Question

I am working on a system that requires interaction with a native C API using P/Invoke. Now I’ve (yet again) stumbled upon a problem which I cannot seem to solve in any way. The original function is designed to return 2 kinds of structures, based on a parameter that specifies which structure to use.

The C header file defines the structures and function as follows:

#pragma pack(1) typedef struct {    DWORD JobId;     DWORD CardNum;    HANDLE hPrinter; } CARDIDTYPE, FAR *LPCARDIDTYPE; #pragma pack()  typedef struct {    BOOL        bActive;    BOOL        bSuccess; } CARD_INFO_1, *PCARD_INFO_1, FAR *LPCARD_INFO_1;  typedef struct {    DWORD       dwCopiesPrinted;    DWORD       dwRemakeAttempts;    SYSTEMTIME  TimeCompleted; } CARD_INFO_2, *PCARD_INFO_2, FAR *LPCARD_INFO_2;  BOOL ICEAPI GetCardId(HDC hdc, LPCARDIDTYPE pCardId); BOOL ICEAPI GetCardStatus(CARDIDTYPE CardId, DWORD level, LPBYTE pData, DWORD cbBuf, LPDWORD pcbNeeded ); 

I have attempted to implement P/Invoke wrappers like this:

[StructLayout(LayoutKind.Sequential, Pack=1)] public class CARDIDTYPE {     public UInt32 JobId;     public UInt32 CardNum;     public IntPtr hPrinter; }  [StructLayout(LayoutKind.Sequential)] public class CARD_INFO_1 {     public bool bActive;     public bool bSuccess; }  [StructLayout(LayoutKind.Sequential)] public class CARD_INFO_2 {     public UInt32 dwCopiesPrinted;     public UInt32 dwRemakeAttempts;     public Win32Util.SYSTEMTIME TimeCompleted; } [DllImport('ICE_API.DLL', CharSet = CharSet.Auto, CallingConvention = CallingConvention.Winapi, SetLastError = true)] public static extern bool GetCardId(HandleRef hDC, [Out]CARDIDTYPE pCardId);  [DllImport('ICE_API.DLL', CharSet = CharSet.Auto, CallingConvention = CallingConvention.Winapi, SetLastError = true)] public static extern bool GetCardStatus(CARDIDTYPE CardId, UInt32 level, [Out] byte[] pData, UInt32 cbBuf, out UInt32 pcbNeeded); 

Calling the ‘GetCardId’ seems to work fine. I get plausible data in CARDIDTYPE instance after calling it. However when I call ‘GetCardStatus’ the problems start. The type of structure that should be returned is defined by the ‘level’ param, and a value of 1 should result in a CARD_INFO_1 structure to be returnes in ‘pData’.

The documentation contains the following C example:

CARD_INFO_1 ci1; DWORD cbNeeded; ci1.bActive = TRUE; if (GetCardStatus(*lpCardID, 1, (LPBYTE)&ci1, sizeof(ci1), &cbNeeded )) { /* success */ } 

My equivalent C# implementation is like this:

uint needed; byte[] byteArray = new byte[Marshal.SizeOf(typeof(CARD_INFO_1))]; if (GetCardStatus(cardId, 1, byteArray, (uint)byteArray.Length, out needed)) { /* success */ } 

When I execute this C# code, the method returns false and Marshal.GetLastWin32Error() return -1073741737 (which does not make much sense to me). I see no reason why this call should fail, and definitely not with this error code. So I suspect I have got something wrong in my P/Invoke wrapper.

I know that using ‘byte[]’ as the type of pData is probably not correct, but according to some googling a ‘LPBYTE’ translates to ‘[Out] byte[]’. I guess the correct way to do this is to have pData as an IntPtr, and create the structure using Marshal.PtrToStructure(…). I have tried this, but the result is the same. Here is the code for this scenario:

[DllImport(@'ICE_API.DLL', CharSet = CharSet.Auto, EntryPoint = '_GetCardStatus@28', CallingConvention = CallingConvention.Winapi, SetLastError = true)] public static extern bool GetCardStatus(CARDIDTYPE CardId, UInt32 level, IntPtr pData, UInt32 cbBuf, out UInt32 pcbNeeded);  uint needed; int memSize = Marshal.SizeOf(typeof(CARD_INFO_1)); IntPtr memPtr = Marshal.AllocHGlobal(memSize); if (!GetCardStatus(cardId, 1, memPtr, (uint)memSize, out needed)) {     int lastError = Marshal.GetLastWin32Error();     // error code is -1073741737 } CARD_INFO_1 info = (CARD_INFO_1)Marshal.PtrToStructure(memPtr, typeof(CARD_INFO_1)); Marshal.FreeHGlobal(memPtr); 

Edit: One thing I forgot to mention is that for some reason the GetCardStatus call fails with an unknown entry point exception if I do not specify EntryPoint = ‘_GetCardStatus@28’. This has not happened to any other function I have wrapped, so it got me wondering a bit.

Answer 1

_GetCardStatus@28 gave me an idea. Unless you are running on 64-bit Windows, you’ve got the number of arguments wrong. Your P/Invoke for GetCardStatus would be _GetCardStatus@20, because it has 5 32-bit arguments. Your C declaration of GetCardStatus seems to accept the cardId by value rather than by reference. Since CARDIDTYPE is 12 bytes long, this would give the correct length of the argument list (28). Moreover, this would explain both your receiving an error code of -1073741737 (C0000057, STATUS_INVALID_PARAMETER) — since you’re not passing a valid cardId — and the access violation — GetCardStatus tries to write to pcbNeeded, which is garbage because the marshaler hasn’t even pushed it!

Ergo:

[DllImport('ICE_API.DLL', CharSet = CharSet.Auto,       CallingConvention = CallingConvention.Winapi, SetLastError = true)]  public static extern bool GetCardStatus (      IntPtr hPrinter, UInt32 cardNum, UInt32 jobId, UInt32 level,      [In, Out] CARD_INFO_1 data, UInt32 cbBuf, out UInt32 pcbNeeded) ; [DllImport('ICE_API.DLL', CharSet = CharSet.Auto,       CallingConvention = CallingConvention.Winapi, SetLastError = true)]  public static extern bool GetCardStatus (      IntPtr hPrinter, UInt32 cardNum, UInt32 jobId, UInt32 level,      [In, Out] CARD_INFO_2 data, UInt32 cbBuf, out UInt32 pcbNeeded) ; 

Note reverse order of the three CARDIDTYPE members: stdcall pushes the parameters left-to-right (i.e. towards lower addresses), and my guess is that a struct is ‘pushed’ as a unit.

Also, if you later close the printer handle with CloseHandle, I’d suggest receiving the handle in CARDIDTYPE into an appropriate SafeHandle, not into a bare IntPtr, and declaring GetCardStatus to receive the safe handle.