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ReadOnlyCollectionBase Class

Provides the abstract (MustInherit in Visual Basic) base class for a strongly typed read-only collection.

For a list of all members of this type, see ReadOnlyCollectionBase Members.

System.Object
   System.Collections.ReadOnlyCollectionBase
      Derived classes

[Visual Basic]
<Serializable>
MustInherit Public Class ReadOnlyCollectionBase
   Implements ICollection, IEnumerable
[C#]
[Serializable]
public abstract class ReadOnlyCollectionBase : ICollection,
   IEnumerable
[C++]
[Serializable]
public __gc __abstract class ReadOnlyCollectionBase : public
   ICollection, IEnumerable
[JScript]
public
   Serializable
abstract class ReadOnlyCollectionBase implements
   ICollection, IEnumerable

Thread Safety

Public static (Shared in Visual Basic) members of this type are safe for multithreaded operations. 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 could 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.

Remarks

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 should 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.

Example

[Visual Basic, C#, C++] The following code example implements the ReadOnlyCollectionBase class.

[Visual Basic] 
Imports System
Imports System.Collections

Public Class ROCollection
   Inherits ReadOnlyCollectionBase

   Public Sub New(sourceList As IList)
      InnerList.AddRange(sourceList)
   End Sub 'New

   Default Public ReadOnly Property Item(index As Integer) As [Object]
      Get
         Return InnerList(index)
      End Get
   End Property

   Public Function IndexOf(value As [Object]) As Integer
      Return InnerList.IndexOf(value)
   End Function 'IndexOf

   Public Function Contains(value As [Object]) As Boolean
      Return InnerList.Contains(value)
   End Function 'Contains

End Class 'ROCollection 


Public Class SamplesCollectionBase

   Public Shared Sub Main()

      ' Create an ArrayList.
      Dim myAL As 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.
      Dim myCol As New ROCollection(myAL)

      ' Display the contents of the collection using the enumerator.
      Console.WriteLine("Contents of the collection (using enumerator):")
      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"))
      Console.WriteLine()

      ' Display the contents of the collection using the Count property and the Item property.
      Console.WriteLine("Contents of the collection (using Count and Item):")
      PrintIndexAndValues2(myCol)

   End Sub 'Main

   Public Shared Sub PrintIndexAndValues(myCol As ROCollection)
      Dim i As Integer = 0
      Dim myEnumerator As System.Collections.IEnumerator = myCol.GetEnumerator()
      While myEnumerator.MoveNext()
         Console.WriteLine("   [{0}]:   {1}", i, myEnumerator.Current)
         i += 1
      End While
      Console.WriteLine()
   End Sub 'PrintIndexAndValues

   Public Shared Sub PrintIndexAndValues2(myCol As ROCollection)
      Dim i As Integer
      For i = 0 To myCol.Count - 1
         Console.WriteLine("   [{0}]:   {1}", i, myCol(i))
      Next i
      Console.WriteLine()
   End Sub 'PrintIndexAndValues2

End Class 'SamplesCollectionBase 


'This code produces the following output.
'
'Contents of the collection (using enumerator):
'   [0]:   red
'   [1]:   blue
'   [2]:   yellow
'   [3]:   green
'   [4]:   orange
'   [5]:   purple
'
'Contains yellow: True
'orange is at index 4.
'
'Contents of the collection (using Count and Item):
'   [0]:   red
'   [1]:   blue
'   [2]:   yellow
'   [3]:   green
'   [4]:   orange
'   [5]:   purple


[C#] 
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 the enumerator.
      Console.WriteLine( "Contents of the collection (using enumerator):" );
      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" ) );
      Console.WriteLine();

      // Display the contents of the collection using the Count property and the Item property.
      Console.WriteLine( "Contents of the collection (using Count and Item):" );
      PrintIndexAndValues2( myCol );

   }
 
   public static void PrintIndexAndValues( ROCollection myCol )  {
      int i = 0;
      System.Collections.IEnumerator myEnumerator = myCol.GetEnumerator();
      while ( myEnumerator.MoveNext() )
         Console.WriteLine( "   [{0}]:   {1}", i++, myEnumerator.Current );
      Console.WriteLine();
   }

   public static void PrintIndexAndValues2( ROCollection myCol )  {
      for ( int i = 0; i < myCol.Count; i++ )
         Console.WriteLine( "   [{0}]:   {1}", i, myCol[i] );
      Console.WriteLine();
   }

}


/* 
This code produces the following output.

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

Contains yellow: True
orange is at index 4.

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

*/


[C++] 
#using <mscorlib.dll>
using namespace System;
using namespace System::Collections;

public __gc class ROCollection : public ReadOnlyCollectionBase  {

public:
    ROCollection( IList* sourceList )  {
        InnerList->AddRange( sourceList );
    }

public:
    __property Object* get_Item( int index )  {
        return( InnerList->Item[index] );
    }

public:
    int IndexOf( Object* value )  {
        return( InnerList->IndexOf( value ) );
    }

public:
    bool Contains( Object* value )  {
        return( InnerList->Contains( value ) );
    }

};

static void PrintIndexAndValues( ROCollection* myCol )  {
    int i = 0;
    System::Collections::IEnumerator* myEnumerator = myCol->GetEnumerator();
    while ( myEnumerator->MoveNext() )
        Console::WriteLine( S"   [{0}]:   {1}", __box(i++), myEnumerator->Current );
    Console::WriteLine();
}

static void PrintIndexAndValues2( ROCollection* myCol )  {
    for ( int i = 0; i < myCol->Count; i++ )
        Console::WriteLine( S"   [{0}]:   {1}", __box(i), myCol->Item[i] );
    Console::WriteLine();
}

int main()  {

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

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

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

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

    // Display the contents of the collection using the Count property and the Item property.
    Console::WriteLine( S"Contents of the collection (using Count and Item):" );
    PrintIndexAndValues2( myCol );

}

/* 
This code produces the following output.

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

Contains yellow: True
orange is at index 4.

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

*/

[JScript] No example is available for JScript. To view a Visual Basic, C#, or C++ example, click the Language Filter button Language Filter in the upper-left corner of the page.

Requirements

Namespace: System.Collections

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family

Assembly: Mscorlib (in Mscorlib.dll)

See Also

ReadOnlyCollectionBase Members | System.Collections Namespace | System.Collections.ArrayList | CollectionBase

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