How to solve the " index was out of range. Must be non-negative and less than the size of the collection." in ASP .NET using C#

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my code is as follows
i want that i select some items from the list box and the selected items are automatically add on to another list box and remove from the first list box .
in asp.net using c#.

What I have tried:

protected void ListBox3_SelectedIndexChanged(object sender, EventArgs e)
{

}
protected void ListBox4_SelectedIndexChanged(object sender, EventArgs e)
{

}
protected void Button6_Click(object sender, EventArgs e)
{
int i;
for (i = 0; i <= ListBox3.Items.Count; i++)
{
if (ListBox3.Items[i].Selected == true)
{
ListBox4.Items.Add(ListBox3.SelectedItem);
ListBox3.Items.Remove(ListBox3.SelectedItem);
}
}

}
}

Posted 23-Jan-18 1:54am

riya rawat

Updated 23-Jan-18 2:02am

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F-ES Sitecore 23-Jan-18 8:06am

You're looping through the number of items in ListBox3 (let's say there are 5) and you arte removing them as they go (let's say you remove 2 so there are 3 left), when you hit i being three it looks for the fourth item but there are only 3. There used to be 5 but you've removed them.

Break the loop when you find a selected item and u use a "while" loop or some other mechanism that loops until nothing in the listbox is selected. An alternative is to add the selected items to a new list and once you've looped through them the listbox, loop through the new list you've made and remove\add in that loop.

Richard Deeming 23-Jan-18 10:54am

Probably simpler just to reverse the loop:

for (int i = ListBox3.Items.Count - 1; i >= 0; i--) { ...

I'd assume, since the condition is tested each time, and the code isn't caching the count anywhere, that it wouldn't try to iterate past the end of the list. (So long as the termination condition has been fixed, as Jochen spotted.)

But iterating forwards will still end up skipping items.

F-ES Sitecore 23-Jan-18 11:35am

Given he is adding the items to another control I'd assume he'd want to preserve the order of items, if so he'd need to insert at the front of the collection too, but if it was the second time the code has run that might also have undesirable results. Things always seem so simple in your mind before you start to actually code :)

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Jochen Arndt · Answer 1 · 2018-01-23T02:02:00

Solution 1

The hint is in the "less than size of the collection" part of the error message because you are iterating up to and including that size:

C#

for (i = 0; i <= ListBox3.Items.Count; i++)

It must be

C#

for (i = 0; i < ListBox3.Items.Count; i++)

[EDIT]
As noted by Richard, your loop might skip items and you should iterating down starting at the last item.
[/EDIT]

Posted 23-Jan-18 2:02am

Jochen Arndt

Updated 23-Jan-18 5:04am

v2

Comments

Richard Deeming 23-Jan-18 10:55am

That's still going to end up skipping items. :)

Jochen Arndt 23-Jan-18 11:02am

Yes. I recognised that too meanhwile.

But the question was "how to solve the index out of range error" :)