Foreach

Programs spend most of their time in loops. In C#, we find the foreach-loop, which has special features, and clearer syntax.

With this loop, we evaluate each element individually—an index is not needed. With no indexes, loops are simpler. This makes programs easier to read and understand.

A foreach-loop

Foreach accesses each element in order—this is called enumeration. We eliminate errors caused by incorrect index handling.

using System;

// Step 1: create an array of 3 strings.
string[] pets = { "dog", "cat", "bird" };

// Step 2: loop with the foreach keyword.
foreach (string value in pets)
{
    // Step 3: access the enumeration variable.
    Console.WriteLine("FOREACH ITEM: " + value);
}
FOREACH ITEM: dog
FOREACH ITEM: cat
FOREACH ITEM: bird

Reverse

To use a reverse foreach-loop, we can call Reverse(). We can then use foreach over the reversed results. Reverse() is an extension method—make sure to include System.Linq.

using System;
using System.Linq;

// An example array.
string[] array =
{
    "bird",
    "frog",
    "dog",
    "snake"
};
// Use the Reverse generic extension method.
foreach (string value in array.Reverse())
{
    Console.WriteLine("REVERSE VALUE: " + value);
}
REVERSE VALUE: snake
REVERSE VALUE: dog
REVERSE VALUE: frog
REVERSE VALUE: bird

Query

This example uses a foreach-loop to evaluate a LINQ expression. The expression sorts an array. Foreach causes a query to be evaluated.

using System;
using System.Linq;

// Step 1: an unsorted string array.
string[] letters = { "d", "c", "a", "b" };

// Step 2: use query to sort the array alphabetically.
var sorted = from letter in letters
             orderby letter
             select letter;

// Step 3: loop with the foreach keyword.
foreach (string value in sorted)
{
    Console.WriteLine(value);
}
a
b
c
d

Yield

This feature allows the position in the control flow to be remembered and resumed when the statement finishes. It is implemented in terms of simpler constructs.

using System;
using System.Collections.Generic;

public class Program
{
    static void Main()
    {
        foreach (int value in X())
        {
            Console.WriteLine(value);
            // Break at value 10.
            if (value == 10)
            {
                break;
            }
        }
    }

    static IEnumerable<int> X()
    {
        // Return infinite numbers.
        int i = 0;
        while (true)
        {
            i++;
            yield return i;
        }
    }
}
1
2
3
4
5
6
7
8
9
10

Var, implicit types

The var keyword can simplify our loop syntax. The example enumerates a Dictionary. The character count in "KeyValuePair" can be reduced.

using System;
using System.Collections.Generic;

// Add items to Dictionary.
var data = new Dictionary<int, int>();
data.Add(1, 2);
data.Add(2, 3);
data.Add(3, 4);

// Use var in foreach-loop.
foreach (var pair in data)
{
    Console.WriteLine("KEY, VALUE: {0}, {1}", pair.Key, pair.Value);
}
KEY, VALUE: 1, 2
KEY, VALUE: 2, 3
KEY, VALUE: 3, 4

List exception

The foreach loop has a drawback. It restricts any mutations made to the collection during the loop. This can impact code that uses Lists.

using System;
using System.Collections.Generic;

List<int> list = new List<int>();
list.Add(1);
list.Add(2);
list.Add(3);
// Loop over list elements using foreach-loop.
foreach (int element in list)
{
    Console.WriteLine(element);
}
// You can't remove elements in a foreach-loop.
try
{
    foreach (int element in list)
    {
        list.Remove(element);
    }
}
catch (Exception ex)
{
    Console.WriteLine(ex.Message);
}
1
2
3
Collection was modified; enumeration operation may not execute.

Loop comparison

Let's compare loops. For has more complexity in its syntax. This gives it more power if you want to modify the collection or examine adjacent elements.

using System;

// Array of color strings.
string[] colors = { "red", "orange", "yellow", "green" };

// Print all colors with for-loop.
Console.WriteLine(":::FOR:::");
for (int i = 0; i < colors.Length; i++)
{
    // Assign string reference based on induction variable.
    string value = colors[i];
    Console.WriteLine(value);
}

// Print all colors with foreach.
Console.WriteLine(":::FOREACH:::");
foreach (var value in colors)
{
    Console.WriteLine(value);
}
:::FOR:::
red
orange
yellow
green
:::FOREACH:::
red
orange
yellow
green

Error, cannot assign

We cannot assign the variable we use in a foreach-loop. To fix this error, we can add another local inside the loop—that local can be reassigned as much as we want.

class Program
{
    static void Main()
    {
        foreach (int value in new int[] { 1, 2, 3 })
        {
            value = 20;
        }
    }
}
Error CS1656
Cannot assign to 'value' because it is a 'foreach iteration variable'

Error, cannot operate

We cannot use a foreach statement on many kinds of variables such as tuples. An implementation of GetEnumerator (or IEnumerable) is always required.

using System;

class Program
{
    static void Main()
    {
        var tuple = new Tuple<int, int>(0, 0);
        // Cannot use foreach on tuple.
        foreach (int value in tuple)
        {
        }
    }
}
Error CS1579
foreach statement cannot operate on variables of type 'Tuple<int, int>'
because 'Tuple<int, int>' does not contain a public instance definition
for 'GetEnumerator'

Error, cannot convert type

When we specify the variable type in foreach, there must exist a conversion to it. Here the string "test" returns chars.

class Program
{
    static void Main()
    {
        string test = "bird";
        // The foreach variable must have a conversion.
        foreach (string value in test)
        {
        }
    }
}
Error CS0030
Cannot convert type 'char' to 'string'

Benchmark, local array

How can we improve the iteration performance of a foreach-loop? The foreach-loop is often less efficient than a simple for-loop.

using System;
using System.Diagnostics;

class Program
{
    int[] _values = { 1, 2, 3 };

    int Method1()
    {
        // Access the field directly in the foreach expression.
        int result = 0;
        foreach (int value in this._values)
        {
            result += value;
        }
        return result;
    }

    int Method2()
    {
        // Store the field into a local variable and then iterate.
        int result = 0;
        var values = this._values;
        foreach (int value in values)
        {
            result += value;
        }
        return result;
    }

    const int _max = 100000000;
    static void Main()
    {
        Program program = new Program();
        // Version 1: use foreach-loop on field.
        var s1 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            program.Method1();
            program.Method1();
        }
        s1.Stop();
        // Version 2: use foreach-loop on local.
        var s2 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            program.Method2();
            program.Method2();
        }
        s2.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000 * 1000) / _max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000 * 1000) / _max).ToString("0.00 ns"));
    }
}
6.77 ns: foreach, field
6.72 ns: foreach, local variable

Foreach is a powerful and elegant loop. It uses the "in" keyword and reads like a sentence—and reduces possible bugs. It handles arrays and works well with queries.