KeyedCollection<TKey,TItem>.Item[TKey] Proprietà
Definizione
Importante
Alcune informazioni sono relative alla release non definitiva del prodotto, che potrebbe subire modifiche significative prima della release definitiva. Microsoft non riconosce alcuna garanzia, espressa o implicita, in merito alle informazioni qui fornite.
Ottiene l'elemento con la chiave specificata.
public:
property TItem default[TKey] { TItem get(TKey key); };public TItem this[TKey key] { get; }member this.Item('Key) : 'ItemDefault Public ReadOnly Property Item(key As TKey) As TItemParametri
- key
- TKey
Chiave dell'elemento da ottenere.
Valore della proprietà
TItem
Elemento con la chiave specificata. Se un elemento con la chiave specificata non viene trovato, viene generata un'eccezione.
Eccezioni
key è null.
Un elemento con la chiave specificata non esiste nella raccolta.
Esempio
Questo esempio di codice mostra il codice minimo necessario per derivare una classe di raccolta da KeyedCollection<TKey,TItem>: l'override del GetKeyForItem metodo e la fornitura di un costruttore pubblico che delega a un costruttore della classe base. L'esempio di codice illustra anche molte delle proprietà e dei metodi ereditati dalle KeyedCollection<TKey,TItem> classi e Collection<T> .
L'esempio di codice chiama sia la KeyedCollection<TKey,TItem>.Item[] proprietà , di sola lettura che recupera per chiave, sia la Collection<T>.Item[] proprietà , che è impostabile e recupera in base all'indice. Illustra come accedere alla seconda proprietà quando gli oggetti della raccolta derivata hanno chiavi integer, indistinguibili dagli interi utilizzati per il recupero indicizzato.
La SimpleOrder classe è un elenco di richieste molto semplice che contiene OrderItem oggetti, ognuno dei quali rappresenta un elemento di riga nell'ordine. La chiave di OrderItem è non modificabile, una considerazione importante per le classi che derivano da KeyedCollection<TKey,TItem>. Per un esempio di codice che usa chiavi modificabili, vedere ChangeItemKey.
using namespace System;
using namespace System::Collections::Generic;
using namespace System::Collections::ObjectModel;
// This class represents a simple line item in an order. All the
// values are immutable except quantity.
//
public ref class OrderItem
{
private:
int _quantity;
public:
initonly int PartNumber;
initonly String^ Description;
initonly double UnitPrice;
OrderItem(int partNumber, String^ description,
int quantity, double unitPrice)
{
this->PartNumber = partNumber;
this->Description = description;
this->Quantity = quantity;
this->UnitPrice = unitPrice;
}
property int Quantity
{
int get() { return _quantity; }
void set(int value)
{
if (value < 0)
throw gcnew ArgumentException("Quantity cannot be negative.");
_quantity = value;
}
}
virtual String^ ToString() override
{
return String::Format(
"{0,9} {1,6} {2,-12} at {3,8:#,###.00} = {4,10:###,###.00}",
PartNumber, _quantity, Description, UnitPrice,
UnitPrice * _quantity);
}
};
// This class represents a very simple keyed list of OrderItems,
// inheriting most of its behavior from the KeyedCollection and
// Collection classes. The immediate base class is the constructed
// type KeyedCollection<int, OrderItem>. When you inherit
// from KeyedCollection, the second generic type argument is the
// type that you want to store in the collection -- in this case
// OrderItem. The first type argument is the type that you want
// to use as a key. Its values must be calculated from OrderItem;
// in this case it is the int field PartNumber, so SimpleOrder
// inherits KeyedCollection<int, OrderItem>.
//
public ref class SimpleOrder : KeyedCollection<int, OrderItem^>
{
// The parameterless constructor of the base class creates a
// KeyedCollection with an internal dictionary. For this code
// example, no other constructors are exposed.
//
public:
SimpleOrder() {}
// This is the only method that absolutely must be overridden,
// because without it the KeyedCollection cannot extract the
// keys from the items. The input parameter type is the
// second generic type argument, in this case OrderItem, and
// the return value type is the first generic type argument,
// in this case int.
//
protected:
virtual int GetKeyForItem(OrderItem^ item) override
{
// In this example, the key is the part number.
return item->PartNumber;
}
};
public ref class Demo
{
public:
static void Main()
{
SimpleOrder^ weekly = gcnew SimpleOrder();
// The Add method, inherited from Collection, takes OrderItem.
