IEnumerable Interface (System.Collections) (2024)

Reference

Definition

Namespace:: System.Collections

Assembly:: System.Runtime.dll

Assembly:: mscorlib.dll

Assembly:: netstandard.dll

Important

Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.

Exposes an enumerator, which supports a simple iteration over a non-generic collection.

Examples

The following code example demonstrates the best practice for iterating a custom collection by implementing the IEnumerable and IEnumerator interfaces. In this example, members of these interfaces are not explicitly called, but they are implemented to support the use of foreach (For Each in Visual Basic) to iterate through the collection. This example is a complete Console app. To compile the Visual Basic app, change the Startup object to Sub Main in the project's Properties page.

using System;using System.Collections;// Simple business object.public class Person{ public Person(string fName, string lName) { this.firstName = fName; this.lastName = lName; } public string firstName; public string lastName;}// Collection of Person objects. This class// implements IEnumerable so that it can be used// with ForEach syntax.public class People : IEnumerable{ private Person[] _people; public People(Person[] pArray) { _people = new Person[pArray.Length]; for (int i = 0; i < pArray.Length; i++) { _people[i] = pArray[i]; } }// Implementation for the GetEnumerator method. IEnumerator IEnumerable.GetEnumerator() { return (IEnumerator) GetEnumerator(); } public PeopleEnum GetEnumerator() { return new PeopleEnum(_people); }}// When you implement IEnumerable, you must also implement IEnumerator.public class PeopleEnum : IEnumerator{ public Person[] _people; // Enumerators are positioned before the first element // until the first MoveNext() call. int position = -1; public PeopleEnum(Person[] list) { _people = list; } public bool MoveNext() { position++; return (position < _people.Length); } public void Reset() { position = -1; } object IEnumerator.Current { get { return Current; } } public Person Current { get { try { return _people[position]; } catch (IndexOutOfRangeException) { throw new InvalidOperationException(); } } }}class App{ static void Main() { Person[] peopleArray = new Person[3] { new Person("John", "Smith"), new Person("Jim", "Johnson"), new Person("Sue", "Rabon"), }; People peopleList = new People(peopleArray); foreach (Person p in peopleList) Console.WriteLine(p.firstName + " " + p.lastName); }}/* This code produces output similar to the following: * * John Smith * Jim Johnson * Sue Rabon * */

Imports System.Collections' Simple business object.Public Class Person Public Sub New(ByVal fName As String, ByVal lName As String) Me.firstName = fName Me.lastName = lName End Sub Public firstName As String Public lastName As StringEnd Class' Collection of Person objects, which implements IEnumerable so that' it can be used with ForEach syntax.Public Class People Implements IEnumerable Private _people() As Person Public Sub New(ByVal pArray() As Person) _people = New Person(pArray.Length - 1) {} Dim i As Integer For i = 0 To pArray.Length - 1 _people(i) = pArray(i) Next i End Sub ' Implementation of GetEnumerator. Public Function GetEnumerator() As IEnumerator _ Implements IEnumerable.GetEnumerator Return New PeopleEnum(_people) End FunctionEnd Class' When you implement IEnumerable, you must also implement IEnumerator.Public Class PeopleEnum Implements IEnumerator Public _people() As Person ' Enumerators are positioned before the first element ' until the first MoveNext() call. Dim position As Integer = -1 Public Sub New(ByVal list() As Person) _people = list End Sub Public Function MoveNext() As Boolean Implements IEnumerator.MoveNext position = position + 1 Return (position < _people.Length) End Function Public Sub Reset() Implements IEnumerator.Reset position = -1 End Sub Public ReadOnly Property Current() As Object Implements IEnumerator.Current Get Try Return _people(position) Catch ex As IndexOutOfRangeException Throw New InvalidOperationException() End Try End Get End PropertyEnd ClassClass App Shared Sub Main() Dim peopleArray() As Person = { _ New Person("John", "Smith"), _ New Person("Jim", "Johnson"), _ New Person("Sue", "Rabon")} Dim peopleList As New People(peopleArray) Dim p As Person For Each p In peopleList Console.WriteLine(p.firstName + " " + p.lastName) Next End SubEnd Class' This code produces output similar to the following:' ' John Smith' Jim Johnson' Sue Rabon

Remarks

IEnumerable is the base interface for all non-generic collections that can be enumerated. For the generic version of this interface see System.Collections.Generic.IEnumerable<T>. IEnumerable contains a single method, GetEnumerator, which returns an IEnumerator. IEnumerator provides the ability to iterate through the collection by exposing a Current property and MoveNext and Reset methods.

It is a best practice to implement IEnumerable and IEnumerator on your collection classes to enable the foreach (For Each in Visual Basic) syntax, however implementing IEnumerable is not required. If your collection does not implement IEnumerable, you must still follow the iterator pattern to support this syntax by providing a GetEnumerator method that returns an interface, class or struct. When using Visual Basic, you must provide an IEnumerator implementation, which is returned by GetEnumerator. When developing with C# you must provide a class that contains a Current property, and MoveNext and Reset methods as described by IEnumerator, but the class does not have to implement IEnumerator.

Methods

GetEnumerator()

Returns an enumerator that iterates through a collection.

Extension Methods

Cast<TResult>(IEnumerable)	Casts the elements of an IEnumerable to the specified type.
OfType<TResult>(IEnumerable)	Filters the elements of an IEnumerable based on a specified type.
AsParallel(IEnumerable)	Enables parallelization of a query.
AsQueryable(IEnumerable)	Converts an IEnumerable to an IQueryable.

Applies to