Trick for young players.

[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.

[Lost User]

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.

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

[twizzle801]

I see the problem... it's my inability to understand why people prefer object oriented languages. I must be too old to play. :(

[Jorgen Andersson]

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. :-D

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

[Lost User]

I used to think that some years ago - couldn't see the point; until I started using them - and fell in love!

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

[Lost User]

Contains uses Equals, and it has to. Firstly, because it can't see an overloaded operator==, secondly because if it somehow could, you could put a double.NaN in a list and never find it again - you'd have a list with one element, but Contains is false for any argument, making it look as though the one element isn't any one value, and thirdly because object.ReferenceEquals would always give false for lists of value types because you'd create new boxed objects all the time. So there's really no good alternative, it has to be this way.

[BobJanova]

_Maxxx_ wrote:

If each object had two booleans, would it act the same?

Depends if it's a value type, and whether Equals is overridden.

_Maxxx_ wrote:

The fact that the Contains method works inconsistently depending on the contents of objects is the problem.

It does not behave differently depending on the contents. It depends differently depending on the type. You can think of it like string interning: there is only one false so when you have two of them, they are always equal, both in value terms and in reference terms. I think this is true for all value types; it's certainly true for all basic types.

_Maxxx_ wrote:

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'

The answer is yes if there is an object in the collection for which Equals with the one you've passed returns true. That's what equality means, and it's usually much more useful than checking for reference equality. You can override == on custom types to make that check whatever you want. By default the behaviour is to check references for reference types and to check values for value types (or at least base types). Choosing which constructor to use based on the values of parameters, instead of the types of parameters, is a WTF all to itself. Edit: Equals, not ==. I always override both of these, and also !=, if I do either, so I get them mixed up sometimes.

[BobJanova]

Like all tools, it is excellent when deployed correctly and a useful technique to know about. It's over-promoted in modern CS though, in my opinion, to the detriment of other useful techniques like functional or event-driven programming.

[JMK89]

Gotta love ReSharper

static void Main()
{
object arg1 = true;
object arg2 = true;

var collection = new List<object> { arg1 };

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

foreach (var t in collection)
{
    //Do fuck all
}

Console.WriteLine("{0} {1}", false, false);

Console.ReadKey();

}

[Lost User]

BobJanova wrote:

Depends if it's a value type, and whether Equals is overridden.

Not sure I even follow you there. An object with two boolean properties is not a value type - or am I misunderstanding you?

BobJanova wrote:

and it's usually much more useful than checking for reference equality.

I have to disagree there, and say it entirely depends on the context. If I want to check if two objects are the same object, i would like to be able to do so in a consistent manner, without having to check to see if the object is a value-type wrapper. similarly, if I want to check if the value of two objects are the same, I would expect to be able to use the same methods regardless as to whether the objects in question have booleans or Customers internally. I reiterate - I understand exactly what is happening, and it is a trick for young players (hence the post). But I still think that there is an inherent (pun absolutely intended) discrepancy n the handling of boxed objects vs the handling of 'vanilla' objects.

BobJanova wrote:

Choosing which constructor to use based on the values of parameters, instead of the types of parameters, is a WTF all to itself.

The AIM of the method in question was was to decide on the constructor based upon the TYPES of parameter vs TYPES of arguments. The parameters were in a collection and the arguments in another.

1. For Each parameter in the constructor it is checking
2. For each argument in the collection
3. If THIS ARGUMENT is already in our output collection, continue
4. If this argument is of the same type (or subtype etc) as the current parameter, add it to the output collection
5. Next Argument
6. Next parameter

Line 3 is where the issue arises, because if there are two value arguments, and they both have the same runtime value, then this If is triggered and the argument ignored for the 2nd and subsequent parameter.

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