[RESOLVED] Declaring Arrays in Structures

[gigemboy]

Trying to utlize a code sample I picked up, and I'm having a problem with an Array declared inside a structure.. the code is :

Public Structure UUID
        Public Data1 As Long
        Public Data2 As Integer
        Public Data3 As Integer
        Public Data4(7) As Byte
    End Structure

The function that is utilizing it refers to it here :

With typUUID
                .Data1 = &H626FC520
                .Data2 = &HA41E
                .Data3 = &H11CF
                .Data4(0) = &HA7
                .Data4(1) = &H31
                .Data4(2) = &H0
                .Data4(3) = &HA0
                .Data4(4) = &HC9
                .Data4(5) = &H8
                .Data4(6) = &H26
                .Data4(7) = &H37
            End With

Says "Arrays declared as structure members cannot be declared with initial size" . If I take the initialization out, i can run it, yet the program breaks at the start of the .Data4(0) in the With statement. Have any ideas??

[dinosaur_uk]

I am quite interested in this too

I managed to make another structure to make it work...but i dont think this is what you want...

Public Structure UUID
        Public Data1 As Long
        Public Data2 As Integer
        Public Data3 As Integer
        Public Data4 As Daty
    End Structure
 
    Public Structure Daty
        Friend D1 As Byte
        Friend D2 As Byte
        Friend D3 As Byte
        Friend D4 As Byte
        Friend D5 As Byte
        Friend D6 As Byte
        Friend D7 As Byte
        Friend D8 As Byte
    End Structure
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim ken As New UUID
        With ken
            .Data1 = &H626FC520
            .Data2 = &HA41E
            .Data3 = &H11CF
            .Data4.D1 = &HA7
            .Data4.D2 = &H31
            .Data4.D3 = &H0
            .Data4.D4 = &HA0
            .Data4.D5 = &HC9
            .Data4.D6 = &H8
            .Data4.D7 = &H26
            .Data4.D8 = &H37
        End With
    End Sub

[dinosaur_uk]

Got it!

You need to redim ...of course!

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim ken As New UUID
        ReDim ken.Data4(7)
        With ken
            .Data1 = &H626FC520
            .Data2 = &HA41E
            .Data3 = &H11CF
            .Data4(0) = &HA7
            .Data4(1) = &H31
            .Data4(2) = &H0
            .Data4(3) = &HA0
            .Data4(4) = &HC9
            .Data4(5) = &H8
            .Data4(6) = &H26
            .Data4(7) = &H37
 
        End With
    End Sub

[gigemboy]

Thx! Redim worked.... simple as that

[jmcilhinney]

An array is a reference type object. When you declare an array without an upper bound, it is just like declaring any reference type variable without using the New keyword. When you specify an upper bound, you are in fact creating a new instance of the Array class. These two lines are equivalent:

Dim myArray1(7) As Byte
Dim myArray2() As Byte = New Byte(7) {}

Since it is illegal to initialise variables when declaring in a structure, it cannot be done that way.

Also note that because you have declared it as a public variable, anyone can simply assign a different array to that variable anyway, like this:[Highlight=VB]Dim myUUID As UUID

myUUID.Data4 = New Byte(100) {}[Highlight=VB]If you want to make sure that the array always has 8 elements you could make it a read-only property like this:

Public Structure UUID
    Public Data1 As Long
    Public Data2 As Integer
    Public Data3 As Integer
    Private m_Data4() As Byte
 
    Public ReadOnoly Property Data4() As Byte()
        Get
            If Me.m_Data4 Is Nothing Then
                ReDim Me.m_Data4(7)
            End If
 
            Return Me.m_Data4
        End Get
    End Property
End Structure

That way you can modify the elements of the array from outside but you cannot assign a different array to the property. Note that you can also declare public variables as ReadOnly. I assume it has the same effect, but I haven't ever tried it to find out.

[jmcilhinney]

If you don't care how many elements the array has then why not provide a constructor that takes the array upper bound as an argument:

Public Sub New(ByVal upperBound As Integer)
    ReDim Me.Data4(upperBound)
End Sub