List of enumerators?

[A.B.]

Hi,
I have various lists I want to go through, at different paces and I
want to control that with a list of enumerators.

I have the following:

List<List<DataEntry>mydata = new List<List<Data>>();
List<List<DataEntry>.Enumeratormyenums = news
List<List<Data>.Enumertor>();

I do
myenums.Add(mydata[0].GetEnumerator());
then
myenums[0].MoveNext();

well it seems that doesn't work... like myenums[0] is a temporary copy
of the enumerator... I don't know but when I do that, myenums[0]
desn't actually move.

e = myenums[0];
e.MoveNext();

works though, meaning e moves, but it doesn't change myenums[0]
( !?? )

Can someone explain me the rational here and a good way to do what I
am trying to achieve?
Thanks!

[Jon Skeet [C# MVP]]

On Mar 21, 1:39 pm, "A.B." <[e-mail]:

I have various lists I want to go through, at different paces and I
want to control that with a list of enumerators.

<snip>

Please post a short but complete program that demonstrates the
problem.
See http://pobox.com/~skeet/csharp/complete.html for what I mean by
that.

Jon

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

A.B. wrote:

Hi,
I have various lists I want to go through, at different paces and I
want to control that with a list of enumerators.

I have the following:

List<List<DataEntry>mydata = new List<List<Data>>();
List<List<DataEntry>.Enumeratormyenums = news
List<List<Data>.Enumertor>();

I do
myenums.Add(mydata[0].GetEnumerator());
then
myenums[0].MoveNext();

well it seems that doesn't work... like myenums[0] is a temporary copy
of the enumerator... I don't know but when I do that, myenums[0]
desn't actually move.

e = myenums[0];
e.MoveNext();

works though, meaning e moves, but it doesn't change myenums[0]
( !?? )

Can someone explain me the rational here and a good way to do what I
am trying to achieve?
Thanks!

What you are experiencing is the usage of a value type. An enumerator is
a structure, which means that whenever you get the enumerator from the
list, you are getting a copy of it. If you want the enumerator in the
list to be updated, you have to put the updated enumerator back into the
list:

e = myenums[0];
e.MoveNext();
myenums[0] = e;

--
Göran Andersson
_____
http://www.guffa.com

[A.B.]

Here's a working code showing what happens
using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
List<List<int>l = new List<List<int>>();
List<List<int>.Enumeratore = new
List<List<int>.Enumerator>();

l.Add(new List<int>());
l.Add(new List<int>());

l[0].Add(1);
l[0].Add(2);
l[1].Add(10);
l[1].Add(11);

e.Add(l[0].GetEnumerator());
e.Add(l[1].GetEnumerator());

e[0].MoveNext();
Console.WriteLine(e[0].Current); // prints 0, I'd like to
print "1"
List<int>.Enumerator f = e[1];
f.MoveNext(); f.MoveNext();
Console.WriteLine("{0} {1}",f.Current,e[1].Current); //
prints 11 0, I'd like to print "11 11"
}
}
}

[Barry Kelly]

A.B. wrote:

List<List<DataEntry>mydata = new List<List<Data>>();
List<List<DataEntry>.Enumeratormyenums = news

myenums.Add(mydata[0].GetEnumerator());

myenums[0].MoveNext();

List<T>.Enumerator is a value type, so myenums[0] will return a copy.
Cast it to IEnumerable<Tinstead.

-- Barry

--
http://barrkel.blogspot.com/

[Jon Skeet [C# MVP]]

On Mar 21, 2:07 pm, Göran Andersson <[e-mail]:

<snip>

What you are experiencing is the usage of a value type. An enumerator is
a structure, which means that whenever you get the enumerator from the
list, you are getting a copy of it.

Note that enumerators aren't *in general* structures. It just happens
that List<T>.Enumerator is a structure.

That's a pretty nasty "gotcha" when converting from .NET 1.1 to
generics...

Jon

[A.B.]

On Mar 21, 10:27 am, Barry Kelly <[e-mail]:

List<T>.Enumerator is a value type, so myenums[0] will return a copy.
Cast it to IEnumerable<Tinstead.

