VB.NET無法轉換類接口

Question

我有一段簡單的代碼如下：

Public Interface ITest(Of T) 
    Function getObj() As T 
End Interface 

Public Class clsTest(Of T As {IOrder, New}) 
    Implements ITest(Of T) 
    Public Function getObj() As T Implements ITest(Of T).getObj 
     Return New T 
    End Function 
End Class 

Public Interface IOrder 
    Property Name As String 
End Interface 

Public Class clsOrder 
    Implements IOrder 
    Public Property Name As String Implements IOrder.Name 
End Class 

當我創建另clsTest對象，如：

Dim test As ITest(Of IOrder) = New clsTest(Of clsOrder) 

我得到了以下錯誤：

'ConsoleApplication1.clsTest(Of ConsoleApplication1.clsOrder)' cannot be converted to 'ConsoleApplication1.ITest(Of ConsoleApplication1.IOrder)'. Consider changing the 'T' in the definition of 'Interface ITest(Of T)' to an Out type parameter, 'Out T'.

任何人都知道原因？如果我確實要將測試作爲ITest（IOrder）申報，我該如何使其工作？謝謝。

Answer 1

製作接口協在T與Out-keyword：

Public Interface ITest(Of Out T) 
    Function getObj() As T 
End Interface 

，那麼這將編譯：

Dim test As ITest(Of IOrder) = New clsTest(Of clsOrder) 

E.利珀特可以解釋它比我好：

• Implementing nested generic Interfaces