In a generic VB.NET structure, how do I access its explicitly provided constructor? - vb.net

Question

First of all, what I want to achieve:
I want to extend a value datatype by providing additional properties, especially to validate ranges provided at declaration time. I want the new datatype to be a value type as well.
Compare with Ada:
subtype Day_Number is Integer range 1 .. 31;
Ideal, but obviously not implementable, would be:
Dim DayNumber As Int64 Range 1 To 31
However, I would be happy with:
Dim DayNumber As RangeInt64(1, 31)
It is of no concern, if initialization takes its time. Once ranges are provided, they are considered to be immutable. The datatype from then on is only used to set/get values like with ordinary value types, only that they are subject of being validated against the initially provided range.
My attempt:
Since I cannot inherit from structures in order to expand on them, I tried to incorporate a structure into a structure as a member.
In a module, I have this structure:
Friend Structure SRangeValueType(Of T)
Private lMinimum As T
Private lMaximum As T
Friend Property Minimum As T
Get
Return lMinimum
End Get
Set(tValue As T)
lMinimum = tValue
End Set
End Property
Friend Property Maximum As T
Get
Return lMaximum
End Get
Set(tValue As T)
lMaximum = tValue
End Set
End Property
Friend Sub New(Minimum As T, Maximum As T)
lMinimum = Minimum
lMaximum = Maximum
End Sub
End Structure
I attempt to use this generic structure as a member of another structure (of concrete type Int64):
Public Structure RangeInt64
Private Range As SRangeValueType(Of Int64)
End Structure
However, this is not using the constructor with the two arguments.
Say I want to initialize Range (the only member of the structure RangeInt64) with the values 100 and 200 for Minimum and Maximum, resp.
I am not allowed to use something like:
Private Range As SRangeValueType(Of Int64)(100,200)
What is the correct syntax to provide my values to the generic constructor?

Answer 1

Normally, if you add a constructor to a structure, you can call it using the New keyword:
Dim x As SRangeValueType(Of Int64) ' Calls the default, infered, parameter-less constructor
Dim y As New SRangeValueType(Of Int64)(100, 200) ' Calls the explicitly defined constructor
However, that's not really the problem. The problem is that you are trying to set the default value of a non-shared field in a structure. That is something which is never allowed. All non-shared fields in structures must default to their default value (i.e. Nothing). For instance:
Public Structure RangeInt64
Private x As Integer = 5 ' Error: Initializers on structure members are valid only for 'Shared' members and constants
Private y As New StringBuilder() ' Error: Non-shared members in a structure cannot be declared 'New'
End Structure
And, as you may already know, you cannot override the default, inferred, parameter-less constructor on a structure either:
Public Structure RangeInt64
Public Sub New() ' Error: Structures cannot declare a non-shared 'Sub New' with no parameters
x = 5
y = New StringBuilder()
End Sub
Private x As Integer
Private y As StringBuilder
End Structure
As such, you are stuck. By design, when the default constructor is used, all fields in the structure must always default to Nothing. However, if you really, really need it to be a structure, and you can't just convert it to a class, and you really need to change it's default value, you could theoretically fake it into working by using a property to wrap the field:
Public Structure RangeInt64
Private _Range As SRangeValueType(Of Int64)
Private _RangeInitialized As Boolean
Private Property Range As SRangeValueType(Of Int64)
Get
If Not _RangeInitialized Then
_Range = New SRangeValueType(Of Int64)(100, 200)
_RangeInitialized = True
End If
Return _Range
End Get
Set(value As SRangeValueType(Of Int64))
_Range = value
End Set
End Property
End Structure
It should go without saying, though, that it's pretty gross and should be avoided if possible.
Update
Now that you've provided more details about what you are trying to accomplish, I think I may have a better solution for you. You're right that structures do not support inheritance, but what they do support is interfaces. So, if all you need is a bunch of range types, one per value-type, all having the same minimum and maximum properties, but all returning different predetermined values, then you could do something like this:
Private Interface IRangeValueType(Of T)
ReadOnly Property Minimum As T
ReadOnly Property Maximum As T
End Interface
Private Structure RangeInt64
Implements IRangeValueType(Of Int64)
Public ReadOnly Property Minimum As Long Implements IRangeValueType(Of Int64).Minimum
Get
Return 100
End Get
End Property
Public ReadOnly Property Maximum As Long Implements IRangeValueType(Of Int64).Maximum
Get
Return 200
End Get
End Property
End Structure