Compiler error with foreach?

[Flinchvoid]

Interesting little problem; go easy on me because I'm just a humble
scripter turned c# developer.

I've a class UserList which I auto-generated with a python script, it
extends CollectionBase and has the usual methods: this[int index],
Add, Insert, Remove and Contains. There are no differences between
this class and the other generated classes I'm using other than the
type of the contained object. The other classes can be foreach'ed
quite happily.

I knocked up a quick console app to test an aspect of this system. I
can iterate the members of the list with an integer index, but not
with a foreach. The method called inside the loop, GetName, does not
alter the underlying object, it just returns a formatted string. Can
anybody shed any light on this peculiar quirk?

// this works

UserList l = dataprovider.GetLocalUsers();

int c = l.Count;

while(c-- > 0){
User u = l[c];
Console.WriteLine(u.GetName(User.NameFormat.Initia lSurname));
}
// this causes a compiler error

// foreach(User u in l)
// {
// Console.WriteLine(u.GetName(u.NameFormat.InitialSu rname));
// }

[Lasse Vågsæther Karlsen]

Flinchvoid wrote:

Interesting little problem; go easy on me because I'm just a humble
scripter turned c# developer.

<snip>

Does your user list collection class implement IEnumerable ?

--
Lasse Vågsæther Karlsen
http://www.vkarlsen.no/
mailto:la***@vkarlsen.no
PGP KeyID: 0x0270466B

[Uri Dor]

extending CollectionBase isn't enough - you need to implement
GetEnumerator - see MSDN on foreach

Flinchvoid wrote:

Interesting little problem; go easy on me because I'm just a humble
scripter turned c# developer.

I've a class UserList which I auto-generated with a python script, it
extends CollectionBase and has the usual methods: this[int index],
Add, Insert, Remove and Contains. There are no differences between
this class and the other generated classes I'm using other than the
type of the contained object. The other classes can be foreach'ed
quite happily.

I knocked up a quick console app to test an aspect of this system. I
can iterate the members of the list with an integer index, but not
with a foreach. The method called inside the loop, GetName, does not
alter the underlying object, it just returns a formatted string. Can
anybody shed any light on this peculiar quirk?

// this works

UserList l = dataprovider.GetLocalUsers();

int c = l.Count;

while(c-- > 0){
User u = l[c];
Console.WriteLine(u.GetName(User.NameFormat.Initia lSurname));
}
// this causes a compiler error

// foreach(User u in l)
// {
// Console.WriteLine(u.GetName(u.NameFormat.InitialSu rname));
// }

[Anders Norås [MCAD]]

Flinchvoid wrote:

I've a class UserList which I auto-generated with a python script, it
extends CollectionBase and has the usual methods: this[int index],
Add, Insert, Remove and Contains. There are no differences between
this class and the other generated classes I'm using other than the
type of the contained object. The other classes can be foreach'ed
quite happily.

You must implement the IEnumerable interface to enable foreach to
enumerate the collection.

Anders Norås
http://dotnetjunkies.com/weblog/anoras/

[Pathogenix]

Okay, there seems to be a concensus here, but I have *other* classes
built in exactly the same way which do use foreach without any
problems.

None of the other collection classes I'm using are any different.
Another test I've run (and just re-run to make sure I'm not
hallucinating) contains the following snippet

EnquirySummaryList l = dp.SearchEnquiryDetails();

foreach(EnquirySummary e in l)
{
Console.WriteLine(e.EnquiryName);
}
EnquirySummaryList is /exactly/ the same as UserList other than the
contained type, and yet it enumerates with foreach. What am I missing?

[Pathogenix]

Having scurried off to MSDN
[http://msdn.microsoft.com/library/de...lasstopic.asp]
, I find that CollectionBase implements IEnumerable. So that's not the
problem. Compiler bug, or the sinister machinations of a dark force
bent on driving me insane?

[Richard Blewett [DevelopMentor]]

What is the compiler error you get?

