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ReadOnlyCollectionBase.GetEnumerator Method

Returns an enumerator that iterates through the ReadOnlyCollectionBase instance.

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

public virtual IEnumerator GetEnumerator ()
public IEnumerator GetEnumerator ()
public function GetEnumerator () : IEnumerator
Not applicable.

Return Value

An IEnumerator for the ReadOnlyCollectionBase instance.

The foreach statement of the C# language (for each in Visual Basic) hides the complexity of the enumerators. Therefore, using foreach is recommended, instead of directly manipulating the enumerator.

Enumerators can be used to read the data in the collection, but they 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, Current is undefined. 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.

If MoveNext passes the end of the collection, the enumerator is positioned after the last element in the collection and MoveNext returns false. When the enumerator is at this position, subsequent calls to MoveNext also return false. If the last call to MoveNext returned false, Current is undefined. 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 its behavior is undefined.

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

This method is an O(1) operation.

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" ) );
      Console.WriteLine();

   }
 
   // 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] );
      Console.WriteLine();
   }

   // 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 );
      Console.WriteLine();
   }

   // 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 );
      Console.WriteLine();
   }

}


/* 
This code produces the following output.

Contents of the collection (using foreach):
   red
   blue
   yellow
   green
   orange
   purple

Contents of the collection (using enumerator):
   red
   blue
   yellow
   green
   orange
   purple

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.

*/


import System.*;
import System.Collections.*;

public class ROCollection extends ReadOnlyCollectionBase
{
    public ROCollection(IList sourceList) 
    {
        get_InnerList().AddRange(sourceList);
    } //ROCollection
   
    /** @property 
     */
    public Object get_Item(int index)
    {
        return get_InnerList().get_Item(index);
    } //get_Item
     
    public int IndexOf(Object value) 
    {
        return get_InnerList().IndexOf(value);
    } //IndexOf
   
    public boolean Contains(Object value) 
    {
        return get_InnerList().Contains(value);
    } //Contains
} //ROCollection

public class SamplesCollectionBase
{
    public static void main(String[] args)
    {
        // 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 for. This is the 
        // preferred method.
        Console.WriteLine("Contents of the collection (using for):");
        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}",
            (System.Boolean)myCol.Contains("yellow"));
        Console.WriteLine("orange is at index {0}.", 
            (Int32)myCol.IndexOf("orange"));
        Console.WriteLine();
    } //main
    
    // Uses the Count property and the Item property.
    public static void PrintIndexAndValues(ROCollection myCol) 
    {
        for(int i = 0; i < myCol.get_Count(); i++) {
            Console.WriteLine("   [{0}]:   {1}",(Int32)i, myCol.get_Item(i));
        } 
        Console.WriteLine();
    } //PrintIndexAndValues
   
    // Uses the for statement which hides the complexity of the enumerator.
    // NOTE: The for statement is the preferred way of enumerating the contents
    // of a collection.
    public static void PrintValues1(ROCollection myCol) 
    {
        for (int iCtr = 0; iCtr < myCol.get_Count(); iCtr++ ) {
            Object obj = myCol.get_Item(iCtr);
            Console.WriteLine("   {0}", obj);
        }
        Console.WriteLine();
    } //PrintValues1
     
    // Uses the enumerator. 
    // NOTE: The for 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.get_Current());
        }
        Console.WriteLine();
    } //PrintValues2
} //SamplesCollectionBase
 
/* 
This code produces the following output.

Contents of the collection (using for):
   red
   blue
   yellow
   green
   orange
   purple

Contents of the collection (using enumerator):
   red
   blue
   yellow
   green
   orange
   purple

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.

*/

Windows 98, Windows Server 2000 SP4, Windows Millennium Edition, 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, 1.1, 1.0

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