upgrade vb6 to vb.net execution engine exception thrown

[swiggin]

Hi there, this is a bit of a beefy problem that i've been having with some code with several tendrils which will need to be loped off.

I am in the process of upgrading a software development kit from vb6 to vb.net. When i run the program i get an executionengineexception when i call the function gen2multitagidentify.

I initialize the structure array IDNUM which is one of the arguments of the function. This is on the advice of the upgrade engine.

 Dim IDNUM(100) As TagIds
        Dim i As Integer
        For i = 0 To 100
            IDNUM(i).Initialize()
        Next
        Dim FFZ As Short
        Dim CountID As Integer
        Try
            FFZ = Gen2MultiTagIdentify(hCom, CountID, IDNUM(0), 255)

As you can see it takes an array of structures which is where i think the problem could be. Here is my structure declaration with the appropriate marshalling attributes.

 <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Ansi)> _
    Public Structure TagIds
        Dim TagType As Byte
        Dim AntNum As Byte
        <VBFixedArray(11)> Dim Ids() As Byte

        Public Sub Initialize()
            ReDim Ids(11)
        End Sub
    End Structure

The gen2multitagidentify function is defined in a .dll file here with a marshalling attribute supplied for the structure:

     Public Declare Function Gen2MultiTagIdentify Lib "\\SBS-CRM\Company\Project\RFID\visual_studio\DEMO VB Source\Mr915ApiV10.dll" (ByVal hCom As Integer, ByRef Count As Integer, <MarshalAs(UnmanagedType.Struct)> ByRef Value As TagIds, ByVal NetAddr As Byte) As Short

Can anyone see where i've gone wrong? The execution engine exception as listed in the msdn glossary provides an incredibly vague and unhelpful description of its causes. I've been working on this problem for 3 days now with no luck. Any help would be much appreicated.

[cicatrix]

I don't see why you need Initialize in your structure. Try removing it completely.
If nothing helps you can also pass a 14 byte array to your function - this is the simplest of forms so it should work.
How the code of the structure looked like in VB6?

[Joacim Andersson]

No, you do need to Initialize your structure, or rather you do need to redim the size of the Ids array.
What you're trying to do is to pass an unmanaged array by passing the first element of that array. In the unmanaged world this is correct since an array is a pointer to the first element. This was also the only way of doing it in VB6 since it used SAFEARRAYs internally.

As you've seen this does not work in the .Net world and the reason for that is that you're telling the marshaller that you only want to marshal one element. So you get the exception because the function expects an array but only gets a single element. Try changing the declaration of the argument to this:
<MarshalAs(UnmanagedType.Struct), Out()> ByVal Value As TagIds()
and pass the whole array instead of a single element. Please note that I changed it to ByVal and added the OutAttribute instead.

[Shaggy Hiker]

I would also suggest reconsidering the design of the structure. All you appear to have is a fixed size array of bytes. The other members are also bytes, so the whole thing could be turned into a slightly different design:

Public Structure TagIds
  Private bts(12) as Byte
  
  Public Property TagType as Byte
   Get
     Return bts(0)
   End Get
   Set (value as Byte)
    bts(0) = value
   End Set
 End Property

 Public Property AntNm as Byte
   Get
     Return bts(1)
   End Get
   Set (value as Byte)
    bts(1) = value
   End Set
 End Property

End Structure

I left one thing out of that design, which is the Ids concept. It could be a third property that returns or sets the remaining bytes in the array, which would be similar to the way you have the data, but based on the name you used, I wondered if there wasn't a better solution than that. The name makes it sound like it is a series of ID items stuck together. Is each byte a different ID? If so, you could add a property that takes an Integer which indexes into the array, and the code would use the whole thing as if each individual item was a different member thought they were actually a single array of bytes. Alternatively, you could do both, with properties that returned individual bytes along with properties that returned chunks of bytes, or even the whole array.