Trouble removing items in arraylist

Question

Case:

I'm adding objects to my arraylist, before i add the object to my arraylist i check if the same does not exist already. After that I try to rewrite the complete arraylist to the text file.

Code:

WaardeObjecten obj = new WaardeObjecten(index.ToString(), allFiles[index].ToString(), values[0], values[1], values[2], values[3], values[4], values[5], values[6], values[7], values[8], values[9], values[10], values[11], values[12], values[13], values[14], values[15], values[16], values[17]);    

//Check if duplcatie
foreach (WaardeObjecten item in listOfWaardeObjecten)
{
      if (item.getIndex() == obj.getIndex())
      {
               listOfWaardeObjecten.Remove(item);
      }
}

//Add to arraylist
listOfWaardeObjecten.Add(obj);

//writedata to log file
writeLogFile();

Code for writing all arraylist elements to .txt file:

//Creating a streamwriter to write to the file with the path of logFileName.
using (FileStream fs = new FileStream(logFileName, FileMode.Truncate, FileAccess.Write))
using (var sw = new StreamWriter(fs))
{
      foreach (WaardeObjecten item in listOfWaardeObjecten)
      {
           sw.WriteLine(item.getTotalString());
      }
}

Now I'm getting the following error:

"An unhandled exception of type 'System.InvalidOperationException' occurred in mscorlib.dll."

"Additional Information: The collection has been changed. The inventory operation may not be performed."

UPDATE

The code im using now to remove duplicates:

//Check if duplcatie
for (int i = listOfWaardeObjecten.Count - 1; i >= 0; i--)
{
     if (i == obj.getIndex())
     {
           listOfWaardeObjecten.RemoveAt(i);
     }
}

//Add to arraylist
listOfWaardeObjecten.Add(obj);

//writedata to log file
writeLogFile();

Code im using to write textfile:

//Creating a streamwriter to write to the file with the path of logFileName.
using (FileStream fs = new FileStream(logFileName, FileMode.Truncate, FileAccess.Write))
using (var sw = new StreamWriter(fs))
{
     foreach (WaardeObjecten item in listOfWaardeObjecten)
     {
          sw.WriteLine(item.getTotalString());
     }
}

Output if i first do 10-15 images with all different values and i rewind to the first image and put all values on "N.v.t.":

