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

Question

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 namedate&time and all i want to show the questions related to chapters.

Vulpes · 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 { 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 Gupta · Answer

Thanks for ur reply :) Can u plz send me the snippet code to understand me clearly Thanks in advance :)

Vulpes · Answer

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.