ReadOnlyCollectionBase.GetEnumerator Metodo

Definizione

Restituisce un enumeratore per lo scorrimento dell'istanza di ReadOnlyCollectionBase.Returns an enumerator that iterates through the ReadOnlyCollectionBase instance.

public:
 virtual System::Collections::IEnumerator ^ GetEnumerator();
public System.Collections.IEnumerator GetEnumerator ();
abstract member GetEnumerator : unit -> System.Collections.IEnumerator
override this.GetEnumerator : unit -> System.Collections.IEnumerator
Public Function GetEnumerator () As IEnumerator

Restituisce

Oggetto IEnumerator per l'istanza di ReadOnlyCollectionBase.An IEnumerator for the ReadOnlyCollectionBase instance.

Implementazioni

Esempi

Nell'esempio di codice seguente viene implementata la classe ReadOnlyCollectionBase.The following code example implements the ReadOnlyCollectionBase class.

using namespace System;
using namespace System::Collections;
public ref class ROCollection: public ReadOnlyCollectionBase
{
public:
   ROCollection( IList^ sourceList )
   {
      InnerList->AddRange( sourceList );
   }

   property Object^ Item [int]
   {
      Object^ get( int index )
      {
         return (InnerList[ index ]);
      }

   }
   int IndexOf( Object^ value )
   {
      return (InnerList->IndexOf( value ));
   }

   bool Contains( Object^ value )
   {
      return (InnerList->Contains( value ));
   }

};

void PrintIndexAndValues( ROCollection^ myCol );
void PrintValues2( ROCollection^ myCol );
int main()
{
   // Create an ArrayList.
   ArrayList^ myAL = gcnew 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 = gcnew ROCollection( myAL );

   // 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.
void PrintIndexAndValues( ROCollection^ myCol )
{
   for ( int i = 0; i < myCol->Count; i++ )
      Console::WriteLine( "   [{0}]:   {1}", i, myCol->Item[ i ] );
   Console::WriteLine();
}


// Uses the enumerator. 
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 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.

*/
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.

*/

Imports System.Collections

Public Class ROCollection
    Inherits ReadOnlyCollectionBase


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


    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


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 For Each. This is the preferred method.
        Console.WriteLine("Contents of the collection (using For Each):")
        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()

    End Sub


    ' Uses the Count property and the Item property.
    Public Shared Sub PrintIndexAndValues(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


    ' Uses the For Each statement which hides the complexity of the enumerator.
    ' NOTE: The For Each statement is the preferred way of enumerating the contents of a collection.
    Public Shared Sub PrintValues1(myCol As ROCollection)
        Dim obj As [Object]
        For Each obj In  myCol
            Console.WriteLine("   {0}", obj)
        Next obj
        Console.WriteLine()
    End Sub


    ' Uses the enumerator. 
    ' NOTE: The For Each statement is the preferred way of enumerating the contents of a collection.
    Public Shared Sub PrintValues2(myCol As ROCollection)
        Dim myEnumerator As System.Collections.IEnumerator = myCol.GetEnumerator()
        While myEnumerator.MoveNext()
            Console.WriteLine("   {0}", myEnumerator.Current)
        End While
        Console.WriteLine()
    End Sub

End Class


'This code produces the following output.
'
'Contents of the collection (using For Each):
'   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.

Commenti

L'istruzione foreach del linguaggio C# (for each in Visual Basic) nasconde la complessità degli enumeratori.The foreach statement of the C# language (for each in Visual Basic) hides the complexity of the enumerators. Pertanto, si consiglia l'utilizzo di foreach, anziché la modifica diretta dell'enumeratore.Therefore, using foreach is recommended, instead of directly manipulating the enumerator.

È possibile utilizzare enumeratori per leggere i dati nella raccolta, ma non per modificare la raccolta sottostante.Enumerators can be used to read the data in the collection, but they cannot be used to modify the underlying collection.

Inizialmente l'enumeratore è posizionato davanti al primo elemento della raccolta.Initially, the enumerator is positioned before the first element in the collection. Anche il metodo Reset riporta l'enumeratore in questa posizione.Reset also brings the enumerator back to this position. In questa posizione, la proprietà Current è indefinita.At this position, Current is undefined. Pertanto, è necessario chiamare il metodo MoveNext per spostare in avanti l'enumeratore, in corrispondenza del primo elemento della raccolta, prima di leggere il valore di Current.Therefore, you must call MoveNext to advance the enumerator to the first element of the collection before reading the value of Current.

Current restituisce lo stesso oggetto finché non viene chiamato il metodo MoveNext o Reset.Current returns the same object until either MoveNext or Reset is called. MoveNext imposta Current sull'elemento successivo.MoveNext sets Current to the next element.

Se MoveNext supera la fine della raccolta, l'enumeratore viene posizionato dopo l'ultimo elemento della raccolta e MoveNext restituisce false.If MoveNext passes the end of the collection, the enumerator is positioned after the last element in the collection and MoveNext returns false. Quando l'enumeratore si trova in questa posizione, le chiamate successive a MoveNext restituiscono anche false.When the enumerator is at this position, subsequent calls to MoveNext also return false. Se l'ultima chiamata a MoveNext restituita false, Current non è definito.If the last call to MoveNext returned false, Current is undefined. Per impostare nuovamente la proprietà Current sul primo elemento della raccolta, è possibile chiamare il metodo Reset seguito da MoveNext.To set Current to the first element of the collection again, you can call Reset followed by MoveNext.

Un enumeratore rimane valido finché la raccolta rimane invariata.An enumerator remains valid as long as the collection remains unchanged. In caso di modifiche alla raccolta, ad esempio aggiunta, modifica o eliminazione di elementi, l'enumeratore sarà reso non valido in modo irreversibile e il comportamento corrispondente non sarà definito.If changes are made to the collection, such as adding, modifying, or deleting elements, the enumerator is irrecoverably invalidated and its behavior is undefined.

L'enumeratore non dispone di accesso esclusivo alla raccolta. L'enumerazione di una raccolta non è quindi una procedura thread-safe.The enumerator does not have exclusive access to the collection; therefore, enumerating through a collection is intrinsically not a thread-safe procedure. Per assicurare la protezione del thread durante l'enumerazione, è possibile bloccare la raccolta durante l'intera enumerazione.To guarantee thread safety during enumeration, you can lock the collection during the entire enumeration. Per consentire l'accesso alla raccolta in lettura e scrittura da parte di più thread, è necessario implementare la propria sincronizzazione.To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.

Questo metodo è un'operazione di O(1).This method is an O(1) operation.

Si applica a

Vedi anche