error very strange

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Argentino Douglas - 13 Nov 2006 20:00 GMT

a function, whose parameter is a ref to a collection of class I
private void func(List i){
for(int n=0;n<MAXVALUE;++n){

for(int m=0;m<MAXVALUE;++m){

Dosomething();
}

i[n].value=5; [******]Eror: can't modify the value of
System.Collections.Generic.List<program.Form1.I>.this[int] because it
is not a variable[/******]
}

Thanks

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Argentino Douglas - 13 Nov 2006 20:09 GMT

Any questions or answers ? Looks like nobody is in now.

> a function, whose parameter is a ref to a collection of class I
> private void func(List i){
[quoted text clipped - 11 lines]
>
> Thanks

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Jon Skeet [C# MVP] - 13 Nov 2006 20:11 GMT

> a function, whose parameter is a ref to a collection of class I
> private void func(List i){
[quoted text clipped - 9 lines]
> is not a variable[/******]
> }

Could you post a short but complete program which demonstrates the
problem?

See http://www.pobox.com/~skeet/csharp/complete.html for details of
what I mean by that.

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Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
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Argentino Douglas - 13 Nov 2006 20:24 GMT

public struct I{
 public int value;
}

private void ValuableHelper(Listi){

 i=new List();

 for(int n=0;n<MAXVALUE;++n){

 for(int m=0;m<MAXVALUE;++m){

 PleaseHelpMe();

 }

 i[n].value=5; //Error is here

 }
}
I am coding it as a window application,
Thanks

Jon Skeet [ C# MVP ] wrote:

> > a function, whose parameter is a ref to a collection of class I
> > private void func(List i){
[quoted text clipped - 20 lines]
> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
> If replying to the group, please do not mail me too

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Jon Skeet [C# MVP] - 13 Nov 2006 21:36 GMT

> public struct I{
> public int value;
[quoted text clipped - 17 lines]
> }
> I am coding it as a window application,

Well, firstly, that's *not* a short but complete program. Please read
http://www.pobox.com/~skeet/csharp/incomplete.html

However, the cause of your problem is that you've got a struct - a
value type - and you're trying to change a value which is returned from
an indexer. You can't do that, and even if you did, it wouldn't do what
you wanted it to.

i[n] returns a value. If you modify that value, it won't change the
contents of the list at all.

Now, if I were a reference type, you'd be modifying the value *referred
to* by the reference in the list, and that would be a different matter.

This kind of problem is just one reason why mutable value types are
generally a bad idea.

Oh, and regards your other post: you can't expect replies *that*
quickly. You only left your original message for 9 minutes before
posting again!

Signature

Jon Skeet - <skeet@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

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error very strange

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