• Article

ReadOnlyCollection<T>.Item Property

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Gets the element at the specified index.

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

Syntax

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

public T this[
    int index
] { get; }

Parameters

  • index
    Type: System.Int32
    The zero-based index of the element to get.

Property Value

Type: T
The element at the specified index.

Exceptions

Exception Condition
ArgumentOutOfRangeException

index is less than zero.

-or-

index is equal to or greater than Count.

Remarks

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.

Examples

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

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