//
weekly->Add(gcnew OrderItem(110072674, "Widget", 400, 45.17));
weekly->Add(gcnew OrderItem(110072675, "Sprocket", 27, 5.3));
weekly->Add(gcnew OrderItem(101030411, "Motor", 10, 237.5));
weekly->Add(gcnew OrderItem(110072684, "Gear", 175, 5.17));
Display(weekly);
// The Contains method of KeyedCollection takes the key,
// type, in this case int.
//
Console::WriteLine("\nContains(101030411): {0}",
weekly->Contains(101030411));
// The default Item property of KeyedCollection takes a key.
//
Console::WriteLine("\nweekly(101030411)->Description: {0}",
weekly[101030411]->Description);
// The Remove method of KeyedCollection takes a key.
//
Console::WriteLine("\nRemove(101030411)");
weekly->Remove(101030411);
Display(weekly);
// The Insert method, inherited from Collection, takes an
// index and an OrderItem.
//
Console::WriteLine("\nInsert(2, New OrderItem(...))");
weekly->Insert(2, gcnew OrderItem(111033401, "Nut", 10, .5));
Display(weekly);
// The default Item property is overloaded. One overload comes
// from KeyedCollection<int, OrderItem>; that overload
// is read-only, and takes Integer because it retrieves by key.
// The other overload comes from Collection<OrderItem>, the
// base class of KeyedCollection<int, OrderItem>; it
// retrieves by index, so it also takes an Integer. The compiler
// uses the most-derived overload, from KeyedCollection, so the
// only way to access SimpleOrder by index is to cast it to
// Collection<OrderItem>. Otherwise the index is interpreted
// as a key, and KeyNotFoundException is thrown.
//
Collection<OrderItem^>^ coweekly = weekly;
Console::WriteLine("\ncoweekly[2].Description: {0}",
coweekly[2]->Description);
Console::WriteLine("\ncoweekly[2] = gcnew OrderItem(...)");
coweekly[2] = gcnew OrderItem(127700026, "Crank", 27, 5.98);
OrderItem^ temp = coweekly[2];
// The IndexOf method inherited from Collection<OrderItem>
// takes an OrderItem instead of a key
//
Console::WriteLine("\nIndexOf(temp): {0}", weekly->IndexOf(temp));
// The inherited Remove method also takes an OrderItem.
//
Console::WriteLine("\nRemove(temp)");
weekly->Remove(temp);
Display(weekly);
Console::WriteLine("\nRemoveAt(0)");
weekly->RemoveAt(0);
Display(weekly);
}
private:
static void Display(SimpleOrder^ order)
{
Console::WriteLine();
for each( OrderItem^ item in order )
{
Console::WriteLine(item);
}
}
};
void main()
{
Demo::Main();
}
/* This code example produces the following output:
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
101030411 10 Motor at 237.50 = 2,375.00
110072684 175 Gear at 5.17 = 904.75
Contains(101030411): True
weekly(101030411)->Description: Motor
Remove(101030411)
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
110072684 175 Gear at 5.17 = 904.75
Insert(2, New OrderItem(...))
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
111033401 10 Nut at .50 = 5.00
110072684 175 Gear at 5.17 = 904.75
coweekly(2)->Description: Nut
coweekly[2] = gcnew OrderItem(...)
IndexOf(temp): 2
Remove(temp)
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
110072684 175 Gear at 5.17 = 904.75
RemoveAt(0)
110072675 27 Sprocket at 5.30 = 143.10
110072684 175 Gear at 5.17 = 904.75
*/
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
// This class represents a very simple keyed list of OrderItems,
// inheriting most of its behavior from the KeyedCollection and
// Collection classes. The immediate base class is the constructed
// type KeyedCollection<int, OrderItem>. When you inherit
// from KeyedCollection, the second generic type argument is the
// type that you want to store in the collection -- in this case
// OrderItem. The first type argument is the type that you want
// to use as a key. Its values must be calculated from OrderItem;
// in this case it is the int field PartNumber, so SimpleOrder
// inherits KeyedCollection<int, OrderItem>.
