CODEWITHSUNDEEP
c#windowsvisual-studio-2017
Mich
Mich

Reputation: 3594

C# Copy ListBox.SelectedItems to another variable

I'm getting this error

System.InvalidOperationException: 'List that this enumerator is bound to has been modified. An enumerator can only be used if the list does not change.'

Because in my code, the Selected Items change during iteration

The code is simple

var items = testListBox.SelectedItems;
foreach (var item in items)
{

}

I want to freeze the items list in a variable, so that if it changes, the foreach loop doesn't change and create this error

The problem is the line var items = testListBox.SelectedItems; doesn't seem to work, because for some reason it still gives me this error

How can I clone the SelectedItems list to another variable so it doesn't crash on the foreach loop if the SelectedItems list changes

Thanks,

Upvotes: 1

Views: 529

Answers (2)

Eric J.
Eric J.

Reputation: 150108

Your code creates an alias called items that refers to SelectedItems. It doesn't make a copy.

It seems like an easy fix would be:

var items = testListBox.SelectedItems.ToList();

Alas, the WinForms collections are so old they don't support IEnumerable<T> and so aren't fully supported by Linq. Instead, first cast the selected items to object (or any other appropriate type):

var items = testListBox.SelectedItems.OfType<object>().ToList();

It's not clear what you might be doing in the foreach loop to cause the error you report. You cannot modify the thing you are enumerating. However, you can enumerate over your copy and modify the original (or vice-versa).

Upvotes: 2

Eric Warburton
Eric Warburton

Reputation: 329

You can do various things to fix this, ex:

foreach (var item in items.ToArray())
{
    items.Add(new Item...);
}

Or

for (int i = 0; i < items.Count; i++)
{
    items.Add(new Item...);
}

See What is the best way to modify a list in a 'foreach' loop? for an in depth answer as to why this happens and what the best ways to modify items in a foreach loop are.

Upvotes: 1

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