Delete inside foreach with linq where

Question

I can see why this is not allowed:

foreach (Thing t in myCollection) {
   if (shouldDelete(t) {
      myCollection.Delete(t);
   }
}

but how about this?

foreach (Thing t in myCollection.Where(o=>shouldDelete(o)) {
   myCollection.Delete(t);
}

I don't understand why this fails. The "Where()" method obviously isn't returning the original collection, so I am not enumerating round the original collection when I try to remove something from it.

Answer 1

Try use this code

myCollection.RemoveAll(x => x.shouldDelete(x));

Answer 2

I don't understand why this fails.

I assume your question then is "why does this fail?" (You forgot to actually ask a question in your question.)

The "Where()" method obviously isn't returning the original collection

Correct. "Where" returns an IEnumerable<T> which represents the collection with a filter put on top of it.

so I am not enumerating round the original collection when I try to remove something from it.

Incorrect. You are enumerating the original collection. You're enumerating the original collection with a filter put on top of it.

When you call "Where" it does not eagerly evaluate the filter and produce a brand new copy of the original collection with the filter applied to it. Rather, it gives you an object which enumerates the original collection, but skips over the items that do not match the filter.

When you're at a store and you say "show me everything", the guy showing you everything shows you everything. When you say "now just show me the apples that are between $1 and $5 a kilogram", you are not constructing an entirely new store that has only apples in it. You're looking at exactly the same collection of stuff as before, just with a filter on it.

Answer 3

You can do:

myCollection.RemoveAll(shouldDelete);