How to remove an item in a Generic List inside a foreach loop

Question

_{Friday, April 30, 2010 2:49 PM | 1 vote}

When using a for loop it's easy to figure out what index to shove into the RemoveAt method on a generic list.  But I'm not sure how to do this in a foreach so easily:

            foreach (Item item in Items)
            {

                if(item == null)
                    // remove it from the Items generic list

                ....

             }

C# Web Developer

All replies (8)

_{Friday, April 30, 2010 2:53 PM ✅Answered | 4 votes}

It's not possible. The underlying collection cannot be modified while it's being enumerated.

The standard approach is to keep track of the items to remove (e.g., in another list), and then after Items has been enumerated, enumerate the removelist, removing each item from Items.

        -Steve

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_{Friday, April 30, 2010 2:56 PM | 1 vote}

For Each loops are read only. You cannot alter the content of the target list in a For Each loop. Use a standard For loop.

_{Friday, April 30, 2010 3:11 PM | 14 votes}

There are actually several ways to do this:

1) You can use a for/next loop instead a for/each.

2) You can iterate through a separate list:

      List<Customer> custList = Customer.Populate();

      foreach (var cust in custList.ToList())
      {
        custList.Remove(cust);
      }

Notice the ToList on the list variable. So this iterates through the list created by the ToList but removes the items from the original list.

Hope this helps.

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_{Tuesday, January 21, 2014 10:48 AM | 4 votes}

Can't you just use linq to do this for you, ie:

Items.RemoveAll(item => item == null);

Then your above example would be:

var Items = new List<string>{"A","b",null,"c"};
foreach (string item in Items)
{
    if(item == null)
        Debug.WriteLine("delete me");
}

Items.RemoveAll(item => item == null);

foreach (string item in Items)
{
    Debug.WriteLine(item);
}

_{Sunday, February 23, 2014 12:21 AM | 1 vote}

Hello

A reverse 'forr' can do the trick.

_{Wednesday, March 19, 2014 7:14 PM}

Brilliant!

_{Wednesday, March 19, 2014 8:51 PM | 2 votes}

Typically this is done with a For loop and you will usually do the for loop in starting with the last element working your way to the 0 element and removing items along the way.

While there are other examples shown here some of them are inefficient.

        static void Main(string[] args)
        {
            var s = new List<string>() { "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z" };

            for (int i = s.Count -1; i >= 0; i--)
            {
                if ((int) Convert.ToChar(s[i])%2 == 1)
                {
                    s.RemoveAt(i);
                }
            }

            Console.ReadKey();
        }

_{Wednesday, March 19, 2014 10:04 PM}

items = items.Where(item => item!=null);

Paul Linton