Using Task with .Net 4.0 vs .Net 4.5? - task-parallel-library

Question

When i ran this piece of code
private void button1_Click(object sender, EventArgs e)
{
Start(sender, e);
}
private void Start(object sender, EventArgs e)
{
for (int i = 0; i < 5; i++)
{
System.Threading.Tasks.Task.Factory.StartNew(() => dosomething(i));
Debug.WriteLine("Called " + i);
}
Debug.WriteLine("Finished");
}
public void dosomething(int i)
{
Debug.WriteLine("Enters " + i);
lock (this)
{
Debug.WriteLine("Working " + i);
Thread.Sleep(100);
}
Debug.WriteLine("Done " + i);
}
output is different with .Net version 4.0 and 4.5. With 4.0 number 5 is repeated I can see the reason value of i is moved to 5 before some of the Tasks executed but same code with 4.5 shows different output.
(output ran with VS 2010 .Net 4.0)
Called 0
Called 1
Enters 1
Working 1
Called 2
Called 3
Called 4
Finished
Enters 0
Done 1
Enters 5
Working 0
Working 5
Done 0
**Enters 5
Working 5
Done 5
Enters 5
Done 5
Working 5
Done 5**
but when i ran with .Net 4.5 (VS 2011 beta) the result is,
(output ran with VS 2011 beta .Net 4.5)
Enters 0
Working 0
Called 0
Called 1
Enters 2
Called 2
Enters 2
Enters 3
Called 3
Called 4
Finished
Done 0
Working 2
Enters 5
Done 2
Working 3
Done 3
Working 5
Done 5
Working 2
Done 2
I couldn't see changes done with Task under CLR 4.5? Can anyone point me what are the changes with .Net 4.5 please.

Answer 1

Your code has a race condition. Let's say the loop finishes executing before any of the tasks gets started. This is entirely possible.
Then i will have a value of 5 in all tasks. This is the bug.
Solution: Copy i to a loop-local variable and use this local in the task's lambda.

Answer 2

Your code has a race condition. What that means is that it can behave in various way, depending on the exact order of operations.
And any small change may affect the order of operations, so it's not unexpected that your code will behave differently under different versions of the framework. Actually, I would expect it to behave differently when run multiple times for the same version of .net.