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LINQ anonymous types and reference issues

Hi experts, I'm fairly new to the whole linq stuff. Today I came across a behaviour I don't really understand.
Frist, see code below, it's an extension method for a dataset.
This one works, no problem.

but, if I don't loop through TableNamesToDelete.ToList() and would instead use TableNamesToDelete directly, I get an exception in the foreach loop, saying the enumeration has been changed while looping.
the same happens when I create an anonymous type in the linq query, like
select new{t.TableName} and use TableNamesToDelete directly.
So, somehow it seems I still have a reference to the datatables in the datasets, which causes the foreach-loop to fail.

I don't really get it, why is there still a reference when I create an anonymous type?
Or am I getting something wrong here?

public static class ExtensionMethods
    {
        public static void RemoveUnnecessaryTables(this DataSet ds)
        {
            var TableNamesToDelete = from t in ds.Tables.Cast<DataTable>()
                                     where t is INoImport
                                     select t.TableName;


            foreach(string TName in TableNamesToDelete.ToList())
            {
                if(ds.Tables.Contains(TName))
                    ds.Tables.Remove(TName);
            }
        }

    }

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ASKER CERTIFIED SOLUTION

sanket_1985

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Arikael

ASKER

so, you say, even when I return an anonymous type in my linq-query, I have somehow a reference to the datatable/dataset?

kaufmed

Use a "for" loop instead and work backwards (see below). As sanket_1985 stated, you cannot modify a collection that you are iterating over--it would screw up the internal iterator for the collection. Use a "for" loop when you want to add/remove from a collection in a loop.

var TableNamesToDelete = (from t in ds.Tables.Cast<DataTable>()
                          where t is INoImport
                          select t.TableName).ToList();

for(int i = TableNamesToDelete.Count - 1; i >= 0; i--)
{
    if(ds.Tables.Contains(TableNamesToDelete[i]))
        ds.Tables.Remove(TableNamesToDelete[i]);
}

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kaufmed

In hindsight, you don't have to "work backwards" in this particular scenario. You would be fine to start at the beginning of TableNamesToDelete and go forward.

Arikael

ASKER

Hi,

I know that I can't modify a collection I'm iterating over :)
But I don't really understand why TableNamesToDelete seems to have a reference to to the dataset/datatable, even when I return a new anonymous type.

SOLUTION

kaufmed

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Arikael

ASKER

thanks

I still find it strange, especially when I return an anonymous type. It's not the behaviour I would expect.