Issue Marshalling C# Structure to call C .DLL - c#

I'm having difficulty trying to marshal a structure I've defined in a C# program that is required for calling an unmanaged C .DLL file I do not have access to the source code for. A sample unmanaged C program C program can call this .DLL with no issue. The problem structure is fa_keylist below. There are multiple sub structures contained in the structure I am having issues with:
From the C header file:
struct fa_keypart {
short kp_start;
short kp_leng;
long kp_flags;
};
struct fa_keydesc {
long k_flags;
long k_nparts;
struct fa_keypart k_part [FA_NPARTS];
};
struct fa_keylist {
long kl_nkeys;
char kl_reserve[4];
struct fa_keydesc *kl_key [FA_NKEYS];
}
In C#, I have this defined as:
[StructLayout(LayoutKind.Sequential)]
public struct fa_keypart
{
public Int16 kp_start;
public Int16 kp_leng;
public Int32 kp_flags;
}
[StructLayout(LayoutKind.Sequential)]
public struct fa_keydesc
{
public Int32 k_flags;
public Int32 k_nparts;
[MarshalAs(UnmanagedType.ByValArray)]
public fa_keypart[] kparts;
};
[StructLayout(LayoutKind.Sequential)]
public struct fa_keylist
{
public Int32 kl_nkeys;
public UInt32 kl_reserve;
[MarshalAs(UnmanagedType.ByValArray)]
public fa_keydesc[] kl_keys;
}
The DLLIMPORT signature for the actual call is defined as:
[STAThread]
[DllImport("F4AGFCFA.dll", EntryPoint = "cobfa_open", CallingConvention = CallingConvention.StdCall)]
public static extern Int32 cobfa_open(
string fileName,
Int32 openFlags,
ref fa_keylist keyList,
Int32 recordLength);
The call to the function is coded as:
handle = cobfa_open(filename, fileFlags, ref keyList, 80);
I've tried a number of different Marshalling options by the way. The current error I receive is an Access Violation (Attempt to read or write protected memory).
Any suggestions would be greatly appreciated.

You need to specify the size for the arrays. Assuming that FA_NPARTS in C is 128, you could do the following:
[StructLayout(LayoutKind.Sequential)]
public struct fa_keydesc
{
public Int32 k_flags;
public Int32 k_nparts;
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 128)]
public fa_keypart[] kparts;
};
UnmanagedType.ByValArray only works with SizeConst set as well.

Related

C# DllImport - Call a function that returns an struct

I am working in C# and I need to call a function in a C++ dll library. This function returns a struct but I can´t get anything.
This is the function I need to call and the struct that returns in C++ library:
ATHENA_API _DEVICE_INFO* __stdcall GetDeviceInfoKeepConnection(_DEVICE_INFO* pDeviceInfo);
typedef struct TD_DEVICE_INFO{
TCHAR chDeviceName[256];
int nCommPort;
int nECGPos;
int nNumberOfChannel;
int nESUType;
int nTymestampType;
int nDeviceHandle;
TCHAR chDeviceID[260];
}_DEVICE_INFO;
This is my C# code trying to call the function:
[DllImport(#"\BAlertSDK\ABM_Athena.dll")]
static extern _DEVICE_INFO GetDeviceInfoKeepConnection(_DEVICE_INFO deviceInfo);
[StructLayout(LayoutKind.Sequential)]
struct _DEVICE_INFO
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
public string chDeviceName;
public int nCommPort;
public int nECGPos;
public int nNumberOfChannel;
public int nESUType;
public int nTymestampType;
public int nDeviceHandle;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
public string chDeviceID;
}
void Test_Click()
{
_DEVICE_INFO d = new _DEVICE_INFO();
_DEVICE_INFO deviceInfo = GetDeviceInfoKeepConnection(d);
}
The only I can get is an empty _DEVICE_INFO object. I think my problem is that I am not defining correctly the DEVICE INFO struct.
I have never worked with dll´s to this level. Can you help me?
Thanks in advance.
Thanks to all!! The problem has solved with this:
Parameter pass by reference and struct charset Unicode.
It seems that the function GetDeviceInfoKeepConnection returns a pointer to _DEVICE_INFO struct. So, in your C# code, you need to change the definition of the function to:
[DllImport(#"\BAlertSDK\ABM_Athena.dll")]
static extern IntPtr GetDeviceInfoKeepConnection(IntPtr deviceInfo);
And then you can access the struct data like this:
void Test_Click()
{
_DEVICE_INFO d = new _DEVICE_INFO();
IntPtr pDeviceInfo = Marshal.AllocHGlobal(Marshal.SizeOf(d));
Marshal.StructureToPtr(d, pDeviceInfo, false);
IntPtr deviceInfo = GetDeviceInfoKeepConnection(pDeviceInfo);
_DEVICE_INFO result = (_DEVICE_INFO)Marshal.PtrToStructure(deviceInfo, typeof(_DEVICE_INFO));
Marshal.FreeHGlobal(pDeviceInfo);
}
Note that, to ensure that the memory is cleaned up after use, you should use the Marshal.FreeHGlobal method to free the memory that was allocated by Marshal.AllocHGlobal.

