Vb.net 自定义字典对象？

我想扩展VB.NET dictionary对象，使其不必抱怨某个项已经在关联数组中

这就是我想做的：

    Public Class BetterDictionary
    Inherits System.Collections.Generic.Dictionary(Of Object, Object)

    Public Sub Store(ByRef key As Object, ByVal value As Object)
        If Me.ContainsKey(key) Then
            Me(key) = value
        Else
            Me.Add(key, value)
        End If
    End Sub

End Class

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我现在的问题是：如何替换（of object，object）行以使字典为泛型/可自定义类型？

将继承的类定义为：

Public Class BetterDictionary(Of TKey,TValue)
    Inherits System.Collections.Generic.Dictionary(Of TKey, TValue)