compare two structs via ==

[titan nyquist]

I wish to compare two structs via == but it does not compile. I can
overload and create my own == but am I missing something that c#
already has implemented?

~titan

[Nicholas Paldino [.NET/C# MVP]]

Re: compare two structs via ==

No, you have not. If you overload ==, then you have to overload != as
well.

On top of that, you should also overload Equals and GetHashCode as well
to produce consistent results.


--
- Nicholas Paldino [.NET/C# MVP]
- mvp@spam.guard. caspershouse.co m

"titan nyquist" <titan.nyquist@ gmail.comwrote in message
news:1180554252 .418734.196230@ z28g2000prd.goo glegroups.com.. .

>I wish to compare two structs via == but it does not compile. I can
overload and create my own == but am I missing something that c#
already has implemented?
>
~titan
>

[=?ISO-8859-1?Q?G=F6ran_Andersson?=]

Re: compare two structs via ==

titan nyquist wrote:

I wish to compare two structs via == but it does not compile. I can
overload and create my own == but am I missing something that c#
already has implemented?

There is no default comparer for structs, as comparing the values as a
binary block of data doesn't always make sense.

If you for example have a struct:

public struct MyStruct {
private string _name;
public MyStruct(string name) { _name = name; }
public string Name { get { return _name; } }
}

If you then create two struct values that contains the same string value:

MyStruct v1 = new MyStruct("1");
MyStruct v2 = new MyStruct(1.ToSt ring());

If you could compare these struct values as binary chunks of data, they
would not be equal, as the structs contains two different references,
eventhough the references points to string values that are equal.

--
Göran Andersson
_____

http://www.guffa.com

[Fabio]

Re: compare two structs via ==

"Göran Andersson" <guffa@guffa.co mha scritto nel messaggio
news:O7I9L9voHH A.2596@TK2MSFTN GP06.phx.gbl...

>
There is no default comparer for structs, as comparing the values as a
binary block of data doesn't always make sense.
>

But why I can do this?

Point p1 = new Point(1, 2);

Point p2 = new Point(1, 2);

Console.WriteLi ne(p1 == p2);

[Christof Nordiek]

Re: compare two structs via ==

"Fabio" <znt.fabio@virg ilio.itschrieb im Newsbeitrag
news:uZzjZ02oHH A.3880@TK2MSFTN GP04.phx.gbl...

"Göran Andersson" <guffa@guffa.co mha scritto nel messaggio
news:O7I9L9voHH A.2596@TK2MSFTN GP06.phx.gbl...
>

>>
>There is no default comparer for structs, as comparing the values as a
>binary block of data doesn't always make sense.
>>

>
But why I can do this?
>
Point p1 = new Point(1, 2);
>
Point p2 = new Point(1, 2);
>
Console.WriteLi ne(p1 == p2);
>

Because an eqaulity operator is defined for Point in Point itself.

Christof

[Marc Gravell]

Re: compare two structs via ==

There is no *default* equality operator. Point declares one itself -
look in "reflector" to see it.

Marc

[Jon Skeet [C# MVP]]

Re: compare two structs via ==

On May 31, 11:18 am, "Fabio" <znt.fa...@virg ilio.itwrote:

There is no default comparer for structs, as comparing the values as a
binary block of data doesn't always make sense.

>
But why I can do this?
>
Point p1 = new Point(1, 2);
Point p2 = new Point(1, 2);
Console.WriteLi ne(p1 == p2);

Because Point overloads the equality operator itself, along with
addition, subtraction etc.

Jon

[Jon Skeet [C# MVP]]

Re: compare two structs via ==

On May 31, 11:32 am, "Jon Skeet [C# MVP]" <s...@pobox.com wrote:

<snip>

Oops. Looks like the automatic reply generator for myself, Marc and
Christof has got out of sync... Time to reboot ourselves again, guys.

(Anticipating some sort of equivalent reply from Marc and Christof
now...)

Jon

[Moty Michaely]

Re: compare two structs via ==

On May 31, 1:34 pm, "Jon Skeet [C# MVP]" <s...@pobox.com wrote:

On May 31, 11:32 am, "Jon Skeet [C# MVP]" <s...@pobox.com wrote:
>
<snip>
>
Oops. Looks like the automatic reply generator for myself, Marc and
Christof has got out of sync... Time to reboot ourselves again, guys.
>
(Anticipating some sort of equivalent reply from Marc and Christof
now...)
>
Jon

Hi,

This proves what the *asynchronous* fellows mentioned =)

[Serializable, StructLayout(La youtKind.Sequen tial),
TypeConverter(t ypeof(PointConv erter)), ComVisible(true )]
public struct Point
{
...
public static bool operator ==(Point left, Point right);
public static bool operator !=(Point left, Point right);
...
}

Moty

[Mark Wilden]

Re: compare two structs via ==

It'd be nice if C# generated a default memberwise equality operator, but I
suppose there's a good reason why it doesn't.

///ark

[=?ISO-8859-1?Q?G=F6ran_Andersson?=]

Re: compare two structs via ==

Mark Wilden wrote:

It'd be nice if C# generated a default memberwise equality operator, but I
suppose there's a good reason why it doesn't.
>

Yes, there is.

Did you miss my post explaining that a memberwise comparision dosen't
always make sense?

--
Göran Andersson
_____

http://www.guffa.com

[Mark Wilden]

Re: compare two structs via ==

"Göran Andersson" <guffa@guffa.co mwrote in message
news:exuQzSBpHH A.4696@TK2MSFTN GP02.phx.gbl...

Did you miss my post explaining that a memberwise comparision dosen't
always make sense?

I saw the post from you talking about a bitwise comparison, but not a
memberwise comparison. In the latter, you'd compare each member, using its
== operator.

Did I miss a post?

///ark

[=?ISO-8859-1?Q?G=F6ran_Andersson?=]

Re: compare two structs via ==

Mark Wilden wrote:

"Göran Andersson" <guffa@guffa.co mwrote in message
news:exuQzSBpHH A.4696@TK2MSFTN GP02.phx.gbl...
>

>Did you miss my post explaining that a memberwise comparision dosen't
>always make sense?

>
I saw the post from you talking about a bitwise comparison, but not a
memberwise comparison. In the latter, you'd compare each member, using its
== operator.
>
Did I miss a post?
>
///ark
>

Ok, I misunderstood your distinction there. Anyway, not even a
memberwise comparison would always make sense.

--
Göran Andersson
_____

http://www.guffa.com

[Mark Wilden]

Re: compare two structs via ==

Memberwise comparison worked pretty well in C++. An object is equal to
another object if all its members are equal to the other object's members
(using the same criterion for member equality).

[=?ISO-8859-1?Q?G=F6ran_Andersson?=]

Re: compare two structs via ==

Mark Wilden wrote:

Memberwise comparison worked pretty well in C++. An object is equal to
another object if all its members are equal to the other object's members
(using the same criterion for member equality).
>

Usually, but not always.

I have a faint memory of reading somewhere that the decision to not
include an automatic memberwise comparison for structs was that it would
automatically add functionality to all structs that you create. If the
comparison doesn't make sense for a struct, you would be forced to
override it even if it didn't make sense to compare the structs at all.
It would not be possible to prevent usage of the comparison at compile
time, it could only throw an exception at run time.

If you want a memberwise comparison for a struct, it's very easy to
write one, and you can most likely make it more efficient than any
automatically generated code, as you can choose the order to compare the
fields so that you first compare the fields that is most likely to differ.

--
Göran Andersson
_____

http://www.guffa.com