Marshalling an integer pointer inside a structure as a callback

I have a C structure used for callback that I need to marshall to C# .NET:
struct CTMDeviceInfo {
enum CTMDeviceType eDeviceType;
char * szDeviceModel;
char * szDeviceSubModel;
int32_t * piDeviceID;
};
This is my C# version:
[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct CTMDeviceInfo
{
public CTMDeviceType deviceType;
[MarshalAs(UnmanagedType.LPStr)]
public string deviceModel;
[MarshalAs(UnmanagedType.LPStr)]
public string deviceSubModel;
public IntPtr deviceId;
};
Which is used inside another structure:
[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct CTMDeviceError
{
public CTMDeviceInfo deviceInfo;
[MarshalAs(UnmanagedType.I4)]
public Int32 resultCode;
[MarshalAs(UnmanagedType.I4)]
public Int32 extendedResultCode;
public IntPtr denomination;
public IntPtr changeDue;
};
My problem is that the "IntPtr deviceId" does not consistently return the correct value every time a callback was made.
I was expecting an integer value of 5, 15 or 16 but it keeps returning random values like 106, 865412, 652272, etc.
I don't know what I did wrong. What I did though is to prevent the callback in my managed code to be garbage collected using GCHandle.
Here is the sequence on how I did it:
From my unmanaged code I have this CDECL callback method:
void ctm_add_device_error_event_handler(CTMDeviceErrorCallback);
typedef void (CTMDeviceErrorCallback) (struct CTMEventInfo, struct CTMDeviceError );
This is my managed code:
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate void OnDeviceErrorCallBack(CTMEventInfo evtInfo, CTMDeviceError deviceError);
[DllImport("libctmclient-0.dll", EntryPoint = "ctm_add_device_error_event_handler", CallingConvention = CallingConvention.Cdecl)]
public static extern void AddDeviceErrorEventHandler([MarshalAs(UnmanagedType.FunctionPtr)] OnDeviceErrorCallBack deviceErrorCallBack);
OnDeviceErrorCallBack deviceErrorCallback;
GCHandle deviceErrorCallbackGCHandle;
deviceErrorCallback = new OnDeviceErrorCallBack(OnDeviceError);
deviceErrorCallbackGCHandle = GCHandle.Alloc(deviceErrorCallback);
AddDeviceErrorEventHandler(deviceErrorCallback);
And this is where the callback is handled:
public void OnDeviceError(CTMEventInfo evtInfo, CTMDeviceError deviceError)
{
int nDeviceId = Marshal.ReadInt32(deviceError.deviceInfo.deviceId);
}
I tried to use unsafe to use pointers directly but the issue is still the same.
public unsafe int *deviceId; //instead of IntPtr
int nDeviceId = 0;
unsafe
{
nDeviceId = *(deviceError.deviceInfo.deviceId);
}
I'm sure that my unmanaged code returned the correct value because I have logs but when it reached in my managed code, somehow another value was returned.
It's like it is reading on a different reference or something.
Hope somewhat could help me because I am stuck for a while now.
Thanks!
Use following c# structure. You can get the two string from the pointer later in the code. :
[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct CTMDeviceInfo
{
public CTMDeviceType deviceType;
public IntPtr deviceModel;
public IntPtr deviceSubModel;
public IntPtr deviceId;
};

