Looping Through Members of a Collection

by Steve Smith

Using the foreach Loop

In earlier lessons, you learned about while loops and for loops. Although you can use these loop statements to loop through the contents of an array or other collection, the foreach loop statment is designed specifically for this purpose. The foreach loop uses the following syntax:

var myList = new List<string>(){ "one", "two", "three" };
foreach (var item in myList)
{
    Console.WriteLine(item);
}

using System;
using System.Collections.Generic;

namespace ConsoleApplication
{
    public class Program
    {
        public static void Main()
        {
            var myList = new List<string>(){ "one", "two", "three" };
            foreach (var item in myList)
            {
                Console.WriteLine(item);
            }
        }
    }
}

The foreach statement begins with the foreach keyword, followed by an expression in parentheses. This expression includes the declaration of a local variable that will have scope within the loop block (var item). Then, the in keyword is specified, followed by the collection to be iterated over (myList). Like other loop statements, it's acceptable to follow the loop with a single statement (without curly braces), but it's recommended to always use the braces for greater clarity.

Tip
Unlike the for loop in Java, C# does not support iterating over collections in the for loop. You will need to use the foreach loop to achieve this functionality.

You may recall from the previous lesson that the List type includes a similarly-named method, ForEach, that works very similarly to this loop (hence the name). If you do find yourself working with a List and needing to only execute a single statement per item in the list, the ForEach method may be a good choice. However, the foreach loop statement works on other collection types, not justs lists. In fact, you can use the foreach loop on any type that implements IEnumerable (or its generic equivalent, or, more accurately, any type that has a GetEnumerator() method), which includes a wide variety of collection types (and arrays).

For example, to echo back to the user the arguments they passed to a console application, you could use foreach over the args array:

public static void Main(string[] args)
{
    foreach (var arg in args)
    {
        Console.WriteLine(arg);
    }
}

The foreach loop is a very simple way to work with every element in a collection. It doesn't require access to an indexer for the collection, making it applicable to many different collection types. However, in many cases you may want indexer-access to the collection within the loop. In that case, you should consider other techniques for looping over collections.

Tip
If all you need to do is perform some operation on each item in a collection, the foreach loop should be your default choice.

Other looping techniques

The for loop can be used to iterate over an array or list, using its length or count property to determine when the loop should end:

// List<T>
var myList = new List<int>() { 43, 55, 100 };
for (int i = 0; i < myList.Count; i++)
{
    // access current element of the list with index of i
    Console.WriteLine(myList[i]);
}

// array
int[] numbers = new[] { 2, 3, 5, 7 };
for (int i = 0; i < numbers.Length; i++)
{
    // access current element of the array with index of i
    Console.WriteLine(numbers[i]);
}

using System;
using System.Collections.Generic;

namespace ConsoleApplication
{
    public class Program
    {
        public static void Main()
        {
            // List<T>
            var myList = new List<int>() { 43, 55, 100 };
            for (int i = 0; i < myList.Count; i++)
            {
                // access current element of the list with index of i
                Console.WriteLine(myList[i]);
            }

            // array
            int[] numbers = new[] { 2, 3, 5, 7 };
            for (int i = 0; i < numbers.Length; i++)
            {
                // access current element of the array with index of i
                Console.WriteLine(numbers[i]);
            }
        }
    }
}

You can also use a while loop, and your own indexer variable. The example below shows a list but the same approach works with arrays.

var myList = new List<int>() { 10, 20, 30 };
int index = 0;
while (index < myList.Count)
{
    Console.WriteLine(myList[index]);
    index++;
}

using System;
using System.Collections.Generic;

namespace ConsoleApplication
{
    public class Program
    {
        public static void Main()
        {
            var myList = new List<int>() { 10, 20, 30 };
            int index = 0;
            while (index < myList.Count)
            {
                Console.WriteLine(myList[index]);
                index++;
            }
        }
    }
}

Tip
Be careful with this approach. If you forget to increment index, you'll end up with an infinite loop.

Next Steps

Write a program that initializes a list integer numbers, and then prints the numbers out along with their sum. Sample output:

Numbers: 2 4 6
Sum: 12