Data Class calculations and alternatives [closed] - vb.net

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I would like to explore your thoughts on the below.
If I have essentually a data class
Public Class Package
Public Property Length as decimal
Public property Width as decimal
Public Property Height as decimal
End Class
Now I want a calculated property so I could add this to the class in 3 ways.
Have the class property calculate the result
Public ReadOnly Property Cubic as decimal
Get
Return Length * Height * Width
End Get
Use a calculation class to calculate the result
Public ReadOnly Property Cubic as decimal
Get
Dim Calc as new CalculationClass
return Calc.Cubic(Length, Width, Height)
End Get
I could also as an alternative have the property dumb and it could be calculated outside the class soon after the class is populated by calling the same calculation class in example 2. But outside the data class.
Private LocalCubic as Decimal
Public Property Cubic
Get
Return LocalCubic
End Get
Set(value as Decimal)
LocalCubic=value
End Set
End Property
Although the example above for the calculated property is simple I would also like to consider it may not be. It could be 200 or more lines long to do the calculation and maybe encombusing other properties in a larger DataClass.
There could also be hundreds of package classes created as the program runs.
My understanding of using each one is (Please comment if wrong)
Everything is contained in the class, readable and easy to work with. Each Package Class has its own copy of the calculation so for a simple calculation no problem. But if I have 500 package classes with the 'calculation' taking say 200 lines. It seems to me that a I am 'wasting' memory buy having 500 copies of the large calculation compared to the other methods.
Only one copy of the actual calculation is used as each class refers to the same calculation object. Every other class (500) is using the same code so it is saving on memory. Probably breaks OOP.
Has the advantages of 2) Major disadvantage is that if the Length, With or Height properties change in value at some time after creation and calculation then the Cubic property will be wrong.
Are there any other advantages / disadvantages to the above approaches?
Best Practice?
How do other programmers tackle this?
Thanks
Edit
I think I need to be a bit clearer in my questions above to make it more definative;
A) Does example 2) violate the OOP methodology?
B) Does using 2) create a new instance of the CalculationClass within each package class. Or does each Package class created reuse the same Calculation class code?

Answer 1

I would just make the volume a property of the Package class assuming the packages are all rectangular. Force the properties to be set in the constructor and calculate the Volume in the constructor.
Public Class Package
Public Property Length As Decimal
Public Property Width As Decimal
Public Property Height As Decimal
Public Property Volume As Decimal
Public Sub New(Lgth As Decimal, Wdth As Decimal, Hght As Decimal)
Length = Lgth
Width = Wdth
Height = Hght
Volume = Length * Width * Height
End Sub
End Class
Private Sub Button1_Click(sener As Object, e As EventArgs) Handles Button1.Click
Dim pk As New Package(7, 5, 10)
MessageBox.Show(pk.Volume.ToString)
End Sub