Marshalling type which references itself

I have following (shortened) function definition in my c++ code:
EXPORT_API Table* OpenTableExport();
where Table is a struct of the form:
typedef struct Table
{
int fCurrKey;
int fTableNo;
int fRecSize;
char fCreating;
Table* fNextTable;
Table* fPrevTable;
MyFileType fFile;
} Table;
So, to PInvoke this function from managed code I naturally tried following:
[DllImport("Export.dll", EntryPoint = "OpenTableExport", CharSet = CharSet.Auto, CallingConvention = CallingConvention.Cdecl)]
public static extern Table OpenTable();
With following table class:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
public class Table
{
public KeyDescription fCurrKey;
public int fTableNo;
public uint fRecSize;
public byte fCreating;
public Table fNextTable;
public Table fPrevTable;
public FileDescription fFile;
}
Now when using the method, I get following exception (translated from german):
The field "fNextTable" of type "Table" can not be marshalled, no marshalling support exists for this type.
Why can't .NET marshall the fNextTable and fPrevTable members of the same type it is marshalling in any case?
I can also just replace the references in the managed class definition with IntPtr... but is this really necessary? (The marshalling then will work more or less).
Define the structure by the following way:
public class Table
{
public KeyDescription fCurrKey;
public int fTableNo;
public uint fRecSize;
public byte fCreating;
public IntPtr fNextTable;
public IntPtr fPrevTable;
public FileDescription fFile;
}
Test IntPtr members for NULL (IntPtr.Zero) and handle them. Probably unmanaged API contains another functions expecting Table*. See also Marshal.PtrToStructure method and another low-level Marshal methods for managed-unmanaged memory exchange.

How to call a function in an unmanaged dll from C# Code?

How can I call the following method from C#, which is in a C++ dll? How Can I recreate the following structure in C#?
Original
Method:
LONG MyMethod (P_HOLO_INFO pInfo, LPVOID pBuffer, LPUSHORT pTracksWritten);
Structure: This method uses the following structure:
typedef struct _HOLO_INFO
{
LONG lHoloType;
LONG lStatus;
HANDLE lThreadHandle;
LONG lErrorCode;
LONG lDriveIndex;
LONG lHeight;
SHORT iFormat;
INT iStartingTrack;
LONG lWrite;
LONG lSkip;
BOOL bSkipZero;
BOOL bInvert;
LONG lMaxConsecutiveErrors;
LONG lMaxTotalErrors;
LONG lMostConsecutiveErrors;
LONG lTotalErrors;
LPBYTE pBuffer;
LPUSHORT pTracksWritten;
LONG bUpsideDown;
} HOLO_INFO, *P_HOLO_INFO;
I worked in C# like this
Method:
[DllImport("My.dll", EntryPoint = "_MyMethod#12")]
public unsafe static extern long MyMethod(ref HOLO_INFO pInfo, Byte[] pBuffer,ref ushort pTracksWritten);
Structure:
This method uses the following structure:
unsafe public struct HOLO_INFO
{
public long lHoloType;
public long lStatus;
public long lThreadHandle;
public ulong lErrorCode;
public long lDriveIndex;
public long lHeight;
public short iFormat;
public int iStartingTrack;
public long lWrite;
public long lSkip;
public bool bSkipZero;
public bool bInvert;
public long lMaxConsecutiveErrors;
public long lMaxTotalErrors;
public long lMostConsecutiveErrors;
public long lTotalErrors;
public Byte* pBuffer;
public long* pTracksWritten;
public long bUpsideDown;
};
I made a call to the method like this:
do
{
result = MyMethod(ref pInfo,ptrBuf,pTracksWritten);
} while (result ==1 );
Because, it returns 1, if it is Active
0, if it completed successfully
3, if it stopped because of error.
if the method is in running state(Active-1). it modifies pInfo and pTracksWritten to update the status information.
Lots of issues:
LONG should be declared as int in C#
HANDLE is IntPtr.
pTracksWritten is missing. You probably need to make it, and pBuffer, an IntPtr and use Marshal.AllocHGlobal to allocate memory for them, depends.
You need the CallingConvention in the [DllImport] declaration to use Cdecl.
Odds of getting this to work are not great if you can't debug the unmanaged code. One basic sanity test is to make sure that Marshal.SizeOf() returns the same length as sizeof() in the unmanaged code. Next verify that passed arguments look good when debugging the native code. Next triple-check the pointer usage in the native code and verify that they are not getting copied.
See Using a Very C# DLL in C++
You can do a 'simple' trick [this answer](answer Using a Very C# DLL in C++) or you can have a look at fullblown embedding as per my answer
Give this a shot:
[DllImport("My.dll", EntryPoint = "_MyMethod#12")]
int MyMethod (HOLO_INFO pInfo, IntPtr pBuffer, IntPtr pTracksWritten);
public class HOLO_INFO
{
public int lHoloType;
public int lStatus;
public IntPtr lThreadHandle;
public int lErrorCode;
public int lDriveIndex;
public int lHeight;
public short iFormat;
public int iStartingTrack;
public int lWrite;
public int lSkip;
public bool bSkipZero;
public bool bInvert;
public int lMaxConsecutiveErrors;
public int lMaxTotalErrors;
public int lMostConsecutiveErrors;
public int lTotalErrors;
public IntPtr pBuffer;
public IntPtr pTracksWritten;
public int bUpsideDown;
}
Depending on how they're allocated, you may need to use Marshal.Copy to access HOLO_INFO.pBuffer and Marshal.PtrToStructure to access HOLO_INFO.pTracksWritten (or Marshal.Copy if it's an array vs. a pointer to a singular value).