Regards

Richard Blewett - DevelopMentor
http://www.dotnetconsult.co.uk/weblog
http://www.dotnetconsult.co.uk

Interesting little problem; go easy on me because I'm just a humble
scripter turned c# developer.

I've a class UserList which I auto-generated with a python script, it
extends CollectionBase and has the usual methods: this[int index],
Add, Insert, Remove and Contains. There are no differences between
this class and the other generated classes I'm using other than the
type of the contained object. The other classes can be foreach'ed
quite happily.

I knocked up a quick console app to test an aspect of this system. I
can iterate the members of the list with an integer index, but not
with a foreach. The method called inside the loop, GetName, does not
alter the underlying object, it just returns a formatted string. Can
anybody shed any light on this peculiar quirk?

// this works

UserList l = dataprovider.GetLocalUsers();

int c = l.Count;

while(c-- > 0){
User u = l[c];
Console.WriteLine(u.GetName(User.NameFormat.Initia lSurname));
}
// this causes a compiler error

// foreach(User u in l)
// {
// Console.WriteLine(u.GetName(u.NameFormat.InitialSu rname));
// }

--
No virus found in this incoming message.
Checked by AVG Anti-Virus.
Version: 7.0.300 / Virus Database: 265.7.1 - Release Date: 19/01/2005

[microsoft.public.dotnet.languages.csharp]

[Pathogenix]

Drat it, I've found it when trying to reproduce the bug for you: I was
trying to access the NameFormat enum in an instance of User rather than
through the class User. VS.net just borked instead of warning me.

I feel better now that logic has been restored.
Thanks to those who took two minutes to puzzle over my ineptitude.

[Jon Skeet [C# MVP]]

"Anders Norås [MCAD]" <an**********@objectware.no> wrote:

Flinchvoid wrote:

I've a class UserList which I auto-generated with a python script, it
extends CollectionBase and has the usual methods: this[int index],
Add, Insert, Remove and Contains. There are no differences between
this class and the other generated classes I'm using other than the
type of the contained object. The other classes can be foreach'ed
quite happily.

You must implement the IEnumerable interface to enable foreach to
enumerate the collection.

Not necessarily. Here's an example which doesn't:

using System;
using System.Collections;

class Foo
{
ArrayList list = new ArrayList();

internal int Count
{
get
{
return list.Count;
}
}

internal string this[int i]
{
get
{
return (string)list[i];
}
}

internal void Add (string x)
{
list.Add(x);
}

public PseudoEnumerator GetEnumerator()
{
return new PseudoEnumerator(this);
}
}

class PseudoEnumerator
{
int current = -1;
Foo foo;

internal PseudoEnumerator (Foo foo)
{
this.foo = foo;
}

public bool MoveNext()
{
current++;
return current < foo.Count;
}

public string Current
{
get
{
return foo[current];
}
}
}

class Test
{
static void Main()
{
Foo f = new Foo();
f.Add ("Hello");
f.Add ("there.");
f.Add ("No");
f.Add ("IEnumerator");
f.Add ("here...");

foreach (string x in f)
{
Console.WriteLine (x);
}
}
}

See the C# language spec for more details.

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too

[Anders Norås [MCAD]]

Thanks Jon, I wasn't aware of this.

Anders Norås
http://dotnetjunkies.com/weblog/anoras/

[cody]

>> You must implement the IEnumerable interface to enable foreach to

enumerate the collection.

Not necessarily. Here's an example which doesn't:

Very interesting but Iam wondering why this works with foreach/IEnumerable
but not with using/IDisposable.

[Jon Skeet [C# MVP]]

cody <de********@gmx.de> wrote:

You must implement the IEnumerable interface to enable foreach to
enumerate the collection.

Not necessarily. Here's an example which doesn't:

Very interesting but Iam wondering why this works with foreach/IEnumerable
but not with using/IDisposable.

Well, because the language specification says it does :)

The reason it's there is to allow people to implement a strongly-typed
enumerator - there's no equivalent need with IDisposable.

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too