“ClosureAndForeach” (Solution)

Answer

In old compiler versions: 3 3 3.

In new compiler versions: 1 2 3.

Explanation

In old compiler versions, the code from the problem will be transformed in the following code:

public void Run()
{
  var actions = new List<Action>();
  DisplayClass c1 = new DisplayClass();
  foreach (int i in Enumerable.Range(1, 3))
  {
    с1.i = i;
    list.Add(c1.Action);
  }
  foreach (Action action in list)
    action();
}

private sealed class DisplayClass
{
  public int i;

  public void Action()
  {
    Console.WriteLine(i);
  }
}

Thus, all elements from the list refer to same delegate. So, we will see 3 same values in the console, they will equal the last value of i.

Some breaking changes were in new compiler versions. The new version of the code:

public void Run()
{
  var actions = new List<Action>();
  foreach (int i in Enumerable.Range(1, 3))
  {
    DisplayClass c1 = new DisplayClass();
    с1.i = i;
    list.Add(c1.Action);
  }
  foreach (Action action in list)
    action();
}

private sealed class DisplayClass
{
  public int i;

  public void Action()
  {
    Console.WriteLine(i);
  }
}

Now, each element of the list refers to own delegate, all printed values will be different.

Examples:

Mono compiler 2.4.4                       : 3 3 3
Mono compiler 3.10.0                      : 1 2 3
Mono compiler 3.10.0        langversion=4 : 1 2 3
MS compiler 3.5.30729.7903                : 3 3 3
MS compiler 4.0.30319.1                   : 3 3 3
MS compiler 4.0.30319.33440               : 1 2 3
MS compiler 4.0.30319.33440 langversion=4 : 1 2 3

Links

• ItDepends.NET: ClosureAndForeach

• Eric Lippert: Closing over the loop variable considered harmful

• Eric Lippert: Closing over the loop variable, part two

Problem