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ReadOnlyCollectionBase Class

Provides the abstract base class for a strongly typed non-generic read-only collection.


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

public abstract class ReadOnlyCollectionBase : ICollection, 

The ReadOnlyCollectionBase type exposes the following members.

Protected methodReadOnlyCollectionBaseInitializes a new instance of the ReadOnlyCollectionBase class.

Public propertyCountGets the number of elements contained in the ReadOnlyCollectionBase instance.
Protected propertyInnerListGets the list of elements contained in the ReadOnlyCollectionBase instance.

Public methodEquals(Object)Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodGetEnumeratorReturns an enumerator that iterates through the ReadOnlyCollectionBase instance.
Public methodGetHashCodeServes as a hash function for a particular type. (Inherited from Object.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)

Public Extension MethodAsParallelEnables parallelization of a query. (Defined by ParallelEnumerable.)
Public Extension MethodAsQueryableConverts an IEnumerable to an IQueryable. (Defined by Queryable.)
Public Extension MethodCast<TResult>Casts the elements of an IEnumerable to the specified type. (Defined by Enumerable.)
Public Extension MethodOfType<TResult>Filters the elements of an IEnumerable based on a specified type. (Defined by Enumerable.)

Explicit interface implemetationPrivate methodICollection.CopyToCopies the entire ReadOnlyCollectionBase to a compatible one-dimensional Array, starting at the specified index of the target array.
Explicit interface implemetationPrivate propertyICollection.IsSynchronizedGets a value indicating whether access to a ReadOnlyCollectionBase object is synchronized (thread safe).
Explicit interface implemetationPrivate propertyICollection.SyncRootGets an object that can be used to synchronize access to a ReadOnlyCollectionBase object.

A ReadOnlyCollectionBase instance is always read-only. See CollectionBase for a modifiable version of this class.

Notes to Implementers

This base class is provided to make it easier for implementers to create a strongly typed read-only custom collection. Implementers are encouraged to extend this base class instead of creating their own. Members of this base class are protected and are intended to be used through a derived class only.

This class makes the underlying collection available through the InnerList property, which is intended for use only by classes that are derived directly from ReadOnlyCollectionBase. The derived class must ensure that its own users cannot modify the underlying collection.

The following code example implements the ReadOnlyCollectionBase class.

using System;
using System.Collections;

public class ROCollection : ReadOnlyCollectionBase  {

   public ROCollection( IList sourceList )  {
      InnerList.AddRange( sourceList );

   public Object this[ int index ]  {
      get  {
         return( InnerList[index] );

   public int IndexOf( Object value )  {
      return( InnerList.IndexOf( value ) );

   public bool Contains( Object value )  {
      return( InnerList.Contains( value ) );


public class SamplesCollectionBase  {

   public static void Main()  {

      // Create an ArrayList.
      ArrayList myAL = new ArrayList();
      myAL.Add( "red" );
      myAL.Add( "blue" );
      myAL.Add( "yellow" );
      myAL.Add( "green" );
      myAL.Add( "orange" );
      myAL.Add( "purple" );

      // Create a new ROCollection that contains the elements in myAL.
      ROCollection myCol = new ROCollection( myAL );

      // Display the contents of the collection using foreach. This is the preferred method.
      Console.WriteLine( "Contents of the collection (using foreach):" );
      PrintValues1( myCol );

      // Display the contents of the collection using the enumerator.
      Console.WriteLine( "Contents of the collection (using enumerator):" );
      PrintValues2( myCol );

      // Display the contents of the collection using the Count property and the Item property.
      Console.WriteLine( "Contents of the collection (using Count and Item):" );
      PrintIndexAndValues( myCol );

      // Search the collection with Contains and IndexOf.
      Console.WriteLine( "Contains yellow: {0}", myCol.Contains( "yellow" ) );
      Console.WriteLine( "orange is at index {0}.", myCol.IndexOf( "orange" ) );


   // Uses the Count property and the Item property.
   public static void PrintIndexAndValues( ROCollection myCol )  {
      for ( int i = 0; i < myCol.Count; i++ )
         Console.WriteLine( "   [{0}]:   {1}", i, myCol[i] );

   // Uses the foreach statement which hides the complexity of the enumerator.
   // NOTE: The foreach statement is the preferred way of enumerating the contents of a collection.
   public static void PrintValues1( ROCollection myCol )  {
      foreach ( Object obj in myCol )
         Console.WriteLine( "   {0}", obj );

   // Uses the enumerator. 
   // NOTE: The foreach statement is the preferred way of enumerating the contents of a collection.
   public static void PrintValues2( ROCollection myCol )  {
      System.Collections.IEnumerator myEnumerator = myCol.GetEnumerator();
      while ( myEnumerator.MoveNext() )
         Console.WriteLine( "   {0}", myEnumerator.Current );


This code produces the following output.

Contents of the collection (using foreach):

Contents of the collection (using enumerator):

Contents of the collection (using Count and Item):
   [0]:   red
   [1]:   blue
   [2]:   yellow
   [3]:   green
   [4]:   orange
   [5]:   purple

Contains yellow: True
orange is at index 4.


.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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

This implementation does not provide a synchronized (thread safe) wrapper for a ReadOnlyCollectionBase, but derived classes can create their own synchronized versions of the ReadOnlyCollectionBase using the SyncRoot property.

Enumerating through a collection is intrinsically not a thread-safe procedure. Even when a collection is synchronized, other threads can still modify the collection, which causes the enumerator to throw an exception. To guarantee thread safety during enumeration, you can either lock the collection during the entire enumeration or catch the exceptions resulting from changes made by other threads.