Question

I have the below code. I am trying to remove the record and it is throwing Exception when it is removing the Record.
“Collection was modified; enumeration operation may not execute.”

Any ideas on how to get rid of the message. Appreciate your time.

//validClaimControlNo has valid ClaimControl Numbers.
List<string> validClaimControlNo = new List<string>();

int count = 0;
foreach (List<Field> f in records)
{
    foreach (Field fe in f)
    {
        if (i == 0)
            if (!(validClaimControlNo.Contains(fe.Value)))
            {
                //if this claim is not in the Valid list, Remove that Record
                records.RemoveAt(count);
            }
        i++;
    }
    i = 0;
    count++;
}

Answer 1

You cant remove items from a collection you are iterating. Adding .ToList() will create a new list and thus make it work.

 foreach (List<Field> f in records.ToList())

An alternative is to iterate the collection backwards (and you wont need the additional list):

for(int i = records.Count - 1; i >= 0; i--)
{
   var f = records[i];

But looking at your code it can be simplified great deal:

//Put the claim numbers into a set for fast lookup
var set = new HashSet<string>(validClaimControlNo);

//Remove undesired items
records.RemoveAll(f => f.Count > 0 && !set.Contains(f[0].Value));