ReadOnlyCollection Generic Class

Provides the base class for a generic read-only collection.

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

public class ReadOnlyCollection<T> : IList<T>, ICollection<T>, 
	IEnumerable<T>, IList, ICollection, IEnumerable
J# supports the use of generic types and methods, but not the declaration of new ones.
JScript does not support generic types and methods.
Not applicable.

Type Parameters


The type of elements in the collection.

An instance of the ReadOnlyCollection generic class is always read-only. 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. See Collection for a modifiable version of this class.

Notes to Implementers: This base class is provided to make it easier for implementers to create a generic read-only custom collection. Implementers are encouraged to extend this base class instead of creating their own.

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

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

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.

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;

public class Example
    public static void Main()
        List<string> dinosaurs = new List<string>();


        ReadOnlyCollection<string> readOnlyDinosaurs = 
            new ReadOnlyCollection<string>(dinosaurs);

        foreach( string dinosaur in readOnlyDinosaurs )

        Console.WriteLine("\nCount: {0}", readOnlyDinosaurs.Count);

        Console.WriteLine("\nContains(\"Deinonychus\"): {0}", 

        Console.WriteLine("\nreadOnlyDinosaurs[3]: {0}", 

        Console.WriteLine("\nIndexOf(\"Compsognathus\"): {0}", 

        Console.WriteLine("\nInsert into the wrapped List:");
        Console.WriteLine("Insert(2, \"Oviraptor\")");
        dinosaurs.Insert(2, "Oviraptor");

        foreach( string dinosaur in readOnlyDinosaurs )

        string[] dinoArray = new string[readOnlyDinosaurs.Count + 2];
        readOnlyDinosaurs.CopyTo(dinoArray, 1);

        Console.WriteLine("\nCopied array has {0} elements:", 
        foreach( string dinosaur in dinoArray )
            Console.WriteLine("\"{0}\"", dinosaur);

/* This code example produces the following output:


Count: 4

Contains("Deinonychus"): True

readOnlyDinosaurs[3]: Compsognathus

IndexOf("Compsognathus"): 3

Insert into the wrapped List:
Insert(2, "Oviraptor")


Copied array has 7 elements:

Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

A ReadOnlyCollection can support multiple readers concurrently, as long as the collection is not modified. Even so, 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.

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0

.NET Compact Framework

Supported in: 2.0

XNA Framework

Supported in: 1.0