使用泛型集合介面中的變異數 (Visual Basic)
Covariant 介面允許其方法傳回與介面中指定的類型相比，其衍生程度較大的類型。 Contravariant 介面允許其方法接受與介面中指定的參數相比，其類型衍生程度較小的參數。
在 .NET Framework 4 中，有數個現有介面已變成 Covariant 和 Contravariant。 這些結構包括 IEnumerable<T> 及 IComparable<T>。 因此，您可以針對衍生類型的集合，重複使用搭配基底類型之泛型集合運作的方法。
如需 .NET Framework 的變體介面清單，請參閱泛型介面的變異數 (Visual Basic)。
轉換泛型集合
下列範例說明在 IEnumerable<T> 介面中支援共變數的好處。 PrintFullName 方法接受 IEnumerable(Of Person) 類型的集合作為參數。 不過，您可以重複用於 IEnumerable(Of Person) 類型的集合，因為 Employee 會繼承 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
比較泛型集合
下列範例說明在 IComparer<T> 介面中支援反變數的好處。 PersonComparer 類別會實作 IComparer(Of Person) 介面。 不過，您可以重複使用此類別來比較一連串類型為 Employee 的物件，因為 Employee 會繼承 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 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
