Creating generic Isolator<T> class for collection modidy inside foreach loop

[Andrus]

Code below causes error in class definition line

.....Isolator<T >' does not implement interface member
'System.Collect ions.IEnumerabl e.GetEnumerator ()'.
'Myapp.Isolator <T>.GetEnumerat or()' cannot implement
'System.Collect ions.IEnumerabl e.GetEnumerator ()' because it does not have
the matching return type of 'System.Collect ions.IEnumerato r'.

I tried to change method signature to

public IEnumerator GetEnumerator()

bu this causes another error

....Isolator<T> ' does not implement interface member
'System.Collect ions.Generic.IE numerable<T>.Ge tEnumerator()'.
'Myapp.Isolator <T>.GetEnumerat or()' cannot implement
'System.Collect ions.Generic.IE numerable<T>.Ge tEnumerator()' because it does
not have the matching return type of
'System.Collect ions.Generic.IE numerator<T>'.

How to make this code work ?

Andrus.

using System;
using System.Collecti ons;
using System.Collecti ons.Generic ;

public class Isolator<T: IEnumerable<T>
{
IEnumerable<Tin ner;

public Isolator(IEnume rable<Tenumerab le)
{
inner = enumerable;
}

public IEnumerator<TGe tEnumerator()
{
ArrayList<Tlist = new ArrayList<T>();
foreach (object o in inner)
list.Add(o);
return list.GetEnumera tor();
}
}

[Marc Gravell]

Re: Creating generic Isolator&lt;T&g t; class for collection modidy insideforeach loop

Because IEnumerable<Tde rives from IEnumerable, you need to provide
*both* forms of GetEnumerator() ; this is usually dona via explicit
implementation. .. just add:

IEnumerator IEnumerable.Get Enumerator()
{ // reuse typed enumerator
return GetEnumerator() ;
}

Marc

[Andrus]

Re: Creating generic Isolator&lt;T&g t; class for collection modidy inside foreach loop

Marc,

Thank you. It works.
I'm planning to use it to remove members in linq invoice rows like

foreach (var row in new Isolator<TRow>( rowBindingSourc e))
{
if (row.Item == "VAT" || row.Item == "ROUNDING")
rowBindingSourc e.Remove(row);
}

Where rowBindingSourc e type is your linq TableList class.

Is this best style ?

In VFP I can use SQL directly from my application code

DELETE FROM rowBindingSourc e WHERE rows IN ("VAT", "ROUNDING")

Why Linq to Objects requires to create loop manually, .i.e forces me to use
procedural programming ?
Why there is no delete command in linq or method like

rowBindingSourc e.Remove( row=row.Item == "VAT" || row.Item ==
"ROUNDING" );

or

from row in rowBindingSourc e
where row=row.Item in ( "VAT", "ROUNDING" )
delete row;

Andrus.

[Jon Skeet [C# MVP]]

Re: Creating generic Isolator&lt;T&g t; class for collection modidy insideforeach loop

On Jun 5, 10:39 am, "Andrus" <kobrule...@hot .eewrote:

Code below causes error in class definition line

<snip>

Marc has provided the reason why this fails, and a fix - but if you're
using C# 3 and .NET 3.5, you can just call ToList() on any
IEnumerable<Tan d it'll have *nearly* the same effect - the main
difference being the time of copying. If that's an issue, consider an
extension method:

public static IEnumerable<TCo pyLazily(this IEnumerable<Tso urce) {
List<Tlist = source.ToList() ;
foreach (T t in list)
{
yield return t;
}
}

It makes life a lot simpler :)

Jon

[Jon Skeet [C# MVP]]

Re: Creating generic Isolator&lt;T&g t; class for collection modidy insideforeach loop

On Jun 5, 11:23 am, "Andrus" <kobrule...@hot .eewrote:

Thank you. It works.
I'm planning to use it to remove members in linq invoice rows like
>
foreach (var row in new Isolator<TRow>( rowBindingSourc e))
{
if (row.Item == "VAT" || row.Item == "ROUNDING")
rowBindingSourc e.Remove(row);
}
>
Where rowBindingSourc e type is your linq TableList class.
>
Is this best style ?

Well, I'd just use:

foreach (var row in rowBindingSourc e.ToList())
{
...
}

Alternatively you could put a call to Where after .ToList() and remove
every matching row in the loop.

