C#: ArrayList.Clear() clears the wrong array

Question

I'm using ArrayList in C# to do some things. I have 2 ArrayLists (align and best) and, in a specific time, I make best=align inside a "for" routine.

The problem is, in the end of the loop, I do align.Clear, but in this time, the array "best" gets cleared too. After the loop, when I have to use the array "best", I get troubles, because it is cleared and I try to access its index.

What's the problem?

Here is a piece of my code:

public string AntColony()
{
   ArrayList align = new ArrayList();
   ArrayList best = new ArrayList();

   for(int z=0;z<n_ants;z++)
   {
      //do the things i have to do
      //full the array "align" with something (this will have two "adds", so, this array is a 2 lines array)

      score = Score(align);
      UpdatePhero(tao, path, score);

      if (score > score_before)
      {
         score_before = score;
         best = align;
      }
      align.Clear(); //clear the array align
   }
   string s = best[0].ToString() + "\r\n\r\n" + best[1].ToString() + "\r\n\r\n Number of matches: " + n_matches + "\r\n\r\n Score: " + score;

   return s;
}

Thank you!

You are making `best` reference to point to `align`. C# is based on references! — Am_I_Helpful, Mar 29 '16 at 18:58
Thanks for your answer. And what I can do to solve this? I mean, what strategy? Anyway, I have to make best=align and clear the align array in the end. — anderici, Mar 29 '16 at 19:04
Your query has already been solved by an author in the Answer section. Good Luck. — Am_I_Helpful, Mar 29 '16 at 19:05
Also check out http://stackoverflow.com/questions/2309694/arraylist-vs-list-in-c-sharp — Yuriy Faktorovich, Mar 29 '16 at 19:05

score 2 · Accepted Answer · answered Mar 29 '16 at 19:03

2

Array variables are reference types. When you call best=align you are not copying the contents of align to array, you are making it so that they point to the same place, i.e. they reference the same memory location.

Try best=align.Clone()

answered Mar 29 '16 at 19:03

rory.ap

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Thanks for your answer! I tried to do best=align.Clone(), but I got something like it is not possible to convert 'object' to 'Convert.Collections.ArrayList'. – anderici Mar 29 '16 at 19:24
It worked doing best = (ArrayList)align.Clone() Thank you very much – anderici Mar 29 '16 at 19:37
Glad I could help! – rory.ap Mar 29 '16 at 19:41

score 0 · Answer 2 · answered Mar 29 '16 at 19:10

Since align is temporary, it can be recreated just before the call to Score and assigned to best when needed:

public string AntColony()
{
    ArrayList best = null;

    for(int z=0;z<n_ants;z++)
    {
      //do the things i have to do
      //full the array "align" with something (this will have two "adds", so, this array is a 2 lines array)

      ArrayList align = new ArrayList();
      score = Score(align);
      UpdatePhero(tao, path, score);

      if (score > score_before)
      {
         score_before = score;
         best = align;
      }
    }

    if (best != null)
    {
        string s = best[0].ToString() + "\r\n\r\n" + best[1].ToString() + "\r\n\r\n Number of matches: " + n_matches + "\r\n\r\n Score: " + score;
        return s;
    }

    // TODO: Report failure here

    return null;
}

Thanks for your answer! Unfortunatelly I need the array align before the loop to do some other things! — anderici, Mar 29 '16 at 19:31

C#: ArrayList.Clear() clears the wrong array

2 Answers2