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ReadOnlyCollection(Of T).Item Property

Gets the element at the specified index.

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

'Declaration
Public ReadOnly Property Item ( _
	index As Integer _
) As T

Parameters

index
Type: System.Int32

The zero-based index of the element to get.

Property Value

Type: T
The element at the specified index.

Implements

IReadOnlyList(Of T).Item

ExceptionCondition
ArgumentOutOfRangeException

index is less than zero.

-or-

index is equal to or greater than Count.

This property provides the ability to access a specific element in the collection by using the following C# syntax: myCollection[index] (myCollection(index) in Visual Basic).

The C# language uses the this keyword to define the indexers instead of implementing the Item property. Visual Basic implements Item as a default property, which provides the same indexing functionality.

Retrieving the value of this property is an O(1) operation.

The following code example demonstrates several members of the ReadOnlyCollection(Of T) class. The code example creates a List(Of T) of strings and adds four dinosaur names to it. The code example then wraps the list in a ReadOnlyCollection(Of T).

After demonstrating the Count, Contains, Item, and IList.IndexOf members, the code example shows that the ReadOnlyCollection(Of T) is just a wrapper for the original List(Of T) by adding a new item to the List(Of T) and displaying the contents of the ReadOnlyCollection(Of T).

Finally, the code example creates an array larger than the collection and uses the CopyTo method to insert the elements of the collection into the middle of the array.

Imports System
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" 
'""

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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