如何以编程方式重新定义变量的类？

Question

Ok, so I have 2 classes, Note and HeldNote. HeldNote inherits from Note and adds only 2 variables, plus a different way of setting visual dimensions. I have an array of an undefined amount of items, with each item defined as a member of class Note.

Occasionally during my program, I want one of these items in the array to be a HeldNote rather than a Note, and I do not know which one until the program is running.

VB.Net seems fine with me declaring array(i) = New HeldNote but then wont let me access the variables that HeldNote has but Note does not. Using Breaks in the program leads me to believe that array(i) is not actually being redefined as a HeldNote rather than a Note.

So, my question is, how can I make array(i) a HeldNote, rather than a Note?

I'm using VB.net 2008.

Edit: I asked my Teacher about this problem and he said he didn't know and told me to look it up. I couldn't anything online (had been looking since before i asked him) so I've come here for help.

Answer 1

A few suggestions:

1) Change your design and have just one class rather than one that inherits from another.

2) Declare your array as type Object. Each element of an Object array can be of a different type. This way you can assign each element's type at runtime.

Dim notearray As Object() = New Object() {New Note(), New HeldNote(), New HeldNote(), New Note()}

Answer 2

This is not a code issue, it's almost a OO question.

"I have an array of an undefined amount of items, with each item defined as a member of class Note."

Then all of them are "Note". You can create class instances so:

Note theNote = new HeldNote()

but you can't "downgrade" a instance on execution declared so:

Note theNote = new Note()".

If you need "randomly" a type change, you can create a ListOf(HeldNote). Then, you can decide the real type so:

Dim aList as new ListOf(HeldNote)
aList.add(new HeldNote())
aList.add(new HeldNote())
aList.add(new HeldNote())

Dim oNote as Note 
oNote = Ctype(aList(0),Note) 

Dim oNote as HeldNote 
oNote = aList(1)
...

Answer 3

The Array of Note (better use a List(Of Note) if you are unsure about the size) can store both Note and HeldNote because the latter derives from the first.

But, when you store an HeldNote in the array it is not automatically transformed in a Note, it is still an HeldNote, what you need is to try casting it back to an HeldNote

For example

Sub Main

    Dim X(2) As Note

    X(0) = New Note() WIth {.Note = "A Message"}
    X(1) = New HeldNote() With {.Note = "Held Message", .Author = "Steve"}


    Dim h As HeldNote

    h = TryCast(X(0), HeldNote)
    If h Is Nothing Then
        Console.WriteLine("X(0) is NOT an HeldNote")
    Else
        Console.WriteLine(h.Author)
    End If

    h = TryCast(X(1), HeldNote)
    If h Is Nothing Then
        Console.WriteLine("X(1) is NOT an HeldNote")
    Else
        Console.WriteLine(h.Author)
    End If
End Sub

Public Class Note
    Public Note As String
End Class

Public Class HeldNote 
    Inherits Note
    Public Author As String
End Class 

In thisway you still have your HeldNote without using an array of Object that is something to avoid because you loose all the strong typing allowed by a specific Note array (In an Object array you could store anything. A Button or a String makes no difference)