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How to call C++ DLL from C# when one of the parameters is an array of pointers

I have a DLL pin C++ which takes two parameters, similar to main() function in C: Number of argurments, followed by an array of pointers to individual parameters:
__declspec(dllexport) void Calculate(int argc, void** argv)
{
if (argc >= 7)
{
auto sourceX = *((int*)(argv[0]));
auto sourceY = *((int*)(argv[1]));
auto iterations = *((int*)(argv[2]));
auto resize = *((bool*)(argv[3]));
auto input = (double*)(argv[4]);
auto targetX = *((int*)(argv[5]));
auto targetY = *((int*)(argv[6]));
// Do computations
}
}
I am able to call the exported function from another C code (after loading the DLL library via a LoadLibrary and GetProcAddress calls) in this manner:
int X, Y, iterations;
bool Resize;
double* Input;
// Initialize variables, allocate data for Input
// ...
void* Params[] = { &X, &Y, &iterations, &Resize, Input, &X, &Y };
_Calculate(7, Params);
However, when I tried calling the DLL from C#, it crashes. Here is the C# snippet that I'm using:
[DllImport("computedll.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Unicode)]
public static extern void Calculate(int argc, void** argv);
public void Compute()
{
int X, Y, iterations;
bool Resize;
double* Input;
// Initialize variables, allocate data for Input
// ...
var Parameters = stackalloc void*[7];
Parameters[0] = &X;
Parameters[1] = &Y;
Parameters[2] = &iterations;
Parameters[3] = &Resize;
Parameters[4] = Input;
Parameters[5] = &X;
Parameters[6] = &Y;
Calculate(7, Parameters);
}
What am I doing wrong here? Is there no way to make this pattern of using array of pointers (void**) work? Thanks in advance.

Built as x64, the following works. It's very nearly your code. I hate using cdecl outside C/C++, so if that's important to you....sorry. Point is...if you are having problems, it's not fundamental to your actual code or concept.
Interopy.h:
extern "C"
{
__declspec(dllexport) void __stdcall Calculate(int argc, void** argv);
}
Source:
#include "Interopy.h"
void __stdcall Calculate(int argc, void** argv)
{
if (argc >= 7)
{
auto sourceX = *((int*)(argv[0]));
auto sourceY = *((int*)(argv[1]));
auto iterations = *((int*)(argv[2]));
auto resize = *((bool*)(argv[3]));
auto input = (double*)(argv[4]);
auto targetX = *((int*)(argv[5]));
auto targetY = *((int*)(argv[6]));
*(int*)argv[5] = 42; //to test
// Do computations
}
}
c# (Platform target x64):
public static class iop
{
[DllImport("InteropyDll1.dll", CallingConvention = CallingConvention.StdCall)]
unsafe public static extern void Calculate(int argc, void** argv);
unsafe static public void Compute()
{
int X, Y, iterations;
bool Resize;
double* Input = (double*)IntPtr.Zero;
// Initialize variables, allocate data for Input
// ...
void **Parameters = stackalloc void*[7];
Parameters[0] = &X;
Parameters[1] = &Y;
Parameters[2] = &iterations;
Parameters[3] = &Resize;
Parameters[4] = Input;
Parameters[5] = &X;
Parameters[6] = &Y;
Calculate(7, Parameters);
}
}
Note that this assembly is now platform dependent. I've solved this in the past with multiple build configurations, but I'm sure there are better ways to go. In any event, you'll want to put this unsafe code in its own assembly for that and other reasons.

my c++ and c# interop crashes in 64 bit, why? pointer size?

