Delete selected parent node and child nodes - Collection was modified; enumeration operation may not execute

Question

I have treeview control with one level of parent and child nodes, each node is having a checkbox for selecting nodes after selection if I click remove button I want to delete child or parent what ever is selected, Im using following code and it returns an error

Code

    protected void btnRemoveOrganisation_Click(object sender, EventArgs e)
    {

        foreach (TreeNode Item in tvwSelectedOrganisations.CheckedNodes)
        {

            if (Item.Parent == null)
            {
                foreach (TreeNode ChildNode in Item.ChildNodes)
                {
                    Item.ChildNodes.Remove(ChildNode);
                }

                tvwSelectedOrganisations.Nodes.Remove(Item);
            }
            else
            {
                Item.Parent.ChildNodes.Remove(Item);
            }

        }

    }

Error

Collection was modified; enumeration operation may not execute.

Modified code

        protected void btnRemoveOrganisation_Click(object sender, EventArgs e)
        {
            TreeNodeCollection SelectedNodes = tvwSelectedOrganisations.CheckedNodes;

            foreach (TreeNode Item in SelectedNodes)
            {

                if (Item.Parent == null)
                {
                    tvwSelectedOrganisations.Nodes.Remove(Item);
                }
                else
                {
                    tvwSelectedOrganisations.FindNode(Item.Parent.ValuePath).ChildNodes.Remove(Item);

                }

                if (SelectedNodes.Count == 0)
                {
                    break;
                }

            }

        }

Solution

int SelectedCount = SelectedNodes.Count;

            for (int i = SelectedCount - 1; i >= 0; i--)
            {

                if (tvwSelectedOrganisations.CheckedNodes[i].Parent == null)
                {
                    int j = tvwSelectedOrganisations.CheckedNodes[i].ChildNodes.Count;
                    tvwSelectedOrganisations.Nodes.Remove(tvwSelectedOrganisations.CheckedNodes[i]);
                    i += j;
                }
                else
                {
                    tvwSelectedOrganisations.FindNode(tvwSelectedOrganisations.CheckedNodes[i].Parent.ValuePath).ShowCheckBox = false;
                    tvwSelectedOrganisations.FindNode(tvwSelectedOrganisations.CheckedNodes[i].Parent.ValuePath).ChildNodes.Remove(tvwSelectedOrganisations.CheckedNodes[i]);

                }

            }

Found the solution, issue was due to the generic list being used, I have updtaed the post with correct code — sudheshna, Sep 14 '11 at 08:53

CharithJ · Accepted Answer · 2011-09-14T04:13:55.030

4

Hope you are using .Net 3.5 or higher.

foreach (TreeNode ChildNode in Item.ChildNodes.ToList())
{
    Item.ChildNodes.Remove(ChildNode);
}

EDIT

If Item.ChildNodes is not Enumerable. Try below then.

for( int i = Item.ChildNodes.Count - 1; i >= 0; i-- )
{
    Item.ChildNodes.Remove(ChildNode);
}

Or

while (Item.ChildNodes.Count > 0)
{
    Item.ChildNodes.Remove(ChildNode);
}

edited Sep 14 '11 at 04:13

answered Sep 13 '11 at 12:50

CharithJ

46,289
20
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im using framework 4, but when I changed the foreach loop, it displayed a build error, "Error 20 'System.Web.UI.WebControls.TreeNodeCollection' does not contain a definition for 'ToList' and no extension method 'ToList' accepting a first argument of type 'System.Web.UI.WebControls.TreeNodeCollection' could be found (are you missing a using directive or an assembly reference?)" – sudheshna Sep 14 '11 at 03:34
Just added more options. – CharithJ Sep 14 '11 at 04:15
Hi Charith, I have tried the for loop and it worked, updated the post with correct solution. Thanks – sudheshna Sep 14 '11 at 08:56
Isn't `.ToList();` part of `System.Linq`? You need to include that if you didn't already, in order for that to work. – qJake Sep 14 '11 at 16:55

score 2 · Answer 2 · answered Sep 13 '11 at 12:50

2

You can't have this line inside the foreach, because you're modifying the collection you're enumerating on:

tvwSelectedOrganisations.Nodes.Remove(Item);

Instead, build a new list of items to remove, then iterate through that list and remove the items outside of (and after) your existing foreach.

answered Sep 13 '11 at 12:50

qJake

16,821
17
83
135

SpikeX and Kalyan, I have tried thisI don't why its removing the same item from my new collection also, I have modified my post to include my new code. Also if I try to delete single node or items in a single node it works, but if select items in multiple nodes it throws same exception – sudheshna Sep 14 '11 at 04:59

score 1 · Answer 3 · answered Sep 13 '11 at 12:51

Definitely, it will throw exception. You are not supposed to delete or remove an item in a collection by looping in the same collection. Instead copy the collection Item.ChildNodes to an empty new Collection, try to loop in original collection and remove what items you want in the new collection, after looping outside the scope of loop again re-assign the modified collection to the original.

Delete selected parent node and child nodes - Collection was modified; enumeration operation may not execute

3 Answers3