Question

i want to know what is the best way to delete elements from a List in c# while doing a foreach.

here is a code sample. first i create a list with some elements and then delte one:

List<int> foo = new List<int>();
foo.Add(1);
foo.Add(2);
foo.Add(3);
foreach (int i in foo)
{
    if (i==2)
    {
        foo.Remove(i);
    }
}

when i run this, i get a InvalidOperationException but how to solve this with a performant way?

Answer 1

If you must remove entries while enumerating, walk the list in backward direction, and remove items that you need to remove.

for (var i = foo.Count-1 ; i >= 0 ; i--) {
    if (MustBeRemoved(foo[i])) {
        foo.RemoveAt(i);
    }
}

Note that this is not required in case of your post, where you know the values that need to be removed.