In VFP I can use SQL directly from my application code
>
DELETE FROM rowBindingSourc e WHERE rows IN ("VAT", "ROUNDING")
>
Why Linq to Objects requires to create loop manually, .i.e forces me to use
procedural programming ?

LINQ is designed to be side-effect free. Deleting is a side-effect.
Moreover, LINQ to Objects is based on IEnumerable, which has no notion
for deleting. What would you expect to happen if you tried to delete
from an array, for example?

Jon

[Anthony Jones]

Re: Creating generic Isolator&lt;T&g t; class for collection modidy inside foreach loop


"Andrus" <kobruleht2@hot .eewrote in message
news:euYezYvxIH A.1772@TK2MSFTN GP03.phx.gbl...

Marc,
>
In VFP I can use SQL directly from my application code
>
DELETE FROM rowBindingSourc e WHERE rows IN ("VAT", "ROUNDING")
>
Why Linq to Objects requires to create loop manually, .i.e forces me to

use

procedural programming ?
Why there is no delete command in linq or method like
>
rowBindingSourc e.Remove( row=row.Item == "VAT" || row.Item ==
"ROUNDING" );
>
or
>
from row in rowBindingSourc e
where row=row.Item in ( "VAT", "ROUNDING" )
delete row;

The Q in LINQ stands for Query.

DELETE is not a query, it modifies the stored data.


--
Anthony Jones - MVP ASP/ASP.NET

[Marc Gravell]

Re: Creating generic Isolator&lt;T&g t; class for collection modidy insideforeach loop

Well, even indirectly, mixing enumeration and editing (removal) of a
list is not a greate idea.

Re the difference between SQL delete and .NET version; well, LINQ is
object orientated (SQL isn't); if you want to do anything in LINQ, you
do it to the instances. This is no different. Apart from this, there are
also issues like:
* concurrency management - hard to do on a block delete
* identity: if the data-context issues a block delete, how does it know
which records the database actually deleted? to update it's own local
cache of objects...
* LINQ can be configured to to the CRUD through SPs; how would it issue
a block-delete through a DELETE sproc that only accepts a PKID?

If this is going to be doing a lot of work, then I would create an SP
and call that (you can expose SPs with arguments etc to LINQ-to-SQL very
easily).

If you are dealing with smaller volumes, and you want a Remove(fn) on
some local class, then just add one!

Marc

[Andrus]

Re: Creating generic Isolator&lt;T&g t; class for collection modidy inside foreach loop

Well, I'd just use:

>
foreach (var row in rowBindingSourc e.ToList())
{
...
}
>
Alternatively you could put a call to Where after .ToList() and remove
every matching row in the loop.

Thank you. Delete needs to scan over whole collection always.
So I changed foreach to use ToArray()

foreach (var row in rowBindingSourc e.ToArray() )
{
if (row.Item == "VAT" || row.Item == "ROUNDING")
rowBindingSourc e.Remove(row);
}

hope this faster than ToList().

Is it reasonable/how to create generic static method which implements this
mass-Remove() for IList<Tcollecti ons ?
Since covariant types are not supported in C#, generic type parameter should
be used probably.

Andrus.

[Jon Skeet [C# MVP]]

Re: Creating generic Isolator&lt;T&g t; class for collection modidy insideforeach loop

On Jun 5, 1:33 pm, "Andrus" <kobrule...@hot .eewrote:

Alternatively you could put a call to Where after .ToList() and remove
every matching row in the loop.

>
Thank you. Delete needs to scan over whole collection always.

Yes, but if you are only *acting* on certain elements, you can call
Where() to only get those items:

var toDelete = rowBindingSourc e.Where(row =row.Item=="VAT " ||
row.Item=="ROUN DING").ToList() ;
foreach (var row in toDelete)
{
rowBindingSourc e.Remove(row);
}

So I changed foreach to use ToArray()

<snip>

hope this faster than ToList().

I'd expect it to be slower in some cases, actually - because it needs
to accurately count the number of items before creating the array.

Is it reasonable/how to create generic static method which implements this
mass-Remove() for IList<Tcollecti ons ?

Yes, that would be easy to do as an extension method.

Since covariant types are not supported in C#, generic type parameter should be used probably.

Absolutely.

Just offhand (untested):

public static void RemoveWhere<T(t his IList<Tsource, Func<T,bool>
predicate)
{
List<TtoRemove = source.Where(pr edicate).ToList ();
foreach (T t in toRemove)
{
source.Remove(t );
}
}

Jon