ReadOnlyCollection<T>::GetEnumerator Method ()


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Returns an enumerator that iterates through the ReadOnlyCollection<T>.

Namespace:   System.Collections.ObjectModel
Assembly:  mscorlib (in mscorlib.dll)

virtual IEnumerator<T>^ GetEnumerator() sealed

The foreach statement of the C# language (for each in Visual C++, For Each in Visual Basic) hides the complexity of the enumerators. Therefore, using foreach is recommended, instead of directly manipulating the enumerator.

Enumerators can be used to read the data in the collection, but they cannot be used to modify the underlying collection.

Initially, the enumerator is positioned before the first element in the collection. At this position, Current is undefined. Therefore, you must call MoveNext to advance the enumerator to the first element of the collection before reading the value of Current.

Current returns the same object until MoveNext is called. MoveNext sets Current to the next element.

If MoveNext passes the end of the collection, the enumerator is positioned after the last element in the collection and MoveNext returns false. When the enumerator is at this position, subsequent calls to MoveNext also return false. If the last call to MoveNext returned false, Current is undefined. You cannot set Current to the first element of the collection again; you must create a new enumerator instance instead.

An enumerator remains valid as long as the collection remains unchanged. If changes are made to the collection, such as adding, modifying, or deleting elements, the enumerator is irrecoverably invalidated and its behavior is undefined.

The enumerator does not have exclusive access to the collection; therefore, enumerating through a collection is intrinsically not a thread-safe procedure. To guarantee thread safety during enumeration, you can lock the collection during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.

Default implementations of collections in System.Collections.Generic are not synchronized.

This method is an O(1) operation.

The following code example uses the enumerator to display the contents of a ReadOnlyCollection<T> that wraps a List<T>. The enumerator is concealed by the foreach statement (For Each in Visual Basic, for each in C++).

using namespace System;
using namespace System::Collections::Generic;
using namespace System::Collections::ObjectModel;

void main()
    List<String^>^ dinosaurs = gcnew List<String^>();


    ReadOnlyCollection<String^>^ readOnlyDinosaurs = 
        gcnew ReadOnlyCollection<String^>(dinosaurs);

    for each(String^ dinosaur in readOnlyDinosaurs )

    Console::WriteLine("\nCount: {0}", readOnlyDinosaurs->Count);

    Console::WriteLine("\nContains(\"Deinonychus\"): {0}", 

    Console::WriteLine("\nreadOnlyDinosaurs[3]: {0}", 

    Console::WriteLine("\nIndexOf(\"Compsognathus\"): {0}", 

    Console::WriteLine("\nInsert into the wrapped List:");
    Console::WriteLine("Insert(2, \"Oviraptor\")");
    dinosaurs->Insert(2, "Oviraptor");

    for each( String^ dinosaur in readOnlyDinosaurs )

    array<String^>^ dinoArray = 
        gcnew array<String^>(readOnlyDinosaurs->Count + 2);
    readOnlyDinosaurs->CopyTo(dinoArray, 1);

    Console::WriteLine("\nCopied array has {0} elements:", 
    for each( String^ dinosaur in dinoArray )
        Console::WriteLine("\"{0}\"", dinosaur);

/* This code example produces the following output:


Count: 4

Contains("Deinonychus"): True

readOnlyDinosaurs[3]: Compsognathus

IndexOf("Compsognathus"): 3

Insert into the wrapped List:
Insert(2, "Oviraptor")


Copied array has 7 elements:

Universal Windows Platform
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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