协变接口允许其方法返回的派生类型比接口中指定的类型多。 逆变接口允许其方法接受的派生类型的参数比接口中指定的参数少。
在 .NET Framework 4 中，多个现有接口变为协变和逆变。 包括 IEnumerable<T> 和 IComparable<T>。 这使您能够将操作基类型泛型集合的方法重复用于派生类型集合。
有关 .NET Framework 中变体接口的列表，请参阅泛型接口中的变体（Visual Basic）。
转换泛型集合
下面的示例演示了接口中 IEnumerable<T> 协变支持的好处。 该方法 PrintFullName 接受类型集合 IEnumerable(Of Person) 作为参数。 但是，你可以将其重新用于IEnumerable(Of Person)类型的集合，因为Employee继承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
比较泛型集合
以下示例演示了接口中 IComparer<T> 逆变支持的优点。
PersonComparer 类实现 IComparer(Of Person) 接口。 但是，可以重复使用这个类来比较一系列Employee类型的对象，因为Employee继承自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
