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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();
}
}
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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... [^]
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So you would expect contains(arg2) to return false?
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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... [^]
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You're obviously a young player.
Run it!
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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... [^]
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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?
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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... [^]
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== 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.
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== 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.
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Quite right.
I referred to this specific case which of course wasn't obvious at all. 
People say nothing is impossible, but I do nothing every day.
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That's exactly the opposite of what would seem reasonable to me.
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bool is a value type, so it is in the collection, because false == false.
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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.
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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.
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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?
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_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.
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Except object isn't a valuetype
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object is not a value type but the thing you've stored in the object is a value type.
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If you get an email telling you that you can catch Swine Flu from tinned pork then just delete it. It's Spam.
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If I may be clear, I understand what is happening and why - the fact remains it is an inconsistency.
If each object had two booleans, would it act the same?
As an'outside observer' the behavior is inconsistent.
In the case where this came up, the framework was processing parameters passed to a constructor, and trying to find the best constructor to use based on the parameters.
The routine in question failed if two booleans were passed because e second was deemed to be the same parameter as the first when their values were equal.
Because this is a framework, and until now nobody had happened to write a constructor with multiple value types of the same type, and subsequently try to use it with those value types having the same value, nobody had noticed the issue.
If lit had been integers rather than booleans it would have been even more interesting - as the chances of the values also being equal wold be that much smaller.
So, I under stand what is happening, but I still regard this as an issue with the potential for causing larger problems I an application.
The fact that the Contains method works inconsistently depending on the contents of objects is the problem.
I ask 'does object a contain object b'
And I expect the answer to be yes or no - and not 'well, if it's an object containing only a value type, then the collection contains at least one similar Object where the value type has the same value, but if it's a reference type then that instance exists I the collection'
This means in principal that I need to know about the objects in any collection beforehand - which especially in a framework environment, I do not.
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Try this:
Sub Main()
Dim arg1 As Object = True
Dim arg2 As Object = True
Dim collection As New List(Of Object)() From { _
arg1 _
}
Dim first As Boolean = (collection.Contains(arg1))
Dim second As Boolean = (collection.Contains(arg2))
Console.WriteLine(String.Format("{0} {1}", first, second))
first = False
second = False
For i As Integer = 0 To collection.Count - 1
If collection(i) = arg1 Then
first = True
End If
If collection(i) = arg2 Then
second = True
End If
Next
Console.WriteLine(String.Format("{0} {1}", first, second))
Console.ReadKey()
End Sub
At least it's consistent. 
People say nothing is impossible, but I do nothing every day.
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LOL - I am genuinely crying (with a little laughter and a lot of WTF!)
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Just pulling your leg a little bit.
Shorthand for ReferenceEquals in VB.Net is IS.
= is the shorthand for Equals.
So the VB code wasn't the same as your C# code. 
I'v seen this error a few times in code that's been converted between VB and C#.
C# == is not the same as VB =, but oh so easy to miss if you only know one of the languages.
Should add that this is a standard error in all automatic converters I've tried.
People say nothing is impossible, but I do nothing every day.
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And then people complain about the confusing use of pointers and pointees in C/C++ ...
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