Question

如果控件与多个接口匹配，我有一个通用的扩展方法。我可以直接在支持两个接口的控件上调用此方法......

Public Function DisplayValue(Of T As {IChoiceControl, IDataSourceControl})(ByVal vctl As T) As String
End Function

Dim pctl As CustomControl = ... //implements above interfaces
Dim pstrDisplayed As String = pctl.DisplayValue() // works

...但是如何构建一个我确定匹配两个接口的对象，以便我可以调用该方法？

Dim pobj As Object = ...
If TypeOf pobj Is IChoiceControl AndAlso TypeOf pobj Is IDataSourceControl Then
    pstrDisplayed = CType(pobj, ?).DisplayValue()
End If

编辑：

定义复合界面（完全如图所示）可以正常工作。感谢。

Public Interface IChoiceDataSourceControl
    Inherits IChoiceControl, IDataSourceControl 
End Interface

If TypeOf pobj Is IChoiceDataSourceControl Then
    pstrDisplayed = CType(pobj, IChoiceDataSourceControl).DisplayValue()
End If

Answer 1

在C＃中，您可以使用dynamic，但是您必须放弃用户友好的扩展方法类似实例的调用：
```
MyExtensionClass.DisplayValue((dynamic)pobj)
```
不幸的是，VB.NET中没有dynamic（除非你使用Option Strict Off，这使Object表现得像dynamic）。

您可以创建包装器：

Class Wrapper
    Implements IChoiceControl
    Implements IDataSourceControl

    Private _choiceControl As IChoiceControl
    Private _dataSource As IDataSourceControl

    Public Sub New(obj As Object)
        _choiceControl = CType(obj, IChoiceControl)
        _dataSource = CType(obj, IDataSourceControl)
    End Sub

    '' Delegate all IChoiceControl methods to _choiceControl
    '' and IDataSourceControl methods to _dataSource
End Class

并使用它：

Dim both As New Both()
Dim bothAsObject = CType(both, Object)
Dim wrappedBothAsObject = New Wrapper(bothAsObject)
wrappedBothAsObject.DisplayValue()

或者您可以创建另一个实现IChoiceControl和IDataSourceControl的接口，实现它并将其用作扩展方法的通用约束。

Answer 2

定义一个从两个接口继承的接口并使你的扩展方法where子句引用它可能更简单：

Public Interface IChoosableDataSourceableThinger 
    Inherits IChoiceControl, IDataSourceControl 
    Public Function DisplayValue As String
End Interface

扩展名：

Public Function DisplayValue(Of T As IChoosableDataSourceableThinger)(ByVal vctl As T) As String

演员：

If TypeOf pobj Is IChoosableDataSourceableThinger Then
    pstrDisplayed = CType(pobj, IChoosableDataSourceableThinger).DisplayValue()
End If

在.NET中调用匹配多个接口的泛型方法？

2 个答案: