C# Difference between Foreach and for (not performance)

Question

Some time ago I read that foreach works with "copies" of objects and thus it can be used for information retrieval instead of its updating. I do not get it as it is entirely possible to loop through list of classes and change its field. Thanks!

Answer 1

What you may have read is that you can't modify a collection while iterating over it using foreach whereas you can (if you're careful) using a for loop. For example:

using System;
using System.Collections.Generic;

class Test
{
    static void Main()
    {
        var list = new List<int> { 1, 4, 5, 6, 9, 10 };


        /* This version fails with an InvalidOperationException
        foreach (int x in list)
        {
            if (x < 5)
            {
                list.Add(100);
            }
            Console.WriteLine(x);
        }
         */

        // This version is okay
        for (int i = 0; i < list.Count; i++)
        {
            int x = list[i];
            if (x < 5)
            {
                list.Add(100);
            }
            Console.WriteLine(x);            
        }
    }
}

If that's not what you were referring to, please give more details - it's hard to explain what you've read without knowing exactly what it said.

Answer 2

You cannot modify the element in a foreach:

var list = new List<string>();
list.AddRange(new string[] { "A", "B", "C" });

foreach (var i in list)
{
    // compilation error: Cannot assign 'i' because it is a 'foreach iteration variable'
    i = "X";
}

Although when working with for you are accessing the element on the list with its index, and not the iterator, so this way you can modify the collection.