Could you show me how I'd do that on the sample code I posted?
Thanks!

[Jon Skeet [C# MVP]]

A.B. <ab*@mytrashmail.comwrote:

On Mar 21, 10:27 am, Barry Kelly <[e-mail]:

List<T>.Enumerator is a value type, so myenums[0] will return a copy.
Cast it to IEnumerable<Tinstead.

Could you show me how I'd do that on the sample code I posted?
Thanks!

You only need to change the declarations for e and f:

List<IEnumerator<int>e = new List<IEnumerator<int>>();
and
IEnumerator<intf = e[1];

Then it works as per your comments.

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

[Ben Voigt]

"A.B." <ab*@mytrashmail.comwrote in message
news:11*********************@l75g2000hse.googlegro ups.com...

Hi,
I have various lists I want to go through, at different paces and I
want to control that with a list of enumerators.

I have the following:

List<List<DataEntry>mydata = new List<List<Data>>();
List<List<DataEntry>.Enumeratormyenums = news
List<List<Data>.Enumertor>();

I do
myenums.Add(mydata[0].GetEnumerator());
then
myenums[0].MoveNext();

well it seems that doesn't work... like myenums[0] is a temporary copy
of the enumerator... I don't know but when I do that, myenums[0]
desn't actually move.

Correct. Accessing an item inside a List always returns a temporary copy,
while access to an array element is in-place. For reference types, a copy
of the reference means you end up working on the same object in the end, but
with value types, a copy is completely independent.

[A.B.]

Tried it but

e.Add(l[0].GetEnumerator());

fails

Error 1 The best overloaded method match for
'System.Collections.Generic.List<System.Collection s.Generic.IEnumerable<int>>.Add(System.Collections .Generic.IEnumerable<int>)'
has some invalid arguments

and

e.Add((IEnumerable<int>)l[0].GetEnumerator());

fails too

Error 1 Cannot convert type
'System.Collections.Generic.List<int>.Enumerator' to
'System.Collections.Generic.IEnumerable<int>'

[Barry Kelly]

A.B. wrote:

Tried it but

e.Add(l[0].GetEnumerator());

fails

Error 1 The best overloaded method match for
'System.Collections.Generic.List<System.Collection s.Generic.IEnumerable<int>>.Add(System.Collections .Generic.IEnumerable<int>)'
has some invalid arguments

I made a couple of mistakes, you should instantiate:

List<IEnumerator<int>>

rather than:

List<IEnumerable<int>>

Sorry.

-- Barry

--
http://barrkel.blogspot.com/

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

A.B. wrote:

Here's a working code showing what happens
using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
List<List<int>l = new List<List<int>>();
List<List<int>.Enumeratore = new
List<List<int>.Enumerator>();

l.Add(new List<int>());
l.Add(new List<int>());

l[0].Add(1);
l[0].Add(2);
l[1].Add(10);
l[1].Add(11);

e.Add(l[0].GetEnumerator());
e.Add(l[1].GetEnumerator());

e[0].MoveNext();

As I said, you are getting a copy of the enumerator. If you want the
enumerator in the list to change, you have to put the enumerator that
you changed back in the list:

Enumerator en = e[0];
en.MoveNext();
e[0] = en;

Console.WriteLine(e[0].Current); // prints 0, I'd like to
print "1"
List<int>.Enumerator f = e[1];
f.MoveNext(); f.MoveNext();
Console.WriteLine("{0} {1}",f.Current,e[1].Current); //
prints 11 0, I'd like to print "11 11"
}
}
}

--
Göran Andersson
_____
http://www.guffa.com

[Jon Skeet [C# MVP]]

Göran Andersson <gu***@guffa.comwrote:

<snip>

As I said, you are getting a copy of the enumerator. If you want the
enumerator in the list to change, you have to put the enumerator that
you changed back in the list:

Enumerator en = e[0];
en.MoveNext();
e[0] = en;

Or make the list a list of boxed versions which can be used directly
via an interface, in which case there's no need to replace anything -
the value inside the box changes instead.

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