ReadOnlyCollection<T>.GetEnumerator Method
Definition
Important
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Returns an enumerator that iterates through the ReadOnlyCollection<T>.
public:
virtual System::Collections::Generic::IEnumerator<T> ^ GetEnumerator();public System.Collections.Generic.IEnumerator<T> GetEnumerator ();abstract member GetEnumerator : unit -> System.Collections.Generic.IEnumerator<'T>
override this.GetEnumerator : unit -> System.Collections.Generic.IEnumerator<'T>Public Function GetEnumerator () As IEnumerator(Of T)Returns
An IEnumerator<T> for the ReadOnlyCollection<T>.
Implements
Examples
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^>();
dinosaurs->Add("Tyrannosaurus");
dinosaurs->Add("Amargasaurus");
dinosaurs->Add("Deinonychus");
dinosaurs->Add("Compsognathus");
ReadOnlyCollection<String^>^ readOnlyDinosaurs =
gcnew ReadOnlyCollection<String^>(dinosaurs);
Console::WriteLine();
for each(String^ dinosaur in readOnlyDinosaurs )
{
Console::WriteLine(dinosaur);
}
Console::WriteLine("\nCount: {0}", readOnlyDinosaurs->Count);
Console::WriteLine("\nContains(\"Deinonychus\"): {0}",
readOnlyDinosaurs->Contains("Deinonychus"));
Console::WriteLine("\nreadOnlyDinosaurs[3]: {0}",
readOnlyDinosaurs[3]);
Console::WriteLine("\nIndexOf(\"Compsognathus\"): {0}",
readOnlyDinosaurs->IndexOf("Compsognathus"));
Console::WriteLine("\nInsert into the wrapped List:");
Console::WriteLine("Insert(2, \"Oviraptor\")");
dinosaurs->Insert(2, "Oviraptor");
Console::WriteLine();
for each( String^ dinosaur in readOnlyDinosaurs )
{
Console::WriteLine(dinosaur);
}
array<String^>^ dinoArray =
gcnew array<String^>(readOnlyDinosaurs->Count + 2);
readOnlyDinosaurs->CopyTo(dinoArray, 1);
Console::WriteLine("\nCopied array has {0} elements:",
dinoArray->Length);
for each( String^ dinosaur in dinoArray )
{
Console::WriteLine("\"{0}\"", dinosaur);
}
}
/* This code example produces the following output:
Tyrannosaurus
Amargasaurus
Deinonychus
Compsognathus
Count: 4
Contains("Deinonychus"): True
readOnlyDinosaurs[3]: Compsognathus
IndexOf("Compsognathus"): 3
Insert into the wrapped List:
Insert(2, "Oviraptor")
Tyrannosaurus
Amargasaurus
Oviraptor
Deinonychus
Compsognathus
Copied array has 7 elements:
""
"Tyrannosaurus"
"Amargasaurus"
"Oviraptor"
"Deinonychus"
"Compsognathus"
""
*/
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
public class Example
{
public static void Main()
{
List<string> dinosaurs = new List<string>();
dinosaurs.Add("Tyrannosaurus");
dinosaurs.Add("Amargasaurus");
dinosaurs.Add("Deinonychus");
dinosaurs.Add("Compsognathus");
ReadOnlyCollection<string> readOnlyDinosaurs =
new ReadOnlyCollection<string>(dinosaurs);
Console.WriteLine();
foreach( string dinosaur in readOnlyDinosaurs )
{
Console.WriteLine(dinosaur);
}
Console.WriteLine("\nCount: {0}", readOnlyDinosaurs.Count);
Console.WriteLine("\nContains(\"Deinonychus\"): {0}",
readOnlyDinosaurs.Contains("Deinonychus"));
Console.WriteLine("\nreadOnlyDinosaurs[3]: {0}",
readOnlyDinosaurs[3]);
Console.WriteLine("\nIndexOf(\"Compsognathus\"): {0}",
readOnlyDinosaurs.IndexOf("Compsognathus"));
Console.WriteLine("\nInsert into the wrapped List:");
Console.WriteLine("Insert(2, \"Oviraptor\")");
dinosaurs.Insert(2, "Oviraptor");
Console.WriteLine();
foreach( string dinosaur in readOnlyDinosaurs )
{
Console.WriteLine(dinosaur);
}
string[] dinoArray = new string[readOnlyDinosaurs.Count + 2];
readOnlyDinosaurs.CopyTo(dinoArray, 1);
Console.WriteLine("\nCopied array has {0} elements:",
dinoArray.Length);
foreach( string dinosaur in dinoArray )
{
Console.WriteLine("\"{0}\"", dinosaur);
}
}
}
/* This code example produces the following output:
Tyrannosaurus
Amargasaurus
Deinonychus
Compsognathus
Count: 4
Contains("Deinonychus"): True
readOnlyDinosaurs[3]: Compsognathus
IndexOf("Compsognathus"): 3
Insert into the wrapped List:
Insert(2, "Oviraptor")
Tyrannosaurus
Amargasaurus
Oviraptor
Deinonychus
Compsognathus
Copied array has 7 elements:
""
"Tyrannosaurus"
"Amargasaurus"
"Oviraptor"
"Deinonychus"
"Compsognathus"
""
*/
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
Public Class Example
Public Shared Sub Main()
Dim dinosaurs As New List(Of String)
dinosaurs.Add("Tyrannosaurus")
dinosaurs.Add("Amargasaurus")
dinosaurs.Add("Deinonychus")
dinosaurs.Add("Compsognathus")
Dim readOnlyDinosaurs As _
New ReadOnlyCollection(Of String)(dinosaurs)
Console.WriteLine()
For Each dinosaur As String In readOnlyDinosaurs
Console.WriteLine(dinosaur)
Next
Console.WriteLine(vbLf & "Count: {0}", _
readOnlyDinosaurs.Count)
Console.WriteLine(vbLf & "Contains(""Deinonychus""): {0}", _
readOnlyDinosaurs.Contains("Deinonychus"))
Console.WriteLine(vbLf & _
"readOnlyDinosaurs(3): {0}", readOnlyDinosaurs(3))
Console.WriteLine(vbLf & "IndexOf(""Compsognathus""): {0}", _
readOnlyDinosaurs.IndexOf("Compsognathus"))
Console.WriteLine(vbLf & "Insert into the wrapped List:")
Console.WriteLine("Insert(2, ""Oviraptor"")")
dinosaurs.Insert(2, "Oviraptor")
Console.WriteLine()
For Each dinosaur As String In readOnlyDinosaurs
Console.WriteLine(dinosaur)
Next
Dim dinoArray(readOnlyDinosaurs.Count + 1) As String
readOnlyDinosaurs.CopyTo(dinoArray, 1)
Console.WriteLine(vbLf & "Copied array has {0} elements:", _
dinoArray.Length)
For Each dinosaur As String In dinoArray
Console.WriteLine("""{0}""", dinosaur)
Next
End Sub
End Class
' This code example produces the following output:
'
'Tyrannosaurus
'Amargasaurus
'Deinonychus
'Compsognathus
'
'Count: 4
'
'Contains("Deinonychus"): True
'
'readOnlyDinosaurs(3): Compsognathus
'
'IndexOf("Compsognathus"): 3
'
'Insert into the wrapped List:
'Insert(2, "Oviraptor")
'
'Tyrannosaurus
'Amargasaurus
'Oviraptor
'Deinonychus
'Compsognathus
'
'Copied array has 7 elements:
'""
'"Tyrannosaurus"
'"Amargasaurus"
'"Oviraptor"
'"Deinonychus"
'"Compsognathus"
'""
Remarks
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.