//
public class SimpleOrder : KeyedCollection<int, OrderItem>
{
// This is the only method that absolutely must be overridden,
// because without it the KeyedCollection cannot extract the
// keys from the items. The input parameter type is the
// second generic type argument, in this case OrderItem, and
// the return value type is the first generic type argument,
// in this case int.
//
protected override int GetKeyForItem(OrderItem item)
{
// In this example, the key is the part number.
return item.PartNumber;
}
}
public class Demo
{
public static void Main()
{
SimpleOrder weekly = new SimpleOrder();
// The Add method, inherited from Collection, takes OrderItem.
//
weekly.Add(new OrderItem(110072674, "Widget", 400, 45.17));
weekly.Add(new OrderItem(110072675, "Sprocket", 27, 5.3));
weekly.Add(new OrderItem(101030411, "Motor", 10, 237.5));
weekly.Add(new OrderItem(110072684, "Gear", 175, 5.17));
Display(weekly);
// The Contains method of KeyedCollection takes the key,
// type, in this case int.
//
Console.WriteLine("\nContains(101030411): {0}",
weekly.Contains(101030411));
// The default Item property of KeyedCollection takes a key.
//
Console.WriteLine("\nweekly[101030411].Description: {0}",
weekly[101030411].Description);
// The Remove method of KeyedCollection takes a key.
//
Console.WriteLine("\nRemove(101030411)");
weekly.Remove(101030411);
Display(weekly);
// The Insert method, inherited from Collection, takes an
// index and an OrderItem.
//
Console.WriteLine("\nInsert(2, New OrderItem(...))");
weekly.Insert(2, new OrderItem(111033401, "Nut", 10, .5));
Display(weekly);
// The default Item property is overloaded. One overload comes
// from KeyedCollection<int, OrderItem>; that overload
// is read-only, and takes Integer because it retrieves by key.
// The other overload comes from Collection<OrderItem>, the
// base class of KeyedCollection<int, OrderItem>; it
// retrieves by index, so it also takes an Integer. The compiler
// uses the most-derived overload, from KeyedCollection, so the
// only way to access SimpleOrder by index is to cast it to
// Collection<OrderItem>. Otherwise the index is interpreted
// as a key, and KeyNotFoundException is thrown.
//
Collection<OrderItem> coweekly = weekly;
Console.WriteLine("\ncoweekly[2].Description: {0}",
coweekly[2].Description);
Console.WriteLine("\ncoweekly[2] = new OrderItem(...)");
coweekly[2] = new OrderItem(127700026, "Crank", 27, 5.98);
OrderItem temp = coweekly[2];
// The IndexOf method inherited from Collection<OrderItem>
// takes an OrderItem instead of a key
//
Console.WriteLine("\nIndexOf(temp): {0}", weekly.IndexOf(temp));
// The inherited Remove method also takes an OrderItem.
//
Console.WriteLine("\nRemove(temp)");
weekly.Remove(temp);
Display(weekly);
Console.WriteLine("\nRemoveAt(0)");
weekly.RemoveAt(0);
Display(weekly);
}
private static void Display(SimpleOrder order)
{
Console.WriteLine();
foreach( OrderItem item in order )
{
Console.WriteLine(item);
}
}
}
// This class represents a simple line item in an order. All the
// values are immutable except quantity.
//
public class OrderItem
{
public readonly int PartNumber;
public readonly string Description;
public readonly double UnitPrice;
private int _quantity = 0;
public OrderItem(int partNumber, string description,
int quantity, double unitPrice)
{
this.PartNumber = partNumber;
this.Description = description;
this.Quantity = quantity;
this.UnitPrice = unitPrice;
}
public int Quantity
{
get { return _quantity; }
set
{
if (value<0)
throw new ArgumentException("Quantity cannot be negative.");
_quantity = value;
}
}
public override string ToString()
{
return String.Format(
"{0,9} {1,6} {2,-12} at {3,8:#,###.00} = {4,10:###,###.00}",
PartNumber, _quantity, Description, UnitPrice,
UnitPrice * _quantity);
}
}
/* This code example produces the following output:
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
101030411 10 Motor at 237.50 = 2,375.00
110072684 175 Gear at 5.17 = 904.75
Contains(101030411): True
weekly[101030411].Description: Motor
Remove(101030411)
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
110072684 175 Gear at 5.17 = 904.75
Insert(2, New OrderItem(...))
