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ArrayList.Contains() -- using objects of different types

Posted on 2004-08-21
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Last Modified: 2007-12-19
I have an ArrayList of objects called Permission. Permission.Equals() has been overriden to allow comparison with int, and works as expected.

However, ArrayList.Contains() continues to return false. Your thoughts or advice are appreciated.

Permission p = new Permission(9);
int pId = 9;
ArrayList plist = new ArrayList();
plist.Add(p);

p.Equals(pId); // true
Object.Equals(p, pId); // true
pId.Equals(p); // false - as expected
Object.Equals(pId, p); // false - as expected
p.GetHashCode(); // 9
pId.GetHashCode(); // 9
plist.Contains(p); // true
plist.Contains(pId); // false

I thought .Contains() used the Equals method of the contained class? Otherwise I don't think i can make this work at all ... I'm clearly not going to rewrite the Equals() method of int?

It seems silly that ArrayList.Contains(suppliedObj) it would use Object.Equals(suppliedObj, ContainedObj)

Thanks for your thoughts,
Sean.
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Question by:SeanStapleton
11 Comments
 
LVL 22

Expert Comment

by:Mohammed Nasman
ID: 11862655
From MSDN help about Contains method:

This method performs a linear search; therefore, the average execution time is proportional to Count. That is, this method is an O(n) operation, where n is Count.

This method determines equality by calling Object.Equals.
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Author Comment

by:SeanStapleton
ID: 11862705
Yes, I've read this. But Object.Equals takes two parameters, and MSDN doesn't specify which order the params are supplied in. Thought it appears I've discovered it.
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Author Comment

by:SeanStapleton
ID: 11862775
To clarify my previous comment

myObj.Equals(object rhs)
 - 2 parameters: this & rhs  (it's often called right hand side b/c there is a left)

Object.Equals(object item1, object item2)
 - 2 parameters: item1 and item2 (can't access this from a static method, though i suppose could be accessing static vars of Object)


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LVL 20

Expert Comment

by:ihenry
ID: 11862883
The documentation means

when this line is executed, plist.Contains(p);
p.Equals(pId) is evaluated, it returns true as expected

and when this line is executed, plist.Contains(pId);
pId.Equals(p) is evaluated, it returns false which is the correct behaviour.
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LVL 20

Expert Comment

by:ihenry
ID: 11862892
sorry wrong typo,

when this line is executed, plist.Contains(p);
Permission.Equals(p) is evaluated, it returns true as expected

and when this line is executed, plist.Contains(pId);
int.Equals(pId) is evaluated, it returns false which is the correct behaviour.
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LVL 1

Author Comment

by:SeanStapleton
ID: 11862934
pList is composed of Permission objects, so you are not quite right ihenry.

It appears that what happens is akin to...

bool Contains(obj rhs)
{
  foreach(object obj in ArrayList.items)
  {
       if (rhs.Equals(obj)) return true;
   }
}

what i had hoped for (and what makes more sense to me) was that it would instead call obj.Equals(rhs), so that by overloading the Equals of Permission I could handle int.
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LVL 20

Accepted Solution

by:
ihenry earned 50 total points
ID: 11863020

I've tried a quick debugging and I think my assumption is correct. I'm quite sure the implementation of ArrayList.Contains would look something similar like this,

public virtual bool Contains(Object item)
{
      if (item==null)
      {
            ....
            ....
      }
      else
      {
            foreach ( object o in items )
                  if (item.Equals(o) )
                        return true;
                  return false;
            }
      }
}

So when an integer parameter is passed, it evaluates int.Equals() instead
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LVL 20

Expert Comment

by:ihenry
ID: 11863033
I to put a breakpoint in the Permission.Equal method

plist.Contains(p); execution stops at the breakpoint
plist.Contains(pId); nothing happen
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LVL 1

Author Comment

by:SeanStapleton
ID: 11863111
ihenry, I agree with your implementation. It's what i discovered here, but just hoped it was otherwise...

My comment that you weren't quite on was a symantecs. Looking again, your two first comments are each half right...

when this line is executed, plist.Contains(p);
p.Equals(plist.item[1..n]) is evaluated, it returns true as expected

and when this line is executed, plist.Contains(pId);
pId.Equals(plist.item[1..n]) is evaluated, it returns false which is the correct behaviour.

You get the points for confirmation and peace of mind. Thanks.
Sean.
0
 
LVL 20

Expert Comment

by:ihenry
ID: 11863144

yes, I was wrong.. thanks for the correction and also for the points.
0
 
LVL 10

Expert Comment

by:eternal_21
ID: 11864931
Why don't you just override ArrayList.Contains, or cast your integers to Permission objects when you call ArrayList.Contains?
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