實現具有可以是不同類型的值屬性的單元類

Question

我試圖設計一個包含行和單元的表類型結構。我打算單元類有一個值屬性。我試圖找出如何創建一個可以返回一些定義的不同類型的值，例如integer，single，date，string。我希望單元格的值能被強類型化，但我不確定如何最好地實現這個功能。

我在代碼的思想至今：

Public Class Cell 
    Private _value as object 
    Public Property Value as Object // How can I define this so that it return a Type 
    Get        // e.g. integer, string, etc 
     Return _value 
    End Get 
    Set(ByVal value as object) 
     _value = value 
    End Set 
End Class 

Public Class Row 
    Dim _Cells as New List(Of Cell) 
    Public Function AddCell(ByVal c as Cell) as Cell 
    _Cells.Add(c) 
    Return _Cells(_Cells.count - 1) 
    End Function 

Answer 1

這裏使用基本的繼承，甚至沒有泛型一個簡單的解決方案。這個想法是，你有一個單一的所有單元格的父類。每個列類型還有一個不同的單元類。這使得可以聲明Row對象中的單元格列表。由於單行內可能存在不同的單元格類型，因此無法將它（單元列表）聲明爲特定單元類型的列表。但在這種情況下，您可以使用單元格的父類。總之這裏是例子：

Public Class Cell 
End Class 

Public Class Cell_Id 
    Inherits Cell 
    Public Value As Integer 
End Class 

Public Class Cell_Name 
    Inherits Cell 
    Public Value As String 
End Class 

Public Class Cell_MonthlyTax 
    Inherits Cell 
    Public Value As Double 
End Class 


Public Class Row 
    Public Cells As New List(Of Cell) 

    Public Sub AddCell(ByVal cell As Cell) 
     Cells.Add(cell) 
    End Sub 
End Class 


Module Module1 

    Sub Main() 
     Dim row1 As New Row() 
     row1.AddCell(New Cell_Id() With {.Value = 1}) 
     row1.AddCell(New Cell_Name() With {.Value = "Name1"}) 
     row1.AddCell(New Cell_MonthlyTax() With {.Value = 12.2}) 
     Dim row2 As New Row() 
     row2.AddCell(New Cell_Id() With {.Value = 2}) 
     row2.AddCell(New Cell_Name() With {.Value = "Name2"}) 
     row2.AddCell(New Cell_MonthlyTax() With {.Value = 47.9}) 
     Dim row3 As New Row() 
     row3.AddCell(New Cell_Id() With {.Value = 3}) 
     row3.AddCell(New Cell_Name() With {.Value = "Name3"}) 
     row3.AddCell(New Cell_MonthlyTax() With {.Value = 73.3}) 

     Dim rows As New List(Of Row)(New Row() {row1, row2, row3}) 

     ' here you loop through rows, in each row select Cell at index 2, cast it down to a specific 
     ' type (yes, you should tell to a compiler that you sure that Cell at index 2 is of Cell_MonthlyTax type) 
     ' After casting you will get intellisence and see that Value is of Double type. 
     ' In cellsOfColumn2 you will get an array of Double (this is not exactly array, but it doesn't matter in our case) 
     Dim cellsOfColumn2 = From row In rows Select DirectCast(row.Cells(2), Cell_MonthlyTax).Value 

     ' here you may work with array of values as you want, say calculate avarange value 
     Dim result = cellsOfColumn2.Average() 
    End Sub 

End Module