How to use foreach loop with changing its range in C#

Question

An exception gets thrown because the ClientStack.Children range changes after a ClientStack.Children member (textblock) gets removed:

foreach (TextBlock textblock in ClientStack.Children)
{
    if ((string)textblock.ToolTip == e.IP)
    {
        ClientStack.Children.Remove(textblock);
        // update foreach loop's range???
    }
}

Is there a way to update this foreach loop's range after a ClientStack.Children member gets removed?

@TimSchmelter ClientStack is a StackPanel and ClientStack.Children are its contents, in my case TextBlocks — demented, Jul 27 '21 at 19:26
not supported as far as I know. You can either iterate by index, or make a list of items to remove and then do the actual removal after the loop. — Garr Godfrey, Jul 27 '21 at 19:30

Tim Schmelter · Answer 1 · 2021-07-27T19:43:21.863

1

You cannot modify a collection while you are enumerating it, so you get that exception.

You could use a backwards for-loop:

for(int i = ClientStack.Children.Count - 1; i >= 0; i--)
{
    if (((string)ClientStack.Children[i]).ToolTip == e.IP)
    {
        ClientStack.Children.RemoveAt(i);
    }
}

Another - little bit less efficient - way was to use a remove-collection:

var removeBlocks = ClientStack.Children.Cast<object>()
    .Where(textblock => ((string)textblock).ToolTip == e.IP)
    .ToList();
removeBlocks.ForEach(ClientStack.Children.Remove);

edited Jul 27 '21 at 19:43

answered Jul 27 '21 at 19:31

Tim Schmelter

429,027
67
649
891

1

I think its `RemoveAt` when removing index based. – CSharpie Jul 27 '21 at 19:34
@CSharpie: Yes, thanks,fixed – Tim Schmelter Jul 27 '21 at 19:37
It is always better to copy-paste code from highly upvoted answers like https://stackoverflow.com/a/1582317 to avoid risking incorrect own implementation... :) – Alexei Levenkov Jul 27 '21 at 19:38
reverse for+RemoveAt is the way to go for everything implementing IList. – lidqy Jul 27 '21 at 19:41

score 0 · Answer 2 · answered Jul 27 '21 at 19:30

There is no way to update the range while iterating. Instead make a copy of the collection and iterate over that, but remove from the original collection.

foreach (TextBlock textblock in ClientStack.Children.ToList())
{
    if ((string)textblock.ToolTip == e.IP)
    {
        ClientStack.Children.Remove(textblock);
    }
}

How to use foreach loop with changing its range in C#

2 Answers2