAll:

var tasks = Enumerable.Range(0, MAX).Select(index =>
  Task.Run(async () => {
    await SomeAsynchronousTasks();
    var item = items[localIndex];
    item.DoSomeProcessing();
    if(b)
        AVolatileList[localIndex] = item;
    else
        AnotherVolatileList[localIndex] = item;
  }));
await Task.WhenAll(tasks);

Answer 2

All your lambdas capture the same variable which is your loop variable. However, all your lambdas are executed only after the loop has finished. At that point in time, the loop variable has the maximum value, hence all your lambdas use it.

Stephen Cleary shows in his answer how to fix it.

Eric Lippert wrote a detailled two-part series about this.