Passing a Structure to C++ API using Marshal.StructureToPtr in C#

I am using API written in C++ in my code (writting in C#).
API requires a parameter as Pointer to Structure.
The Structure consists of "Int"s and Char Arrays:
for example
unsafe public struct ToBePassed
{
Int32 Num1;
Int32 Num2;
Char[] Data; // or fixed Char Data[255];
}
I can not directly pass the structure pointer to API because in that case, I am getting error as "Pointers cannot reference Marshaled structures". Code get compiled successfully but this Error comes when I execute (Debug) the code.
Now I have two options:
1st:- Passing Structure by Ref: I want to ask does an API requiring A Structure Pointer can receive the address when I pass the structure by ref. Note that API will return Data in "Char[] Data".
2nd:- Using Marshal.StructureToPtr: This will convert Structure Pointer to IntPtr. Again the Doubt is same, Will that API receive it correctly?
Thanks for Your Time!
Regards,
Swanand
If it only requires pointer, you can allocate some unmanaged memory, marshal the structure to the memory, and pass that pointer to your function. Then afterwards you could marshal back to the structure (if you wish) and free the memory. Before you marshal anything, you need to properly define the structure. Something like this:
[StructLayout(
LayoutKind.Sequential, //must specify a layout
CharSet = CharSet.Ansi)] //if you intend to use char
public struct ToBePassed
{
public Int32 Num1;
public Int32 Num2;
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 255)]
public Char[] Data; //specify the size using MarshalAs
}
[DllImport("...")]
public static extern void APICall(IntPtr argPtr);
public static void CallFunction(ToBePassed managedObj)
{
IntPtr unmanagedAddr = Marshal.AllocHGlobal(Marshal.SizeOf(managedObj));
Marshal.StructureToPtr(managedObj, unmanagedAddr, true);
APICall(unmanagedAddr);
Marshal.PtrToStructure(unmanagedAddr, managedObj);
Marshal.FreeHGlobal(unmanagedAddr);
unmanagedAddr = IntPtr.Zero;
}
[edit]
To simulate variable length arrays, allocate unmanaged memory within the structure and initialize as usual.
[StructLayout(LayoutKind.Sequential)]
public struct SomeStruct
{
public Int32 X;
public Int32 Y;
}
[StructLayout(LayoutKind.Sequential)]
public struct VLA
{
public Int32 intArrayLength;
public Int32 SomeStructArrayLength;
public IntPtr intArray;
public IntPtr SomeStructArray;
}
public static VLA CreateVLA(int[] intArray, SomeStruct[] SomeStructArray)
{
var vla = new VLA()
{
intArrayLength = intArray.Length,
SomeStructArrayLength = SomeStructArray.Length,
intArray = Marshal.AllocHGlobal(intArray.Length * Marshal.SizeOf(typeof(int))),
SomeStructArray = Marshal.AllocHGlobal(SomeStructArray.Length * Marshal.SizeOf(typeof(SomeStruct))),
};
Marshal.Copy(intArray, 0, vla.intArray, intArray.Length);
//there's no overload to copy arbitrary arrays, do it manually
for (int i = 0; i < SomeStructArray.Length; i++)
{
Marshal.StructureToPtr(
SomeStructArray[i],
vla.SomeStructArray + i * Marshal.SizeOf(typeof(SomeStruct)),
true);
}
return vla;
}

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