Query example. Here we use IEnumerable as the return value of a query expression. We use it with foreach-loops, and also call extension methods on it.
Part 1 An IEnumerable is returned from a query expression. A query that selects ints will be of type IEnumerable int.
Part 2 On an IEnumerable variable, we can use the foreach-loop. This loop iterates with simple syntax.
using System;
using System.Collections.Generic;
using System.Linq;
// Part 1: query expression.
IEnumerable<int> result = from value in Enumerable.Range(0, 5)
select value;
// Part 2: loop over IEnumerable.
foreach (int value in result)
{
Console.WriteLine($"IENUMERABLE: {value}");
}IENUMERABLE: 0
IENUMERABLE: 1
IENUMERABLE: 2
IENUMERABLE: 3
IENUMERABLE: 4
Extension methods. In System.Linq, many extension methods that act upon IEnumerable are available. These are used in many C# programs.
Part 1 Here we see the Average extension, which computes the average value in an IEnumerable (like an int array).
Part 2 We can apply many transformations to an IEnumerable instance, including the ToList and ToArray conversions.
using System;
using System.Collections.Generic;
using System.Linq;
int[] values = new int[] { 2, 3, 1 };
// Part 1: use Average extension method on IEnumerable.
double average = values.Average();
Console.WriteLine($"AVERAGE: {average}");
// Part 2: convert IEnumerable with ToList extension.
List<int> list = values.ToList();
Console.WriteLine($"TOLIST: {list.Count}");AVERAGE: 2
TOLIST: 3
Arguments. Many classes implement IEnumerable. We can pass them directly to methods that receive IEnumerable arguments. The type parameter must be the same.
Next Display() receives an IEnumerable argument. We can pass Lists or arrays to it.
Also We can implement IEnumerable on a type to provide support for the foreach-loop. This is done through the GetEnumerator method.
using System;
using System.Collections.Generic;
class Program
{
static void Main()
{
Display(new List<bool> { true, false, true });
}
static void Display(IEnumerable<bool> argument)
{
foreach (bool value in argument)
{
Console.WriteLine(value);
}
}
}True
False
True
2D array. IEnumerable works with a 2D array. It enables a foreach-loop over the values in a 2D or jagged array. We create a custom method that returns IEnumerable.
using System;
using System.Collections.Generic;
class Program
{
static int[,] _grid = new int[15, 15];
static void Main()
{
// Initialize some elements in 2D array.
_grid[0, 1] = 4;
_grid[4, 4] = 5;
_grid[14, 2] = 3;
// Sum values in 2D array.
int sum = 0;
foreach (int value in GridValues())
{
sum += value;
}
// Write result.
Console.WriteLine("SUMMED 2D ELEMENTS: " + sum);
}
public static IEnumerable<int> GridValues()
{
// Use yield return to return all 2D array elements.
for (int x = 0; x < 15; x++)
{
for (int y = 0; y < 15; y++)
{
yield return _grid[x, y];
}
}
}
}SUMMED 2D ELEMENTS: 12
GetEnumerator error. In a class, the foreach-loop is not by default supported. A GetEnumerator method (often part of the IEnumerable interface) is required.
So We can implement IEnumerable to fix this error on a class. This provides the GetEnumerator method.
using System;
class Example
{
}
class Program
{
static void Main()
{
Example example = new Example();
// This does not compile: GetEnumerator is required.
foreach (string element in example)
{
Console.WriteLine(true);
}
}
}error CS1579: foreach statement cannot operate on
variables of type 'Example' because 'Example' does
not contain a public definition for 'GetEnumerator'
Implement IEnumerable. This example implements the IEnumerable interface on an Example class. The class contains a List, and for GetEnumerator, we use the List's GetEnumerator method.
So We forward the details of the implementation to the List. Our IEnumerable implementation relies on another.
Here We call the Example class constructor, which populates the _elements field.
Finally The foreach-loop in the Main method implicitly (secretly) calls the GetEnumerator method. So "HERE" is written.
Result The program compiles. The foreach loop refers to the List's Enumerator and loops over the elements of the List.
using System;
using System.Collections;
using System.Collections.Generic;
class Example : IEnumerable<string>
{
List<string> _elements;
public Example(string[] array)
{
this._elements = new List<string>(array);
}
IEnumerator<string> IEnumerable<string>.GetEnumerator()
{
Console.WriteLine("HERE");
return this._elements.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return this._elements.GetEnumerator();
}
}
class Program
{
static void Main()
{
Example example = new Example(new string[] { "cat", "dog", "bird" });
// The foreach-loop calls the generic GetEnumerator method.// ... It then uses the List's Enumerator.
foreach (string element in example)
{
Console.WriteLine(element);
}
}
}HERE
cat
dog
bird
Range. Some methods, like Enumerable.Range(), make it easier to create IEnumerable collections. We do not need to create a separate array.
Detail We test Enumerable.Range, which returns an IEnumerable collection. We enumerate its return value with foreach.
Info Other methods, like Enumerable.Repeat and Empty, are available. They can be useful in certain programs.
using System;
using System.Linq;
class Program
{
static void Main()
{
// Get IEnumerable from Enumerable.Range and loop over it.
foreach (int value in Enumerable.Range(100, 2))
{
Console.WriteLine("RANGE(0, 2): {0}", value);
}
}
}RANGE(0, 2): 100
RANGE(0, 2): 101
Benchmark, IEnumerable. A single IEnumerable method can reduce code size—this has speed advantages. For numeric methods, though, using an array directly is usually a faster approach.
Version 1 We loop over the 3 elements in an int array with for each, and sum them. No IEnumerable is involved.
Version 2 We use AsEnumerable to get an IEnumerable of the int array. Then we use foreach over the IEnumerable.
Result In 2021 on .NET 5 for Linux, the IEnumerable foreach-loop is much slower. Prefer foreach for this type of code.
using System;
using System.Diagnostics;
using System.Linq;
const int _max = 1000000;
int[] values = { 10, 20, 30 };
// Version 1: loop over array directly.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
int sum = 0;
foreach (int value in values)
{
sum += value;
}
if (sum == 0)
{
return;
}
}
s1.Stop();
// Version 2: convert array to IEnumerable and loop over IEnumerable elements.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
int sum = 0;
var enumerable = values.AsEnumerable();
foreach (int value in enumerable)
{
sum += value;
}
if (sum == 0)
{
return;
}
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns")); 2.53 ns int[], foreach
38.80 ns IEnumerable int, foreach
Yield. This keyword is placed before "return." It is used in methods that return IEnumerable. We can use yield to "save the state" of the function after the return.
Foreach. This kind of loop has advantages. It results in simpler, clearer syntax. We no longer need to track indexes with variables (which often have confusing names).
IOrderedEnumerable. If we use an orderby clause in a query expression, we receive an IOrderedEnumerable. This can be used in the same way as an IEnumerable—we can loop over it.
Summary. This generic interface provides an abstraction for looping over elements. It provides foreach-loop support. And it allows us to use LINQ extensions.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Feb 9, 2024 (edit link).
