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ReadOnlyCollectionBase.GetEnumerator Method

Returns an enumerator that can iterate through the ReadOnlyCollectionBase instance.

[Visual Basic]
Public Overridable Function GetEnumerator() As IEnumerator _
   Implements IEnumerable.GetEnumerator
[C#]
public virtual IEnumerator GetEnumerator();
[C++]
public: virtual IEnumerator* GetEnumerator();
[JScript]
public function GetEnumerator() : IEnumerator;

Return Value

An IEnumerator for the ReadOnlyCollectionBase instance.

Implements

IEnumerable.GetEnumerator

Remarks

Enumerators only allow reading the data in the collection. Enumerators cannot be used to modify the underlying collection.

Initially, the enumerator is positioned before the first element in the collection. Reset also brings the enumerator back to this position. At this position, calling Current throws an exception. Therefore, you must call MoveNext to advance the enumerator to the first element of the collection before reading the value of Current.

Current returns the same object until either MoveNext or Reset is called. MoveNext sets Current to the next element.

After the end of the collection is passed, the enumerator is positioned after the last element in the collection, and calling MoveNext returns false. If the last call to MoveNext returned false, calling Current throws an exception. To set Current to the first element of the collection again, you can call Reset followed by MoveNext.

An enumerator remains valid as long as the collection remains unchanged. If changes are made to the collection, such as adding, modifying or deleting elements, the enumerator is irrecoverably invalidated and the next call to MoveNext or Reset throws an InvalidOperationException. If the collection is modified between MoveNext and Current, Current will return the element that it is set to, even if the enumerator is already invalidated.

The enumerator does not have exclusive access to the collection; therefore, 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.

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

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

See Also

ReadOnlyCollectionBase Class | ReadOnlyCollectionBase Members | System.Collections Namespace | System.Collections.IEnumerator

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