Exception for foreach List List Initialize List Remove List RemoveAll

`InvalidOperationException`

This exception has occurred. It reports a "collection was modified" error in the program. We are trying to remove elements from a C# List.

With some code changes, we can fix the problem. It is important to know that the foreach-loop has some restrictions. We cannot change the underlying collection in while foreach is running.

First example

We see a program that can cause this exception. The example demonstrates a List and a foreach-loop that tries to remove an item, but raises an exception.

Start We create a List of 3 integers with a list initializer. We then enter a foreach-loop over the list.

Next The loop iterates over each item in the list. We call the Remove method, and it tries to modify the collection during the loop.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        var list = new List<int>() { 10, 20, 30 };
        // Try to remove an element in a foreach list.
        foreach (int value in list)
        {
            Console.WriteLine("ELEMENT: {0}", value);
            list.Remove(value);
        }
    }
}ELEMENT: 10

Unhandled Exception: System.InvalidOperationException:
   Collection was modified; enumeration operation may not execute.
   at System.ThrowHelper. ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections. Generic.List`1.Enumerator.MoveNextRare()
   at System.Collections. Generic.List`1.Enumerator.MoveNext()
   at Program.Main()...

Example 2

This example removes list elements, without raising an exception. We can call Remove() on the List in a for-loop (with indexes).

Info RemoveAt is another method we could call. It will accept the index of the item we want to remove—it might be faster.

Tip This might be the best way to remove items from a List without causing an exception. RemoveAll() receives a lambda.

Here This program removes all elements in the list where the value is equal to 20.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        var list = new List<int>() { 10, 20, 30 };
        // Remove elements from the list in a for-loop.
        for (int i = 0; i < list.Count; i++)
        {
            int value = list[i];
            // Remove if value is 20.
            if (value == 20)
            {
                list.Remove(value);
            }
        }
        // Write list after element removed.
        foreach (int value in list)
        {
            Console.WriteLine("ELEMENT: {0}", value);
        }
    }
}ELEMENT: 10
ELEMENT: 30

Summary

An InvalidOperationException is not useful. But the "collection was modified" error message can be informative. RemoveAll can be used to safely remove items from a List.

And with a for-loop, we can remove elements by their indexes. RemoveAt can be used. Alternatively, another new list of elements can be created from the elements we wish to keep.