Collection was modified IGrouping of Invoices

Question

I have a List<IGrouping<string, InvoiceCount>> to check positions in a invoice. It works great but when a Position is already in another Invoice the position should be removed but it will come the exception

"Collection was modified; enumeration operation may not execute."

and I have no idea to solve this

 private static void ValidateInvoices(List<IGrouping<string, InvoiceCount>> invo)
        {
            var validationErrors = new StringBuilder();
            foreach (IGrouping<string, InvoiceCount> invoice in invo)
            {
                for (var line = 1; line < invoice.Count(); line++)
                {
                    if (!invoice.ElementAt(line).IsInSameInvoice(invoice.First()))
                    {
                        validationErrors.Append("Validation error in Invoice" + invoice.Key + " in line " + (line + 1) + Environment.NewLine);
                        if (invo.Contains(invoice))
                            try
                            {
                                invo.Remove(invoice);
                            }
                            catch (Exception ex)
                            {
                                throw ex;
                            }
                    }
                }
            }
            if (validationErrors.Length > 0)
                throw new Exception(validationErrors.ToString());
        }

Answer 1

You need to refresh the collection after removing element. Try to change your code like below and see if it works:

Code:

var newList = invo;
foreach (IGrouping<string, InvoiceCount> invoice in invo)
{
       for (var line = 1; line < invoice.Count(); line++)                
       {
           if (YOUR CONDITION HERE)
           {
              newList.Remove(invoice );
              invo= newList;
           }
       }
}

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Note: How to modify a collection during iteration