Trick for young players.

Lost User

Look at the prog below - a collection has one(object)bool in it. And I want to find out if one of them exists in the collection. Using collection.Contains() would seem like a good idea. But isn't!

class Program
{
static void Main(string[] args)
{
object arg1 = true;
object arg2 = true;

		List<object> collection = new List<object>() { arg1 };

		bool first = (collection.Contains(arg1));
		bool second = (collection.Contains(arg2));
		Console.WriteLine(string.Format("{0} {1}", first, second));

		first = false;
		second = false;

		for (int i = 0; i < collection.Count; i++)
		{
			if (collection\[i\] == arg1)
			{
				first = true;
			}
			if (collection\[i\] == arg2)
			{
				second = true;
			}
		}

		Console.WriteLine(string.Format("{0} {1}", first, second));

			Console.ReadKey();
	}
}

MVVM# - See how I did MVVM my way ___________________________________________ Man, you're a god. - walterhevedeich 26/05/2011 .\\axxx (That's an 'M')

Guirec

What would you expect? Looks all fine... arg2 is not contained in the collection but collection[0] is equal (not the same but nonetheless equal) to arg2

Seulement, dans certains cas, n'est-ce pas, on n'entend guère que ce qu'on désire entendre et ce qui vous arrange le mieux... [^]

Lost User

So you would expect contains(arg2) to return false?

MVVM# - See how I did MVVM my way ___________________________________________ Man, you're a god. - walterhevedeich 26/05/2011 .\\axxx (That's an 'M')

Guirec

For sure! your collection is a list of objects and the arg2 object is simply not in that collection...

Seulement, dans certains cas, n'est-ce pas, on n'entend guère que ce qu'on désire entendre et ce qui vous arrange le mieux... [^]

Lost User

You're obviously a young player. Run it!

MVVM# - See how I did MVVM my way ___________________________________________ Man, you're a god. - walterhevedeich 26/05/2011 .\\axxx (That's an 'M')

Guirec

Wtf? Goddamn Sh*t! I would not expect that either.... I am a young player!!

Seulement, dans certains cas, n'est-ce pas, on n'entend guère que ce qu'on désire entendre et ce qui vous arrange le mieux... [^]

Lost User

Guirec Le Bars wrote:

Wtf? Goddamn Sh*t!

That's almost exactly what I said when I realised what was going on!!!! This was found in the middle of an MVVM framework that was causing me an issue. Good one, eh?

MVVM# - See how I did MVVM my way ___________________________________________ Man, you're a god. - walterhevedeich 26/05/2011 .\\axxx (That's an 'M')

Guirec

Then you're not much elder than me :)

Seulement, dans certains cas, n'est-ce pas, on n'entend guère que ce qu'on désire entendre et ce qui vous arrange le mieux... [^]

J4amieC

Not sure what is confusing you, the implementation of Contains obviously uses the Equals object override, so sucessfully finds arg1 and arg2. However, your list is a list of object (rather than a list of bool) so when it comes to using the equality operator instead of Equals, then you're comparing object references (not values!) and clearly arg1 is not arg2.

Jorgen Andersson

== resolves to System.Object.ReferenceEquals while contains determines equality by using the default equality comparer, as defined by the object's implementation.

People say nothing is impossible, but I do nothing every day.

Jorgen Andersson

I understand the difference and still I make that mistake occasionally. I guess my head isn't screwed on tightly enough.

People say nothing is impossible, but I do nothing every day.

Lost User

J4amieC wrote:

Not sure what is confusing you,

Seriously? Honestly? you can't see the confusion? Object1.equals(object2) == true; When object1 and object2 are different objects that have the same value? Assuming you didn't know what the objects were, are you telling me you honestly can't see a source of confusion?

MVVM# - See how I did MVVM my way ___________________________________________ Man, you're a god. - walterhevedeich 26/05/2011 .\\axxx (That's an 'M')

J4amieC

_Maxxx_ wrote:

Object1.equals(object2) == true;

When object1 and object2 are different objects that have the same value?

Exactly, the point is I understand valuetype equality semantics from referencetype equality semantics. I agree they're somewhat confusing to new programmers.

Lost User

Except object isn't a valuetype

MVVM# - See how I did MVVM my way ___________________________________________ Man, you're a god. - walterhevedeich 26/05/2011 .\\axxx (That's an 'M')

BobJanova

bool is a value type, so it is in the collection, because false == false.

BobJanova

object is not a value type but the thing you've stored in the object is a value type.

BobJanova

== will resolve to ReferenceEquals for reference types, and if there is no == operator override. For a value type (like bool) it will do a content equality.

Jorgen Andersson

Quite right. I referred to this specific case which of course wasn't obvious at all. :sigh:

People say nothing is impossible, but I do nothing every day.

Sentenryu

you clearly are forgetting about polymorphism :laugh: this is just one of the confusions he can cause... (see the << and >> operators in c++)

I'm brazilian and english (well, human languages in general) aren't my best skill, so, sorry by my english. (if you want we can speak in C# or VB.Net =p)

OriginalGriff

:thumbsup:

If you get an email telling you that you can catch Swine Flu from tinned pork then just delete it. It's Spam.