Remove items from IEnumerable<T>

c# linq ienumerable
Maxim picture Maxim · Jul 6, 2010 · Viewed 85k times · Source

I have 2 IEnumerable collections.

IEnumerable<MyClass> objectsToExcept

and 

IEnumerable<MyClass> allObjects.

objectsToExcept may contain objects from allObjects.

I need to remove from allObjects objects in objectsToExcept. For example:

foreach (var myClass in objectsToExcept)
{
allObjects.Remove(myClass);
}

Or

allObject.Except(objectsToExcept)

But it doesn't work. The count after the methods have execute indicate that no items have been removed.

Answer

Jon Skeet picture Jon Skeet · Jul 6, 2010

I don't see how the first version would compile, and the second version won't do anything unless you use the result. It doesn't remove anything from the existing collection - indeed, there may not even be an in-memory collection backing it. It just returns a sequence which, when iterated over, will return the appropriate values.

If you are using the result, e.g.

IEnumerable<MyClass> others = allObjects.Except(objectsToExcept);
foreach (MyClass x in others)
{
    ...
}

then it should be fine if you've overridden GetHashCode and Equals or if you're happy to use reference equality. Are you trying to remove logically-equal values, or do the same references occur in both sequences? Have you overridden GetHashCode and Equals, and if so, are you sure those implementations work?

Basically it should be fine - I suggest you try to create a short but complete program that demonstrates the problem; I suspect that while doing so, you'll find out what's wrong.

Related questions

IEnumerable vs List - What to Use? How do they work?

I have some doubts over how Enumerators work, and LINQ. Consider these two simple selects: List<Animal> sel = (from animal in Animals join race in Species on animal.SpeciesKey equals race.SpeciesKey select animal).Distinct().ToList(); or IEnumerable&…

Read More
Returning IEnumerable<T> vs. IQueryable<T>

What is the difference between returning IQueryable<T> vs. IEnumerable<T>, when should one be preferred over the other? IQueryable<Customer> custs = from c in db.Customers where c.City == "<City>" select …

Read More
How to check if IEnumerable is null or empty?

I love string.IsNullOrEmpty method. I'd love to have something that would allow the same functionality for IEnumerable. Is there such? Maybe some collection helper class? The reason I am asking is that in if statements the code looks cluttered …

Read More