Get Current, Default, Available Values

[si_ben]

Hellow,

Question: I do a request on a Capability, how come the Length of my ItemList is always 1?

Struct Def :

_
Friend Structure TwEnumeration
Public ItemType As Short
Public NumItems As Integer
Public CurrentIndex As Integer
Public DefaultIndex As Integer
_
Public ItemList() As Short
End Structure
_________________________________________________________________________
Fill The Structure :

Dim pvalEnum As New TwEnumeration
Dim NumItems As Integer
Dim CurrentIndex As Integer
Dim DefaultIndex As Integer
Dim valueBool As Boolean
Dim value As Short
Dim index As Short

pvalEnum = Marshal.PtrToStructure(Twain.GlobalLock(thisCap.Handle),GetType(TwEnumeration))

NumItems = pvalEnum.NumItems
CurrentIndex = pvalEnum.CurrentIndex
DefaultIndex = pvalEnum.DefaultIndex

For index = 0 To pvalEnum.ItemList.Length – 1
If (pvalEnum.ItemType = TwType.UInt16) Then
value = CType(pvalEnum.ItemList(index), TwType)

ElseIf (pvalEnum.ItemType = TwType.Bool) Then
valueBool = CBool(pvalEnum.ItemList(index))
End If
Next
_________________________________________________________________________

The Problem Is — ANYTHING I request
DScap(appid, appid, TwDG.Control, TwDAT.Capability, TwMSG.Get, cap)
Always returns ItemList of 1

Help?

[spike]

Looks like the problem is marshalling the ItemList.
You declared it:
_
Public ItemList() As Short

(a) I’m guessing ItemList has a length of 1 because you didn’t specify the length so VB defaults the length to 1?
(b) that isn’t at all what’s really there in the structure.

I’ll tell you what’s really there, and maybe you can figure out how to marshall it in VB.NET 😉

At the position of the ItemList field in the structure E, there are E.NumItems items, of the type implied by E.ItemType.
(I think the alignment is Pack 2 also, but I’m not 100% sure about that.)
So if E.NumItems = 5 and E.ItemType = TWTY_INT16, then E.ItemList is an (unmanaged) array of 5 16-bit signed integers.
If E.ItemType = TWTY_FIX32, then E.ItemList is an array of TW_FIX32 structures.

So both the length and element type of the ItemList array are dynamically determined by the NumItems and ItemType fields.
Note that ItemList is not a handle or pointer! It is a variable-sized field at the end of the structure.
This is an old, hackish C idiom, and is used in several places in the TWAIN API.

I’m at a loss to recommend how to marshall that. You could declare several classes – TwInt16Enum, TwFix32Enum, etc. to get the ItemList typed correctly, but the length of the array? Maybe this SO thread helps:
http://stackoverflow.com/questions/5902103/how-do-i-marshal-a-struct-that-contains-a-variable-sized-array-to-c

[si_ben]

This is potentially really helpful thank you… Just reading it and the site maybe helpful. I think I get it yes your right the size is ALWAYS 1.

When I delcare it though

Public ItemList() As Short — I cannot Declare it as
Public ItemList(1) As Short it doesn’t let me but — this might help. I will have
many hours tomorrow to try so thanks again, very much!

[si_ben]

For Testing purposes declared a SizeConst :=10

There was more than one Value, thanks

I will post my more permanent solution once i find one…

[si_ben]

So far my solution to this problem is just to Declare the Structures with a Constant Field large enough to hold all the values and use the NumItem to iterate through each element that has been associated with a value :

_
Friend Structure TwArray
Public ItemType As Short
Public NumItems As Integer

_
Public ItemList() As Short
End Structure
___________________________________________________________

‘/** TWAIN.H Declaration **’/
”/* Container for array of values */
”typedef struct {
” TW_UINT16 ItemType;
” TW_UINT32 NumItems;
” TW_UINT8 ItemList[1];
”} TW_ARRAY, FAR * pTW_ARRAY;
___________________________________________________________

The only other way I could see approaching this problem was to Send the
Operation Twice Example :

CONTROL / CAPABILITY / GET / cap –> Retrieving the Value of NumItems

Creating a Sub-Routine inside the Structure Something like :

Public Sub SetItemList(ByVal _numItems As Integer)
ReDim ItemList(_numItems)
End Sub

CONTROL / CAPABILITY / GET / cap –> Retrieving the Value of NumItems
______________________________________________________________

Problem with this approach is that for some reason it does not accept this
as a Constant size for the ItemList, therefore; When the query was made again
the ItemList size was still 1. Like you mentioned something to do with Unmanaged types.
I didn’t consider it a big deal, just set the SizeConst:=200 since from what I can tell
there are only 165 Defined Capabilities in the TWAIN.H File.

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