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
111033401 10 Nut at .50 = 5.00
110072684 175 Gear at 5.17 = 904.75
coweekly[2].Description: Nut
coweekly[2] = new OrderItem(...)
IndexOf(temp): 2
Remove(temp)
110072674 400 Widget at 45.17 = 18,068.00
110072675 27 Sprocket at 5.30 = 143.10
110072684 175 Gear at 5.17 = 904.75
RemoveAt(0)
110072675 27 Sprocket at 5.30 = 143.10
110072684 175 Gear at 5.17 = 904.75
*/
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
' This class represents a very simple keyed list of OrderItems,
' inheriting most of its behavior from the KeyedCollection and
' Collection classes. The immediate base class is the constructed
' type KeyedCollection(Of Integer, OrderItem). When you inherit
' from KeyedCollection, the second generic type argument is the
' type that you want to store in the collection -- in this case
' OrderItem. The first generic argument is the type that you want
' to use as a key. Its values must be calculated from OrderItem;
' in this case it is the Integer field PartNumber, so SimpleOrder
' inherits KeyedCollection(Of Integer, OrderItem).
'
Public Class SimpleOrder
Inherits KeyedCollection(Of Integer, OrderItem)
' This is the only method that absolutely must be overridden,
' because without it the KeyedCollection cannot extract the
' keys from the items. The input parameter type is the
' second generic type argument, in this case OrderItem, and
' the return value type is the first generic type argument,
' in this case Integer.
'
Protected Overrides Function GetKeyForItem( _
ByVal item As OrderItem) As Integer
' In this example, the key is the part number.
Return item.PartNumber
End Function
End Class
Public Class Demo
Public Shared Sub Main()
Dim weekly As New SimpleOrder()
' The Add method, inherited from Collection, takes OrderItem.
'
weekly.Add(New OrderItem(110072674, "Widget", 400, 45.17))
weekly.Add(New OrderItem(110072675, "Sprocket", 27, 5.3))
weekly.Add(New OrderItem(101030411, "Motor", 10, 237.5))
weekly.Add(New OrderItem(110072684, "Gear", 175, 5.17))
Display(weekly)
' The Contains method of KeyedCollection takes TKey.
'
Console.WriteLine(vbLf & "Contains(101030411): {0}", _
weekly.Contains(101030411))
' The default Item property of KeyedCollection takes the key
' type, Integer.
'
Console.WriteLine(vbLf & "weekly(101030411).Description: {0}", _
weekly(101030411).Description)
' The Remove method of KeyedCollection takes a key.
'
Console.WriteLine(vbLf & "Remove(101030411)")
weekly.Remove(101030411)
Display(weekly)
' The Insert method, inherited from Collection, takes an
' index and an OrderItem.
'
Console.WriteLine(vbLf & "Insert(2, New OrderItem(...))")
weekly.Insert(2, New OrderItem(111033401, "Nut", 10, .5))
Display(weekly)
' The default Item property is overloaded. One overload comes
' from KeyedCollection(Of Integer, OrderItem); that overload
' is read-only, and takes Integer because it retrieves by key.
' The other overload comes from Collection(Of OrderItem), the
' base class of KeyedCollection(Of Integer, OrderItem); it
' retrieves by index, so it also takes an Integer. The compiler
' uses the most-derived overload, from KeyedCollection, so the
' only way to access SimpleOrder by index is to cast it to
' Collection(Of OrderItem). Otherwise the index is interpreted
' as a key, and KeyNotFoundException is thrown.
'
Dim coweekly As Collection(Of OrderItem) = weekly
Console.WriteLine(vbLf & "coweekly(2).Description: {0}", _
coweekly(2).Description)
Console.WriteLine(vbLf & "coweekly(2) = New OrderItem(...)")
coweekly(2) = New OrderItem(127700026, "Crank", 27, 5.98)
Dim temp As OrderItem = coweekly(2)
' The IndexOf method, inherited from Collection(Of OrderItem),
' takes an OrderItem instead of a key.
'
Console.WriteLine(vbLf & "IndexOf(temp): {0}", _
weekly.IndexOf(temp))
' The inherited Remove method also takes an OrderItem.
