Re: No Equals on interfaces

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From: Michi Henning (michi_at_zeroc.com)
Date: 09/06/04 

Date: Mon, 06 Sep 2004 11:02:37 +1000
To: "Herfried K. Wagner [MVP]" <hirf-spam-me-here@gmx.at>

Herfried K. Wagner [MVP] wrote:

> * Michi Henning <michi@zeroc.com> scripsit:
>
>>the following code won't compile, but I believe it should:
>>
>>Interface X
>>End Interface
>>
>>Module M
>> Sub Main(ByVal args As String())
>> Dim iref As X
>> iref.Equals(iref) ' 'Equals' is not a member of 'vb.I'.
>> End Sub
>>End Module
>>
>>The compiler complains that X does not have a member Equals.
>>(The same complaint is issued for the other methods of System.Object:
>>GetHashCode, GetType, and ToString.)
>>
>>This seems wrong, given that C# compiles the equivalent code
>>without complaint.
>
>
> That's simply different behavior in C# and VB.NET, and I think that the
> behavior shown by VB.NET is more correct. Interfaces in .NET do /not/
> have a base type if they don't inherit from another interface.

Nonsense. From the C# language specification for predefined types:

object: The ultimate base type of all other types

> In C#, the compiler doesn't complain for some reason, but the code you
> listed above is rather useless because it will throw a
> 'NullReferenceException'.

Oh, *please*. The point is that the expression iref.Equals(iref) will not
compile, not what happens at run time.

> None of the methods you can access from C#
> are useful because they are all instance members and interfaces cannot
> be instantiated.

Again, nonsense. I can pass a reference to a class that is derived from I
to another method. Are you *really* suggesting that this method then should
not be allowed to invoke Equals on the reference?

Moreover, you ignored the following:

>>Also, the following works fine:
>>
>>Dim iref As X
>>Dim oref As Object = iref
>>oref.Equals(oref)
>>
>>Clearly, if there is an implicit conversion from X to Object,
>>then X must have the members of Object (otherwise, implicit
>>conversion would have to be illegal.)

According to what you say, the implicit conversion to object (which
is allowed by the compiler) would then lead to me holding a reference
that allows me to call Equals on it, yet does not support the Equals
method. Again, that's simply nonsense. *All* reference types are derived
from object. Whether the formal type of a references is for an interface
or to a class is irrelevant. The VB compiler is simply broken in this
respect, period.

>>Does anyone know whether this problem is fixed for Whidbey?
>
>
> There is no problem in VB.NET. It's IMO a C# problem.

Please read the language specification. This isn't a C# problem,
it's a bug in VB.

Cheers,

Michi.

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