How to Remove Item Dynamically from List in Foreach Loop

Question

The code throws an error with reason as we easily see.

Error:{"Collection was modified; enumeration operation may not execute."}

But there must be a way to do that, with foreach (not for), with using List<>. Please do not tell me Deep Copy methods like making

var cloneList = NumberList.ToList()

and removing item from it. Because if we have 1 million elements, we will make it 2 million for removing etc. I don't need this.

The code snippet is here;

public class HomeController : Controller
{
    // GET: Home
    public ActionResult Index()
    {
        var numberList = new List<int>();
        numberList.AddRange(new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9 });
        var totalNumber = 0;
        foreach (var item in numberList)
        {
            totalNumber += item;
            if (item == 5)
            {
                numberList.Remove(item);
            }
        }
        return View(totalNumber);
    }
}

Please help me about this brain burner code snippet..

Thank you.

UPDATE : The question is not about getting Sum() of the List elements at the final point. The core of question is, how to remove item from list while foreach looping.

Answer 1

You are modifieng your collection inside the foreach loop so in next iteration the collection element number change and foreach fail to iterate, try this

public ActionResult Index()
{
    var numberList = new List<int>();
    numberList.AddRange(new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9 });
    var totalNumber = 0;
    var numberListTemp = numberList;
    foreach (var item in umberListTemp)
    {
        totalNumber += item;
        if (item == 5)
        {
            numberList.Remove(item);
        }
    }
    return View(totalNumber);
}

Answer 2

First calculate the totals and then remove the items:

var totals = numberList.Sum();  
numberList.RemoveAll(i=>i==5);

Answer 3

If the double iteration and deep copy is the problem here is my approach:

        ...
        var numberList = new List<int>();
        numberList.AddRange(new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9 });
        var totalNumber = 0;
        foreach (var item in YieldingNotFive(numberList))
        {
            totalNumber += item;
        }
        ...

    private IEnumerable<int> YieldingNotFive(List<int> numberList)
    {
        foreach (int item in numberList)
        {
            if (item != 5)
            {
                yield return item;
            }
        }
        yield break;
    }

or

    var numberList = new List<int>();
    numberList.AddRange(new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9 });
    var totalNumber = 0;
    foreach (var item in numberList.Where(x => x != 5))
    {
        totalNumber += item;
    }

Answer 4

Try to add a counter for a loop and when arrived in your condition remove the element.

public class HomeController : Controller
{
    // GET: Home
    public ActionResult Index()
    {
        var numberList = new List<int>();
        numberList.AddRange(new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9 });
        var totalNumber = 0;
        var count = 0;
        foreach (var item in numberList)
        {
            totalNumber += item;
            if (item == 5)
            {
                numberList.Remove(count);
            }
            count = count + 1;
        }
        return View(totalNumber);
    }
}

But i don't know the name of the functions for remove from index.

Answer 5

I found the answer. From the below you can see the performance detail between tradinional foreach and Parallel.ForEach.

I hope this will be useful.

class Program
{
    static void Main(string[] args)
    {           
        List<string> colorList = new List<string>();
        colorList.AddRange(new List<string>() {
                                  "1. Red",
                                  "2. Green",
                                  "3. Blue",
                                  "4. Yellow",
                                  "5. White",
                                  "6. Black",
                                  "7. Violet",
                                  "8. Brown",
                                  "9. Orange",
                                  "10. Pink"
        });
        Console.WriteLine("Traditional foreach loop\n");
        //start the stopwatch for "for" loop
        var sw = Stopwatch.StartNew();
        foreach (string color in colorList)
        {
            Console.WriteLine("{0}, Thread Id= {1}", color, Thread.CurrentThread.ManagedThreadId);
            Thread.Sleep(10);
        }
        Console.WriteLine("Foreach loop execution time = {0} seconds\n", sw.Elapsed.TotalSeconds);

        //-------------------------------------------------------------------------------------------------------------------------------------------------------

        Console.WriteLine("Using Parallel.ForEach");
        //start the stopwatch for "Parallel.ForEach"
        sw = Stopwatch.StartNew();
        int index = 0;
        Parallel.ForEach(colorList, (color, state) =>
        {

            ++index;
            if (index > colorList.Count - 1)
            {
                state.Stop();
            }

            Console.WriteLine("{0}, Thread Id= {1}", color, Thread.CurrentThread.ManagedThreadId);
            if (color == "2. Green")
            {
                colorList.RemoveRange(3, 2);
            }
            Thread.Sleep(10);
        }
        );
        Console.WriteLine("Parallel.ForEach() execution time = {0} seconds", sw.Elapsed.TotalSeconds);
        Console.Read();
    }
}