Problem removing row in datatable while enumerating

Question

I get the following error while I try to delete a row while looping through it.

C#: Collection was modified; enumeration operation may not execute

I've been doing some research for a while, and I've read some similar posts here, but I still haven't found the right answer.

foreach (DataTable table in JobsDS.Tables)
{

  foreach (DataRow row in table.Rows)
  {
    if (row["IP"].ToString() != null && row["IP"].ToString() != "cancelled")
    {
        string newWebServiceUrl = "http://" + row["IP"].ToString() + "/mp/Service.asmx";
        webService.Url = newWebServiceUrl;
        string polledMessage = webService.mpMethod(row["IP"].ToString(), row["ID"].ToString());

        if (polledMessage != null)
        {
            if (polledMessage == "stored")
            {               
                removeJob(id);
            }

        }
    }
}

}

any help would be greatly appreciated

Answer 1

Instead of using foreach, use a reverse for loop:

for(int i = table.Rows.Count - 1; i >= 0; i--)
{
    DataRow row = table.Rows[i];
    //do your stuff
}

Removing the row indeed modifies the original collection of rows. Most enumerators are designed to explode if they detect the source sequence has changed in the middle of an enumeration - rather than try to handle all the weird possibilities of foreaching across something that is changing and probably introduce very subtle bugs, it is safer to simply disallow it.

Answer 2

You cannot modify a collection inside of a foreach around it.

Instead, you should use a backwards for loop.