Question

I cannot use the instance of my task (t) within my task’s action delegate in my code directly below. I get the following error:

Use of unassigned local variable ‘t’

Code:

Task t = Task.Factory.StartNew(() =>
{
    MessageBox.Show(t.Id.ToString());
});

Now, it works if I do the following:

Task t = null;
t = Task.Factory.StartNew(() =>
{
    MessageBox.Show(t.Id.ToString());
});

Could someone please explain why this is the case?

Answer 1

C# compiler does not know anything about Task.Factory.StartNew. As far as the compiler is concerned, the access to t could happen at any time after calling StartNew, including the time before t has been assigned.

Your second code snippet has a race condition: if the task on a concurrent thread gets around to displaying message box before the assignment is complete, you will see a null reference exception.

Try this experiment:

private static Task Wrapper(Action f) {
    var res = Task.Factory.StartNew(f);
    Thread.Sleep(1000);
    return res;
}

Now replace the direct call of Task.Factory.StartNew with a call of Wrapper in your second snippet, and watch the program crash.

Task t = null;
t = Wrapper(() => {
    MessageBox.Show(t.Id.ToString());
});