Utilizzo della varianza nelle interfacce per le raccolte generiche (C# e Visual Basic)
Un'interfaccia covariante consente ai metodi di restituire tipi più derivati di quelli specificati nell'interfaccia.Un'interfaccia controvariante consente ai metodi di accettare parametri di tipi meno derivati di quelli specificati nell'interfaccia.
In .NET Framework 4, diverse interfacce esistenti diventano covarianti e controvarianti,tra cui IEnumerable<T> e IComparable<T>.In questo modo è possibile riutilizzare i metodi che operano con raccolte generiche di tipi di base per raccolte di tipi derivati.
Per un elenco di interfacce variant in .NET Framework, vedere Varianza nelle interfacce generiche (C# e Visual Basic).
Conversione di raccolte generiche
Nell'esempio seguente vengono illustrati i vantaggi del supporto della covarianza nell'interfaccia IEnumerable<T>.Il metodo PrintFullName accetta una raccolta del tipo IEnumerable<Person> (IEnumerable(Of Person) in Visual Basic) come parametro.È tuttavia possibile riutilizzarlo per una raccolta del tipo IEnumerable<Employee> (IEnumerable(Of Person) in Visual Basic) perché Employee eredita Person.
' Simple hierarchy of classes.
Public Class Person
Public Property FirstName As String
Public Property LastName As String
End Class
Public Class Employee
Inherits Person
End Class
' The method has a parameter of the IEnumerable(Of Person) type.
Public Sub PrintFullName(ByVal persons As IEnumerable(Of Person))
For Each person As Person In persons
Console.WriteLine(
"Name: " & person.FirstName & " " & person.LastName)
Next
End Sub
Sub Main()
Dim employees As IEnumerable(Of Employee) = New List(Of Employee)
' You can pass IEnumerable(Of Employee),
' although the method expects IEnumerable(Of Person).
PrintFullName(employees)
End Sub
// Simple hierarchy of classes.
public class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
public class Employee : Person { }
class Program
{
// The method has a parameter of the IEnumerable<Person> type.
public static void PrintFullName(IEnumerable<Person> persons)
{
foreach (Person person in persons)
{
Console.WriteLine("Name: {0} {1}",
person.FirstName, person.LastName);
}
}
public static void Test()
{
IEnumerable<Employee> employees = new List<Employee>();
// You can pass IEnumerable<Employee>,
// although the method expects IEnumerable<Person>.
PrintFullName(employees);
}
}
Confronto di raccolte generiche
Nell'esempio seguente vengono illustrati i vantaggi del supporto della controvarianza nell'interfaccia IComparer<T>.La classe PersonComparer implementa l'interfaccia IComparer<Person> (IComparer(Of Person) in Visual Basic).È tuttavia possibile riutilizzare questa classe per confrontare una sequenza di oggetti del tipo Employee perché Employee eredita Person.
' Simple hierarhcy of classes.
Public Class Person
Public Property FirstName As String
Public Property LastName As String
End Class
Public Class Employee
Inherits Person
End Class
' The custom comparer for the Person type
' with standard implementations of Equals()
' and GetHashCode() methods.
Class PersonComparer
Implements IEqualityComparer(Of Person)
Public Function Equals1(
ByVal x As Person,
ByVal y As Person) As Boolean _
Implements IEqualityComparer(Of Person).Equals
If x Is y Then Return True
If x Is Nothing OrElse y Is Nothing Then Return False
Return (x.FirstName = y.FirstName) AndAlso
(x.LastName = y.LastName)
End Function
Public Function GetHashCode1(
ByVal person As Person) As Integer _
Implements IEqualityComparer(Of Person).GetHashCode
If person Is Nothing Then Return 0
Dim hashFirstName =
If(person.FirstName Is Nothing,
0, person.FirstName.GetHashCode())
Dim hashLastName = person.LastName.GetHashCode()
Return hashFirstName Xor hashLastName
End Function
End Class
Sub Main()
Dim employees = New List(Of Employee) From {
New Employee With {.FirstName = "Michael", .LastName = "Alexander"},
New Employee With {.FirstName = "Jeff", .LastName = "Price"}
}
' You can pass PersonComparer,
' which implements IEqualityComparer(Of Person),
' although the method expects IEqualityComparer(Of Employee)
Dim noduplicates As IEnumerable(Of Employee) = employees.Distinct(New PersonComparer())
For Each employee In noduplicates
Console.WriteLine(employee.FirstName & " " & employee.LastName)
Next
End Sub
// Simple hierarchy of classes.
public class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
public class Employee : Person { }
// The custom comparer for the Person type
// with standard implementations of Equals()
// and GetHashCode() methods.
class PersonComparer : IEqualityComparer<Person>
{
public bool Equals(Person x, Person y)
{
if (Object.ReferenceEquals(x, y)) return true;
if (Object.ReferenceEquals(x, null) ||
Object.ReferenceEquals(y, null))
return false;
return x.FirstName == y.FirstName && x.LastName == y.LastName;
}
public int GetHashCode(Person person)
{
if (Object.ReferenceEquals(person, null)) return 0;
int hashFirstName = person.FirstName == null
? 0 : person.FirstName.GetHashCode();
int hashLastName = person.LastName.GetHashCode();
return hashFirstName ^ hashLastName;
}
}
class Program
{
public static void Test()
{
List<Employee> employees = new List<Employee> {
new Employee() {FirstName = "Michael", LastName = "Alexander"},
new Employee() {FirstName = "Jeff", LastName = "Price"}
};
// You can pass PersonComparer,
// which implements IEqualityComparer<Person>,
// although the method expects IEqualityComparer<Employee>.
IEnumerable<Employee> noduplicates =
employees.Distinct<Employee>(new PersonComparer());
foreach (var employee in noduplicates)
Console.WriteLine(employee.FirstName + " " + employee.LastName);
}
}