 2. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\A-1101-OSBL\99_History\fotos\A 1101 laadarm04.JPG : Discipline: Preset1 | Soort: Preset1 | Afdruk: Preset1 | Test1: Preset1 | Test2: Preset1 | Test3: Preset1 | Test4: Preset1 | Test5: Preset1 | Test6: Preset1.
 4. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\A-1101-OSBL\99_History\fotos\A 1101 laadarm06.JPG : Discipline: Preset1 | Soort: Preset1 | Afdruk: Preset1 | Test1: Preset1 | Test2: Preset1 | Test3: Preset1 | Test4: Preset1 | Test5: Preset1 | Test6: Preset1.
 6. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\A-1101-OSBL\99_History\fotos\A 1101 laadarm08.JPG : Discipline: Preset1 | Soort: Preset1 | Afdruk: Preset1 | Test1: Preset1 | Test2: Preset1 | Test3: Preset1 | Test4: Preset1 | Test5: Preset1 | Test6: Preset1.
 8. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\A-1101-OSBL\99_History\fotos\Detail tbv blisemafleiding A-1101.jpg : Discipline: Preset1 | Soort: Preset1 | Afdruk: Preset1 | Test1: Preset1 | Test2: Preset1 | Test3: Preset1 | Test4: Preset1 | Test5: Preset1 | Test6: Preset1.
 10. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\A-1101-OSBL\99_History\fotos\IMG-20131108-01010.jpg : Discipline: Preset1 | Soort: Preset1 | Afdruk: Preset1 | Test1: Preset1 | Test2: Preset1 | Test3: Preset1 | Test4: Preset1 | Test5: Preset1 | Test6: Preset1.
 1. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\A-1101-OSBL\99_History\fotos\A 1101 laadarm02.JPG : Discipline: N.v.t. | Soort: N.v.t. | Afdruk: N.v.t. | Test1: N.v.t. | Test2: N.v.t. | Test3: N.v.t. | Test4: N.v.t. | Test5: N.v.t. | Test6: N.v.t..
  3. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\A-1101-OSBL\99_History\fotos\A 1101 laadarm05.JPG : Discipline: N.v.t. | Soort: N.v.t. | Afdruk: N.v.t. | Test1: N.v.t. | Test2: N.v.t. | Test3: N.v.t. | Test4: N.v.t. | Test5: N.v.t. | Test6: N.v.t..
  5. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\A-1101-OSBL\99_History\fotos\A 1101 laadarm07.JPG : Discipline: N.v.t. | Soort: N.v.t. | Afdruk: N.v.t. | Test1: N.v.t. | Test2: N.v.t. | Test3: N.v.t. | Test4: N.v.t. | Test5: N.v.t. | Test6: N.v.t..
  7. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\A-1101-OSBL\99_History\fotos\beschadigde isolatie.jpg : Discipline: N.v.t. | Soort: N.v.t. | Afdruk: N.v.t. | Test1: N.v.t. | Test2: N.v.t. | Test3: N.v.t. | Test4: N.v.t. | Test5: N.v.t. | Test6: N.v.t..
  9. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\A-1101-OSBL\99_History\fotos\Foto tbv bliksemafleiding 1.jpg : Discipline: N.v.t. | Soort: N.v.t. | Afdruk: N.v.t. | Test1: N.v.t. | Test2: N.v.t. | Test3: N.v.t. | Test4: N.v.t. | Test5: N.v.t. | Test6: N.v.t..
  11. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\C-1106-final\99_History\Inspecties\DSC00864.JPG : Discipline: N.v.t. | Soort: N.v.t. | Afdruk: N.v.t. | Test1: N.v.t. | Test2: N.v.t. | Test3: N.v.t. | Test4: N.v.t. | Test5: N.v.t. | Test6: N.v.t..
  12. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\C-1107-fnal\99_History\Inspecties\DSC00864.JPG : Discipline: N.v.t. | Soort: N.v.t. | Afdruk: N.v.t. | Test1: N.v.t. | Test2: N.v.t. | Test3: N.v.t. | Test4: N.v.t. | Test5: N.v.t. | Test6: N.v.t..
  13. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\H-1104-final\99_History\Inspecties\100_0671.JPG : Discipline: N.v.t. | Soort: N.v.t. | Afdruk: N.v.t. | Test1: N.v.t. | Test2: N.v.t. | Test3: N.v.t. | Test4: N.v.t. | Test5: N.v.t. | Test6: N.v.t..
  14. C:\Users\Gebruiker\Desktop\MCA JPG2\05_Mechanical\01_Equipment\1100-final\P-1111-final\99_History\Onderhouds rapportages\2014 wk50  div schades\P-1111 eerste schadepomp  2.JPG : Discipline: N.v.t. | Soort: N.v.t. | Afdruk: N.v.t. | Test1: N.v.t. | Test2: N.v.t. | Test3: N.v.t. | Test4: N.v.t. | Test5: N.v.t. | Test6: N.v.t..

As you can see not all the values are updates to "N.v.t.".

Hope anyone can help!

You can't modify a collection that you're iterating over with foreach. Try a for loop and iterate backwards. — Equalsk
– Equalsk, Commented Oct 24, 2017 at 11:18
So i have to use a for loop instead of for each? and remove at (i)? How do i call the getIndex() function of that object aswell? — Niels
– Niels, Commented Oct 24, 2017 at 11:19
Yes, that should work, and start at the end and work backwards with i--. — Equalsk
– Equalsk, Commented Oct 24, 2017 at 11:20
listOfWaarObjecten[2] returns a WaardeObject, which has no knowledge of it's index. It really shouldn't be necessary, since you already know that the index (i) is 2. — ivospijker
– ivospijker, Commented Oct 24, 2017 at 11:24

Amit Kumar · Accepted Answer · 2017-10-24 11:29:29Z

5

You can not modify/ remove item from collection when using foreach loop on collections. But if you want to remove item from collection then use for loop instead.

for (int i = listOfWaardeObjecten.Count-1; i >= 0; i--)
{
   if (i == obj.getIndex())
      {
         listOfWaardeObjecten.RemoveAt(i);
      }
}

In case if you don't want to use for loop then, you can use break statement with foreach loop. See here.

edited Oct 24, 2017 at 11:29

answered Oct 24, 2017 at 11:20

Amit Kumar

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14 Comments

ivospijker Over a year ago

As stated by Equalsk above, you will need to iterate counting down over the collection, since the index numbers will change after a deletion. So use for (int i = lostOfWaardeObjecten.Count - 1; i >= 0; i--) { ... }

Niels Over a year ago

For some reason the code seems fine but its not working in my program yet, can i update the post? or do i need to make a new one? Cant see why it is not working.

Niels Over a year ago

I've tried 5 images, and with all 5 images(duplicates) it went into the IF-condition.

Niels Over a year ago

It says i can not use .Where in an arraylist.

Niels Over a year ago

I can switch it up so list, however i never used list, give me a second.

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