List that this enumerator is bound to has been modified. An

Aman Gupta

11y5.7kVisual Basic .NET

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

i am getting this exception in my code.

my program is for offline exam where admin set the exam name,date&time and all

i want to show the questions related to chapters.

3 Replies

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Vulpes11y ago
Accepted answer
Sure, consider first this program:

using System;
using System.Collections.Generic;

class Program
{
static void Main()
{
var list = new List{1, 2, 3, 4};

// try to remove even numbers from the list using foreach

foreach(int i in list)
{
if (i % 2 == 0) list.Remove(i);
}
}
}

If you try to run it, you get the following exception because you're attempting to modify the list in the foreach loop:

Unhandled Exception: System.InvalidOperationException: Collection was modified;
enumeration operation may not execute.
at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resour
ce)
at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
at Program.Main()

If you now change the program to use an ordinary 'for' loop, there's no exception:

using System;
using System.Collections.Generic;

class Program
{
static void Main()
{
var list = new List{1, 2, 3, 4};

// try to remove even numbers from the list using for

for(int i = 0; i < list.Count; i++)
{
if (list[i] % 2 == 0)
{
list.RemoveAt(i);
--i;
}
}

// check it worked

foreach(int i in list) Console.WriteLine(i);
Console.ReadKey();
}
}

The output, as expected, is:

1
3
Aman Gupta11y ago
Thanks for ur reply :)

Can u plz send me the snippet code to understand me clearly

Thanks in advance :)
Vulpes11y ago
It sounds like you may be using a foreach statement to iterate through a list but are attempting to modify the list within the loop.

If that's the case, then I'd use an 'ordinary' for statement instead of foreach.