CODEWITHSUNDEEP
c#datatable
hikizume
hikizume

Reputation: 528

Problem removing row in datatable while enumerating

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

Upvotes: 6

Views: 6381

Answers (5)

Evren Kuzucuoglu
Evren Kuzucuoglu

Reputation: 3885

Somehow removeJob(id) changes one of the IEnumerables your enumerating (table.Rows or JobsDS.Tables, from the name of the method I guess it would be the latter), maybe via DataBinding.

I'm not sure the backwards for is going to work directly because it seems you're removing an element enumerated in the outer foreach from within the inner foreach. It's hard to tell without more info about what happens in removeJob(id).

Upvotes: 0

Alex Mendez
Alex Mendez

Reputation: 5150

Also, if you depend on the order that you process the rows and a reverse loop does not work for you. You can add the rows that you want to delete to a List and then after you exit the foreach loop you can delete the rows added to the list. For example,

foreach (DataTable table in JobsDS.Tables) 
{ 
  List<DataRow> rowsToRemove = new List<DataRow>();
  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); 
                rowsToRemove.Add(row);
            } 

        } 
    } 
  }
  rowsToRemove.ForEach(r => removeJob(r["ID"].ToString()));
}

Upvotes: 0

LaGrandMere
LaGrandMere

Reputation: 10379

If you want to remove Elements from a loop on a list of Elements, the trick is to use a for loop, start from the last Element and go to the first Element.

In your example :

int t_size = table.Rows.Count -1;

for (int i = t_size; i >= 0; i--)
{
   DataRow row = table.Rows[i];
   // your code ...
}

Edit : not quick enough :)

Upvotes: 3

SLaks
SLaks

Reputation: 887867

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

Instead, you should use a backwards for loop.

Upvotes: 3

Rex M
Rex M

Reputation: 144162

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.

Upvotes: 15

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