Can't i remove element list during foreach loop - list

Question

I want to check the age of person by passing in a delegate type in a method and iterating through list.
If i find that person is a child....I want to remove him from the list...because i already performed a test on him and don't want to perform test again.
But i am getting exception after i am remove element from the list...Exception is that i cannot modify the collection during foreach loop.Is there is any way by which i can achieve this because i don't want to perform operation on same person again when age of person is already decided in last operation.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace DelegateApp
{
/// <summary>
/// A class to define a person
/// </summary>
public class Person
{
public string Name { get; set; }
public int Age { get; set; }
}
class Program
{
//Our delegate
public delegate bool FilterDelegate(Person p);
static void Main(string[] args)
{
//Create 4 Person objects
Person p1 = new Person() { Name = "John", Age = 41 };
Person p2 = new Person() { Name = "Jane", Age = 69 };
Person p3 = new Person() { Name = "Jake", Age = 12 };
Person p4 = new Person() { Name = "Jessie", Age = 25 };
//Create a list of Person objects and fill it
List<Person> people = new List<Person>() { p1, p2, p3, p4 };
DisplayPeople("Children:", people, IsChild);
DisplayPeople("Adults:", people, IsAdult);
DisplayPeople("Seniors:", people, IsSenior);
Console.Read();
}
/// <summary>
/// A method to filter out the people you need
/// </summary>
/// <param name="people">A list of people</param>
/// <param name="filter">A filter</param>
/// <returns>A filtered list</returns>
static void DisplayPeople(string title, List<Person> people, FilterDelegate filter)
{
Console.WriteLine(title);
foreach (Person p in people)
{
if (filter(p))
{
Console.WriteLine("{0}, {1} years old", p.Name, p.Age);
people.Remove(p);
}
}
Console.Write("\n\n");
}
//==========FILTERS===================
static bool IsChild(Person p)
{
return p.Age <= 18;
}
static bool IsAdult(Person p)
{
return p.Age >= 18;
}
static bool IsSenior(Person p)
{
return p.Age >= 65;
}
}
}

Answer 1

Use a for loop instead of a foreach loop. You can't use foreach loops if you're changing the list as you're looping through it. When using the for loop, you should count backwards through your loop so that you're removing elements from the end.
for (int i = people.Count - 1; i >= 0; i--)
{
Person p = people[i];
if (filter(p))
{
Console.WriteLine("{0}, {1} years old", p.Name, p.Age);
people.RemoveAt(i);
}
}
The reason to go backwards is that otherwise you will skip over some elements. For example, suppose you want to remove elements 4 and 5 of your original list. If you count forwards, then when your loop counter is 4, you'll remove element 4. So then element 5 becomes the new element 4, and element 6 becomes the new element 5. Then on the next iteration of your loop, your counter increments to 5. Now this iteration of the loop is operating on your current element 5 (which was originally your element 6). So your original element 5 got skipped over entirely. Counting backwards avoids this situation. If you were going forward through your list, you'd have to decrement your loop counter each time you removed an element and that's more confusing.