泛型类与特定的实例化

Question

我有一个泛型类：

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 

我想超负荷新的K是字符串的具体情况：

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

这将导致错误：

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

它看起来并不像我正在以正确的方式进行，但希望我已经设法解释我想要的这样做。

Answer 1

你需要做这样的事情：

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

泛型类型在应用程序运行时即时创建。当您参考GenericClass(Of String, Integer)时，它会将该类的自定义版本加载到内存中。当您稍后参考GenericClass(Of String, Long)时，它会将另一个单独的自定义类加载到内存中。在加载类之前，您不能访问任何成员，包括构造函数。在你调用其中一个构造函数之前，你总是必须参考GenericClass(Of String, V)。

有几个选项可供您使用。没有一个比原始代码更可读。主要问题是您只能通过指定K和V类型来引用该类。

---

方法1：静态方法

使用共享/静态Create方法来创建对象。

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 

创建对象有：

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

---

方法2：继承类

创建一个继承类，强制执行K类型为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：静态类

您可以创建一个

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 

您与创建对象静态类（Private Sub New防止它被瞬时化）或包含Create方法的模块。这个与GenericClass具有相同的名称，但没有类型说明符。

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 

您与创建对象抛出一个异常

您可以创建，如果它的正确使用抛出异常的构造器。这里的问题是，IDE不reccognise此限制，将让构造函数无效K类型调用。

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 

您创建的对象：

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

但是，这将引发异常：

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