pinvoke from vb.net using structs in call - c++

Question

I'm really struggling with the following pinvoke call. I've tried numerous different ways of doing this but still no joy.
The call runs through, but I get a 'Bad parameter' message back with suggests there is something wrong with the struct, since I've ran through a c++ example of this code and the parameters are all correct.
I'd be so grateful for any help, I've googled myself mad already!
in the c++ header file:
int __stdcall DVSNET_OpenChannel(HANDLE hServer,unsigned long nChannel,DVSNET_CHANNEL_INFO *pChannelInfo,HANDLE *phChannel);
typedef struct tagDVSNET_CHANNEL_INFO
{
unsigned long lStructSize;
unsigned long dwStreamNo;
unsigned long nProtocol;
HWND hWndDisplay;
unsigned long bPlayStart;
unsigned long dwBackFrameCount;
unsigned long dwFlag;
} DVSNET_CHANNEL_INFO;
My definitions for the import:
<DllImport("DVSNETClient.dll")> _
Public Shared Function DVSNET_OpenChannel(ByVal hServer As System.IntPtr, ByVal nChannel As UInteger, ByRef pChannelInfo As IntPtr, ByRef phChannel As IntPtr) As Integer
End Function
<StructLayout(LayoutKind.Sequential)> _
Public Structure tagDVSNET_CHANNEL_INFO
Public lStructSize As UInteger
Public dwStreamNo As UInteger
Public nProtocol As UInteger
Public hWndDisplay As IntPtr
Public bPlayStart As UInteger
Public dwBackFrameCount As UInteger
Public dwFlag As UInteger
End Structure
My Calling code:
Private Sub OpenChannel()
Dim intRet As Integer
Dim ChannelInfo As New tagDVSNET_CHANNEL_INFO
Dim HWD As New System.IntPtr
ChannelInfo.lStructSize = System.Runtime.InteropServices.Marshal.SizeOf(ChannelInfo)
ChannelInfo.nProtocol = 0
ChannelInfo.dwStreamNo = 0
ChannelInfo.dwBackFrameCount = 10
ChannelInfo.hWndDisplay = HWD
ChannelInfo.bPlayStart = 0 ' dont display
'Channelinfo.dwFlag =
' Initialize unmanged memory to hold the struct.
Dim ptr As IntPtr = Marshal.AllocHGlobal(Marshal.SizeOf(ChannelInfo))
System.Runtime.InteropServices.Marshal.StructureToPtr(ChannelInfo, ptr, True)
Dim nChannel As UInteger = 1
intRet = TotemPoleLib.DVSNET_OpenChannel(hServer, nChannel, ptr, hChannel)
'... snip ...
End Sub
Many thanks in advance!

Answer 1

I don't know VB.Net syntax, but if you understand C# then here are the proper P/Invoke declarations for DVSNET_CHANNEL_INFO and DVSNET_OpenChannel:
[StructLayout(LayoutKind.Sequential)]
struct DVSNET_CHANNEL_INFO
{
uint lStructSize;
uint dwStreamNo;
uint nProtocol;
IntPtr hWndDisplay;
uint bPlayStart;
uint dwBackFrameCount;
uint dwFlag;
}
static class DVSNETClient
{
[DllImport("DVSNETClient.dll")]
public static extern int DVSNET_OpenChannel(
IntPtr hServer,
uint nChannel,
ref DVSNET_CHANNEL_INFO pChannelInfo,
ref IntPtr phChannel
);
}
Your DVSNET_CHANNEL_INFO appears fine, but the 3rd argument of DVSNET_OpenChannel should simply be a ref DVSNET_CHANNEL_INFO; all the marshaling is done for you automatically, no real need for the Marshal class here. The only thing you need to do is initialize ChannelInfo.lStructSize to the value of Marshal.SizeOf(typeof(DVSNET_CHANNEL_INFO)) before calling DVSNET_OpenChannel. It would probably be sensible to do this in a non-default constructor for DVSNET_CHANNEL_INFO, but given that it's a struct, there is of course no way to enforce use of that constructor over the implicit default constructor.
Note that semantically it's possible that pChannelInfo and/or phChannel should be out rather than ref, but it's not possible to tell by the signature of DVSNET_OpenChannel. In any case, using ref will work regardless, though it will be less efficient than using out if out is indeed warranted.