在泛型集合的接口中使用变体 (Visual Basic)
协变接口允许其方法返回的派生类型多于接口中指定的派生类型。 逆变接口允许其方法接受派生类型少于接口中指定的类型的参数。
在.NET Framework 4 中，多个现有接口已变为协变和逆变接口。 包括 IEnumerable<T> 和 IComparable<T>。 这使你可将对基类型的泛型集合进行操作的那些方法重用于派生类型的集合。
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
下例阐释了 IEnumerable<T> 接口中的协变支持的益处。 PrintFullName 方法接受 IEnumerable(Of Person) 类型的集合作为参数。 但可将该方法重用于 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
下例阐释了 IComparer<T> 接口中的逆变支持的益处。 PersonComparer 类实现 IComparer(Of Person) 接口。 但可以重用此类来比较 Employee 类型的对象序列，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 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
