ReadOnlyCollection<T>.GetEnumerator Method
Microsoft Silverlight will reach end of support after October 2021. Learn more.
Returns an enumerator that iterates through the ReadOnlyCollection<T>.
Namespace: System.Collections.ObjectModel
Assembly: mscorlib (in mscorlib.dll)
Syntax
'Declaration
Public Function GetEnumerator As IEnumerator(Of T)
public IEnumerator<T> GetEnumerator()
Return Value
Type: System.Collections.Generic.IEnumerator<T>
An IEnumerator<T> for the ReadOnlyCollection<T>.
Implements
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.
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++).
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
Public Class Example
Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
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)
outputBlock.Text &= vbCrLf
For Each dinosaur As String In readOnlyDinosaurs
outputBlock.Text &= dinosaur & vbCrLf
Next
outputBlock.Text &= String.Format(vbLf & "Count: {0}", _
readOnlyDinosaurs.Count) & vbCrLf
outputBlock.Text &= String.Format(vbLf & "Contains(""Deinonychus"" & vbCrLf: {0}", _
readOnlyDinosaurs.Contains("Deinonychus"))
outputBlock.Text &= String.Format(vbLf & _
"readOnlyDinosaurs(3): {0}", readOnlyDinosaurs(3)) & vbCrLf
outputBlock.Text &= String.Format(vbLf & "IndexOf(""Compsognathus"" & vbCrLf: {0}", _
readOnlyDinosaurs.IndexOf("Compsognathus"))
outputBlock.Text &= vbLf & "Insert into the wrapped List:" & vbCrLf
outputBlock.Text &= String.Format("Insert(2, ""Oviraptor"")") & vbCrLf
dinosaurs.Insert(2, "Oviraptor")
outputBlock.Text &= vbCrLf
For Each dinosaur As String In readOnlyDinosaurs
outputBlock.Text &= dinosaur & vbCrLf
Next
Dim dinoArray(readOnlyDinosaurs.Count + 1) As String
readOnlyDinosaurs.CopyTo(dinoArray, 1)
outputBlock.Text &= String.Format(vbLf & "Copied array has {0} elements:", _
dinoArray.Length) & vbCrLf
For Each dinosaur As String In dinoArray
outputBlock.Text &= String.Format("""{0}""", dinosaur) & vbCrLf
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"
'""
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
public class Example
{
public static void Demo(System.Windows.Controls.TextBlock outputBlock)
{
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);
outputBlock.Text += "\n";
foreach (string dinosaur in readOnlyDinosaurs)
{
outputBlock.Text += dinosaur + "\n";
}
outputBlock.Text += String.Format("\nCount: {0}", readOnlyDinosaurs.Count) + "\n";
outputBlock.Text += String.Format("\nContains(\"Deinonychus\"): {0}",
readOnlyDinosaurs.Contains("Deinonychus")) + "\n";
outputBlock.Text += String.Format("\nreadOnlyDinosaurs[3]: {0}",
readOnlyDinosaurs[3]) + "\n";
outputBlock.Text += String.Format("\nIndexOf(\"Compsognathus\"): {0}",
readOnlyDinosaurs.IndexOf("Compsognathus")) + "\n";
outputBlock.Text += "\nInsert into the wrapped List:" + "\n";
outputBlock.Text += String.Format("Insert(2, \"Oviraptor\")") + "\n";
dinosaurs.Insert(2, "Oviraptor");
outputBlock.Text += "\n";
foreach (string dinosaur in readOnlyDinosaurs)
{
outputBlock.Text += dinosaur + "\n";
}
string[] dinoArray = new string[readOnlyDinosaurs.Count + 2];
readOnlyDinosaurs.CopyTo(dinoArray, 1);
outputBlock.Text += String.Format("\nCopied array has {0} elements:",
dinoArray.Length) + "\n";
foreach (string dinosaur in dinoArray)
{
outputBlock.Text += String.Format("\"{0}\"", dinosaur) + "\n";
}
}
}
/* 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"
""
*/
Version Information
Silverlight
Supported in: 5, 4, 3
Silverlight for Windows Phone
Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0
XNA Framework
Supported in: Xbox 360, Windows Phone OS 7.0
Platforms
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.
See Also