covariant 介面可讓其方法傳回的衍生類型比介面中指定的類型還要多。 Contravariant 介面可讓其方法接受的衍生型別參數，比介面中指定的參數還要少。
在 .NET Framework 4 中，數個現有的介面會變成共變數和反變數。 這些包括 IEnumerable<T> 和 IComparable<T>。 這可讓您將用於基底型別的泛型集合的方法重複用於衍生型別的集合。
如需 .NET Framework 中的變體介面清單，請參閱泛型介面中的變異數（Visual Basic）。
轉換泛型集合
下列範例說明 介面中 IEnumerable<T> 共變數支援的優點。 方法 PrintFullName 接受型別的 IEnumerable(Of Person) 集合做為參數。 不過，您可以針對型別的 IEnumerable(Of Person) 集合重複使用它，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
下列範例說明介面中 IComparer<T> 反變數支援的優點。
PersonComparer 類別會實作 IComparer(Of Person) 介面。 不過，您可以重複使用這個類別來比較型別的物件 Employee 序列，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
