In (Generic Modifier) (Visual Basic)

Artikkeli
09/15/2021

For generic type parameters, the In keyword specifies that the type parameter is contravariant.

Remarks

Contravariance enables you to use a less derived type than that specified by the generic parameter. This allows for implicit conversion of classes that implement variant interfaces and implicit conversion of delegate types.

For more information, see Covariance and Contravariance.

Rules

You can use the In keyword in generic interfaces and delegates.

A type parameter can be declared contravariant in a generic interface or delegate if it is used only as a type of method arguments and not used as a method return type. ByRef parameters cannot be covariant or contravariant.

Covariance and contravariance are supported for reference types and not supported for value types.

In Visual Basic, you cannot declare events in contravariant interfaces without specifying the delegate type. Also, contravariant interfaces cannot have nested classes, enums, or structures, but they can have nested interfaces.

Behavior

An interface that has a contravariant type parameter allows its methods to accept arguments of less derived types than those specified by the interface type parameter. For example, because in .NET Framework 4, in the IComparer<T> interface, type T is contravariant, you can assign an object of the IComparer(Of Person) type to an object of the IComparer(Of Employee) type without using any special conversion methods if Employee inherits from Person.

A contravariant delegate can be assigned another delegate of the same type, but with a less derived generic type parameter.

Example - contravariant generic interface

The following example shows how to declare, extend, and implement a contravariant generic interface. It also shows how you can use implicit conversion for classes that implement this interface.

' Contravariant interface.
Interface IContravariant(Of In A)
End Interface

' Extending contravariant interface.
Interface IExtContravariant(Of In A)
    Inherits IContravariant(Of A)
End Interface

' Implementing contravariant interface.
Class Sample(Of A)
    Implements IContravariant(Of A)
End Class

Sub Main()
    Dim iobj As IContravariant(Of Object) = New Sample(Of Object)()
    Dim istr As IContravariant(Of String) = New Sample(Of String)()

    ' You can assign iobj to istr, because
    ' the IContravariant interface is contravariant.
    istr = iobj
End Sub

Example - contravariant generic delegate

The following example shows how to declare, instantiate, and invoke a contravariant generic delegate. It also shows how you can implicitly convert a delegate type.

' Contravariant delegate.
Public Delegate Sub DContravariant(Of In A)(ByVal argument As A)

' Methods that match the delegate signature.
Public Shared Sub SampleControl(ByVal control As Control)
End Sub

Public Shared Sub SampleButton(ByVal control As Button)
End Sub

Private Sub Test()

    ' Instantiating the delegates with the methods.
    Dim dControl As DContravariant(Of Control) =
        AddressOf SampleControl
    Dim dButton As DContravariant(Of Button) =
        AddressOf SampleButton

    ' You can assign dControl to dButton
    ' because the DContravariant delegate is contravariant.
    dButton = dControl

    ' Invoke the delegate.
    dButton(New Button())
End Sub

Jaa