泛型類與特定的實例化

Question

我有一個泛型類：

Public Class MyClass(Of K,V) 

    Private _Dictionary As Dictionary(Of K,V) 

    Sub New 
    _Dictionary = New Dictionary(Of K,V) 
    End Sub 

End Class 

我想超負荷新的K是字符串的具體情況：

Sub New(sc As StringComparer) 
    _Dictionary = New Dictionary(Of String,V)(sc) 
End Sub 

這將導致錯誤：

Dictionary(Of String, V) cannot be converted to Dictionary(Of K, V) 

它看起來並不像我正在以正確的方式進行，但希望我已經設法解釋我想要的這樣做。

Answer 1

你需要做這樣的事情：

Public Class [MyClass](Of K,V) 

    Private _Dictionary As Dictionary(Of K,V) 

    Public Sub New() 
     _Dictionary = New Dictionary(Of K, V) 
    End Sub 

    Public Sub New(comparer As IEqualityComparer(Of K)) 
     _Dictionary = New Dictionary(Of K, V)(comparer) 
    End Sub 

End Class 

'This class now optional as per comment 
Public Class MyStringClass(Of V) 
    Inherits [MyClass](Of String, V) 

    Public Sub New(sc As StringComparer) 
     MyBase.New(sc) 
    End Sub 

End Class 

Answer 2

泛型類型在應用程序運行時即時創建。當您參考GenericClass(Of String, Integer)時，它會將該類的自定義版本加載到內存中。當您稍後參考GenericClass(Of String, Long)時，它會將另一個單獨的自定義類加載到內存中。在加載類之前，您不能訪問任何成員，包括構造函數。在你調用其中一個構造函數之前，你總是必須參考GenericClass(Of String, V)。

有幾個選項可供您使用。沒有一個比原始代碼更可讀。主要問題是您只能通過指定K和V類型來引用該類。

---

方法1：靜態方法

使用共享/靜態Create方法來創建對象。

Public Class GenericClass(Of K, V) 

    Private _Dictionary As Dictionary(Of K, V) 

    Sub New() 
     _Dictionary = New Dictionary(Of K, V) 
    End Sub 

    Public Shared Function Create(sc As StringComparer) As GenericClass(Of String, V) 
     Dim NewObject As New GenericClass(Of String, V) 
     ' TODO: Initialise object. 
     Return NewObject 
    End Function 
End Class 

創建對象有：

Dim sc As StringComparer = New StringComparer() 
Dim MyObject As GenericClass(Of String, ValueType) = GenericClass(Of String, ValueType).Create(sc) 

---

方法2：繼承類

創建一個繼承類，強制執行K類型爲String。

Dim sc As StringComparer = New StringComparer() 
Dim MyObject As New NotSoGenericClass(Of ValueType)(sc) 

或

Dim sc As StringComparer = New StringComparer() 
Dim MyObject As GenericClass(Of String, ValueType) = New NotSoGenericClass(Of ValueType)(sc) 

---

方法3：靜態類

您可以創建一個

Public Class NotSoGenericClass(Of V) 
    Inherits GenericClass(Of String, V) 

    Public Sub New(sc As StringComparer) 
     MyBase.New() 
     ' TODO: Initialise object. 
    End Sub 
End Class 

您與創建對象靜態類（Private Sub New防止它被瞬時化）或包含Create方法的模塊。這個與GenericClass具有相同的名稱，但沒有類型說明符。

Dim sc As StringComparer = New StringComparer() 
Dim MyObject As GenericClass(Of String, ValueType) = GenericClass.Create(Of ValueType)(sc) 

---

方法4：

Public Class GenericClass 

    Private Sub New() 
    End Sub 

    Public Shared Function Create(Of V)(sc As StringComparer) As GenericClass(Of String, V) 
     Dim NewObject As New GenericClass(Of String, V) 
     ' TODO: Initialise object. 
     Return NewObject 
    End Function 
End Class 

您與創建對象拋出一個異常

您可以創建，如果它的正確使用拋出異常的構造器。這裏的問題是，IDE不reccognise此限制，將讓構造函數無效K類型調用。

Sub New(sc As StringComparer) 
    If GetType(K) IsNot GetType(String) Then Throw New InvalidCastException("Type K must be String for this constructor.") 
    _Dictionary = New Dictionary(Of K, V) 
End Sub 

您創建的對象：

Dim sc As StringComparer = New StringComparer() 
Dim MyObject As New GenericClass(Of String, ValueType)(sc) 

但是，這將引發異常：

Dim sc As StringComparer = New StringComparer() 
Dim MyObject As New GenericClass(Of Object, ValueType)(sc)