'
Console.WriteLine(vbLf & "Remove(temp)")
weekly.Remove(temp)
Display(weekly)
Console.WriteLine(vbLf & "RemoveAt(0)")
weekly.RemoveAt(0)
Display(weekly)
End Sub
Private Shared Sub Display(ByVal order As SimpleOrder)
Console.WriteLine()
For Each item As OrderItem In order
Console.WriteLine(item)
Next item
End Sub
End Class
' This class represents a simple line item in an order. All the
' values are immutable except quantity.
'
Public Class OrderItem
Public ReadOnly PartNumber As Integer
Public ReadOnly Description As String
Public ReadOnly UnitPrice As Double
Private _quantity As Integer = 0
Public Sub New(ByVal partNumber As Integer, _
ByVal description As String, _
ByVal quantity As Integer, _
ByVal unitPrice As Double)
Me.PartNumber = partNumber
Me.Description = description
Me.Quantity = quantity
Me.UnitPrice = unitPrice
End Sub
Public Property Quantity() As Integer
Get
Return _quantity
End Get
Set
If value < 0 Then
Throw New ArgumentException("Quantity cannot be negative.")
End If
_quantity = value
End Set
End Property
Public Overrides Function ToString() As String
Return String.Format( _
"{0,9} {1,6} {2,-12} at {3,8:#,###.00} = {4,10:###,###.00}", _
PartNumber, _quantity, Description, UnitPrice, _
UnitPrice * _quantity)
End Function
End Class
' This code example produces the following output:
'
'110072674 400 Widget at 45.17 = 18,068.00
'110072675 27 Sprocket at 5.30 = 143.10
'101030411 10 Motor at 237.50 = 2,375.00
'110072684 175 Gear at 5.17 = 904.75
'
'Contains(101030411): True
'
'weekly(101030411).Description: Motor
'
'Remove(101030411)
'
'110072674 400 Widget at 45.17 = 18,068.00
'110072675 27 Sprocket at 5.30 = 143.10
'110072684 175 Gear at 5.17 = 904.75
'
'Insert(2, New OrderItem(...))
'
'110072674 400 Widget at 45.17 = 18,068.00
'110072675 27 Sprocket at 5.30 = 143.10
'111033401 10 Nut at .50 = 5.00
'110072684 175 Gear at 5.17 = 904.75
'
'coweekly(2).Description: Nut
'
'coweekly(2) = New OrderItem(...)
'
'IndexOf(temp): 2
'
'Remove(temp)
'
'110072674 400 Widget at 45.17 = 18,068.00
'110072675 27 Sprocket at 5.30 = 143.10
'110072684 175 Gear at 5.17 = 904.75
'
'RemoveAt(0)
'
'110072675 27 Sprocket at 5.30 = 143.10
'110072684 175 Gear at 5.17 = 904.75
Commenti
Questa proprietà consente di accedere a un elemento specifico dell'insieme utilizzando la sintassi seguente: myCollection[key] (myCollection(key) in Visual Basic).
Nota
Questa proprietà è distinta dalla proprietà ereditata Collection<T>.Item[] , che ottiene e imposta gli elementi in base all'indice numerico. Tuttavia, se TKey è di tipo Int32, questa proprietà maschera la proprietà ereditata. In tal caso, è possibile accedere alla proprietà ereditata eseguendo il cast di al KeyedCollection<TKey,TItem> relativo tipo di base. Ad esempio, KeyedCollection<int, MyType> (KeyedCollection(Of Integer, MyType) in Visual Basic, KeyedCollection<int, MyType^> in C++) può essere eseguito il cast a Collection<MyType> (Collection(Of MyType) in Visual Basic, Collection<MyType^> in C++).
Se ha KeyedCollection<TKey,TItem> un dizionario di ricerca, key viene usato per recuperare l'elemento dal dizionario. Se non è presente alcun dizionario di ricerca, la chiave di ogni elemento viene estratta usando il GetKeyForItem metodo e confrontata con la chiave specificata.
Il linguaggio C# usa la parola chiave per definire gli indicizzatori anziché implementare la Item[] proprietà . Visual Basic implementa Item[] come proprietà predefinita, che fornisce la stessa funzionalità di indicizzazione.
Il recupero del valore di questa proprietà è un'operazione O(1) se ha KeyedCollection<TKey,TItem> un dizionario di ricerca; in caso contrario, è un'operazione O(n), dove n è Count.
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