Efficiently deleting item from within 'foreach'

Question

For now, the best I could think of is:

bool oneMoreTime = true;
while (oneMoreTime)
{
    ItemType toDelete=null;
    oneMoreTime=false;
    foreach (ItemTyp         


        

Answer 1

You cannot delete from a collection inside a foreach loop (unless it is a very special collection having a special enumerator). The BCL collections will throw exceptions if the collection is modified while it is being enumerated.

You could use a for loop to delete individual elements and adjust the index accordingly. However, doing that can be error prone. Depending on the implementation of the underlying collection it may also be expensive to delete individual elements. For instance deleting the first element of a List will copy all the remaning elements in the list.

The best solution is often to create a new collection based on the old:

var newCollection = collection.Where(item => !ShouldBeDeleted(item)).ToList();

Use ToList() or ToArray() to create the new collection or initialize your specific collection type from the IEnumerable returned by the Where() clause.