Why can I not iterate over something with linq that can be iterated over with foreach?

Question

I have a collection of table rows (type DataRowCollection). It is perfectly valid for me to write:

foreach(var row in myDataRowCollection)
{
  // Something with row
}

Yet the following will not compile:

myDataRowCollection.ToList();

or even

myDataRowCollection.Select(...

What are these System.Linq extensions expecting that DataRowCollection doesn't implement?

Not compile means? is it throwing any exception? if yes then provide error. — Prasad Telkikar, Sep 27 '19 at 13:20
It simple isn't valid code, if I write .Select (even after including the System.Linq namespace) it has a red line under it, it's not valid, it won't build. — , Sep 27 '19 at 13:21
Not sure why I received a downvote for this - I don't mind downvotes if they're explained with constructive criticism but can't see what's wrong here!? — , Sep 27 '19 at 13:22
`myDataRowCollection.Cast().ToList()` ... extensions are for `IEnumerable` not `IEnumerable` — Selvin, Sep 27 '19 at 13:25
related https://stackoverflow.com/questions/4974159/convert-datarowcollection-to-ienumerablet — xdtTransform, Sep 27 '19 at 13:29

H H · Accepted Answer · 2019-09-27T13:32:16.963

5

foreach() is pattern based (docs), it does not actually rely on IEnumerable. GetEnumerator() is enough.

class A // note: no interface
{
    public IEnumerator GetEnumerator() { ...} 
}

Linq on the other hand is based on extension methods for IEnumerable<T>.

Your DataRowCollection implements IEnumerable but not IEnumerable<DataRow>. It is too old for Linq.

But there are some helper methods available, myDataRowCollection.AsEnumerable().Select(...) should work.

edited Sep 27 '19 at 13:32

answered Sep 27 '19 at 13:22

H H

Ah OK, thanks Henk. My question is less about why foreach works and more about why the linq extensions don't though, given the fact the foreach does. This type does implement `IEnumerable` – Sep 27 '19 at 13:25
@xdtTransform it also implements IEnumerable. It implements both. – Sep 27 '19 at 13:28
https://learn.microsoft.com/en-us/dotnet/api/system.data.internaldatacollectionbase?view=netframework-4.8 – Sep 27 '19 at 13:28
Thanks for the update Henk, that explains it. Much appreciated, will mark as answer when I can. – Sep 27 '19 at 13:29
deleted my comment to make a more clear one but Henk edited his answer faster. For Work around please refer to https://stackoverflow.com/questions/4974159/convert-datarowcollection-to-ienumerablet . – xdtTransform Sep 27 '19 at 13:32
Henk also added details of a workaround haha Thank you @xdtTransform - appreciate the help – Sep 27 '19 at 13:33
I prefer the "Why" and the "How to work around" to be different to prevent ppl from closing Why question as duplicate of work around. – xdtTransform Sep 27 '19 at 13:34

1 Answers1