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ReadOnlyCollection<T> Constructor

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Initializes a new instance of the ReadOnlyCollection<T> class that is a read-only wrapper around the specified list.

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

Syntax

'Declaration
Public Sub New ( _
    list As IList(Of T) _
)

public ReadOnlyCollection(
    IList<T> list
)

Parameters

Exceptions

Exception Condition
ArgumentNullException

list is nulla null reference (Nothing in Visual Basic).

Remarks

To prevent any modifications to list, expose list only through this wrapper.

A collection that is read-only is simply a collection with a wrapper that prevents modifying the collection; therefore, if changes are made to the underlying collection, the read-only collection reflects those changes.

This constructor is an O(1) operation.

Examples

The following code example demonstrates this constructor and 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> by passing it to the constructor.

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.