C# foreach modifying list. Collection has been modified; the enumeration operation may not be performed

Question

I need help to understando this isssue... In this code:

listaVideoTemp = listaVideo;
foreach (var itemVideo in listaVideo)
{
  if (itemVideo.ID == id)
  {
   listaVideoTemp.RemoveAt(i);                            

   listaServidorTemp = listaServidor;
                            
   foreach (var itemServidor in listaServidor)
   {
     listaVideoServidorTemp = itemServidor.PropListaVideo;

     foreach (var itemVideoServidor in itemServidor.PropListaVideo)
     {
       if (itemVideoServidor == id)
         listaVideoServidorTemp.RemoveAt(x);
       x++;
     }
    }
   }
   i++;
}

When execute listaVideoServidorTemp.RemoveAt(x); the item from itemServidor.PropListaVideo also is removed. Can anyone help me?

Exception: Collection has been modified; the enumeration operation may not be performed.

Thanks.

`listaVideoTemp = listaVideo` does not make a copy, it points both variables to the same reference. — Heretic Monkey, Feb 22 '21 at 22:27
You can't modify a collection while you're iterating through it: `listaVideoTemp.RemoveAt(i);` — Rufus L, Feb 22 '21 at 22:27
Try `listaVideoTemp = listaVideo.ToList();` Note you'll likely want to do `listaVideo = listaVideoTemp;`after the loop. — juharr, Feb 22 '21 at 22:28
C# generic `List` are [reference types](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/reference-types) - in other words, after the assigment, `listaVideoServidorTemp` is the __same object__ as `itemServidor.PropListaVideo` and changes to one effect the other. If you wanted a copy, you needed to do `listaVideoServidorTemp = new List(itemServidor.propListaVideo);`. This seems like an [XY Problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem) - are you trying to count something in `x` and `i` the really hard way? — NetMage, Feb 22 '21 at 22:30
What is the purpose of `listaVideoTemp`? What is the purpose of `listaServidorTemp`? — NetMage, Feb 22 '21 at 22:40

score -2 · Accepted Answer · answered Feb 22 '21 at 22:29

The error is telling you that you can't modify a collection while you're iterating through it. In the following line, listaVideoTemp is a reference to listaVideo, which is the item you're iterating through:

listaVideoTemp.RemoveAt(i);

One way to resolve this would be to create a copy of the list instead:

var listaVideoTemp = listaVideo.ToList();

C# foreach modifying list. Collection has been modified; the enumeration operation may not be performed

1 Answers1