i got a native 32 bit dll (no source) which runs as a plugin in an application i use. I've done another native dll myself which will communicate with that plugin in order to create and update the plugin's controls.
From that dll i've exported the functions I need in order to control the plugin from my c# application (with p/invoke).
here's the code:
h file:
#pragma once
#include "include\SpoutControls.h"
extern "C" { __declspec(dllexport) void InitializeControls(char *sendername, int *numControls, char** names, int *types, float* floats, float* toggles, float* press, char** text); }
extern "C" { __declspec(dllexport) bool UpdateControls(const char** text, float *floats, float *toggles, float *press, int *numControls); }
extern "C" { __declspec(dllexport) void CloseControls(); }
//
extern "C" __declspec(dllexport) int ReleaseMemory(float *pArray)
{
delete[] pArray;
//delete[] Usize;
return 0;
};
the cpp:
#include "SpoutControls4vvvv.h"
//SpoutControls and the functions
//CreateControl, OpenControls, CheckControls, CloseControls
//are declared in SpoutControls.h, which comes with the 32 bit plugin dll
SpoutControls spoutcontrols;
void InitializeControls(char *sendername, int *numControls, char** names, int *types, float* floats, float* toggles, float* press, char** text) {
int Vcontrols = numControls[0];
int Tcontrols = numControls[1];
int Pcontrols = numControls[2];
int Scontrols = numControls[3];
int all = Vcontrols + Tcontrols + Pcontrols + Scontrols;
int v=0, t=0, p=0, s = 0;
for (int controlID = 0; controlID < all; controlID++) {
if (types[controlID] == 0) {
spoutcontrols.CreateControl(names[controlID], "float",0.0,1.0, floats[v]);
v++;
}
if (types[controlID] == 1) {
spoutcontrols.CreateControl(names[controlID], "bool", toggles[t]);
t++;
}
if (types[controlID] == 2) {
spoutcontrols.CreateControl(names[controlID], "event", press[p]);
p++;
}
if (types[controlID] == 3) {
spoutcontrols.CreateControl(names[controlID], "text", text[s]);
s++;
}
}
spoutcontrols.OpenControls(sendername);
}
bool UpdateControls(const char** text, float *floats, float *toggles, float *press, int *numControls) {
int Vcontrols = numControls[0];
int Tcontrols = numControls[1];
int Pcontrols = numControls[2];
int Scontrols = numControls[3];
int all = Vcontrols + Tcontrols + Pcontrols + Scontrols;
int v = 0, t = 0, p = 0, s = 0;
if (spoutcontrols.CheckControls(myControls)) {
for (int controlID = 0; controlID < all; controlID++) {
if (myControls[controlID].type == 10) {
floats[v] = myControls[controlID].value;
v++;
}
if (myControls[controlID].type == 0) {
toggles[t] = myControls[controlID].value;
t++;
}
if (myControls[controlID].type == 1) {
press[p] = myControls[controlID].value;
p++;
}
if (myControls[controlID].type == 100) {
text[s] = myControls[controlID].text.data();
s++;
}
}
return true;
}
return false;
}
void CloseControls() {
spoutcontrols.CloseControls();
}
and here's the c# code:
public unsafe class SystemSpoutSenderNode: IDisposable
{
[System.Runtime.InteropServices.DllImport("SpoutControls4vvvv.dll")]
private static extern void InitializeControls(IntPtr sendername, IntPtr numControls,String[] names, IntPtr types, IntPtr floats, IntPtr toggles, IntPtr press, String[] text);
[System.Runtime.InteropServices.DllImport("SpoutControls4vvvv.dll")]
private static extern int CloseControls();
[System.Runtime.InteropServices.DllImport("SpoutControls4vvvv.dll", CallingConvention = CallingConvention.Cdecl)]
private static extern bool UpdateControls([In, Out] String[] text, [In, Out] float[] floats, [In, Out] float[] toggles, [In, Out] float[] press, IntPtr numControls);
[System.Runtime.InteropServices.DllImport("SpoutControls4vvvv.dll")]
private static extern int ReleaseMemory(IntPtr ptr);
public void Evaluate(int SpreadMax)
{
//countControls determines number of controls per type (string,float,toggle,click)
int[] controls = countControls(FType);
//sumControls will just add up all elements in controls
int all = sumControls(controls);
//in my code these arrays will get filled with values, deleted here for readability
String[] names = new String[all];
int[] types = new int[all];
float[] floats = new float[controls[0]];
float[] toggles = new float[controls[1]];
float[] press = new float[controls[2]];
String[] text = new String[controls[3]];
//initialze return arrays
String[] Rtext = new String[controls[3]];
float[] Rfloats = new float[controls[0]];
float[] Rtoggles = new float[controls[1]];
float[] Rpress = new float[controls[2]];
//allocate pointers
IntPtr SndrNamePtr = NativeUtf8FromString(FSenderName);
IntPtr BinPtr = Marshal.AllocHGlobal(4*sizeof(int));
IntPtr TypePtr = Marshal.AllocHGlobal(all*sizeof(int));
IntPtr FloatPtr = Marshal.AllocHGlobal(controls[0]*sizeof(float));
IntPtr TogglePtr = Marshal.AllocHGlobal(controls[1]*sizeof(float));
IntPtr PressPtr = Marshal.AllocHGlobal(controls[2]*sizeof(float));
try
{
//copy control info + defaults to pointer
Marshal.Copy(controls, 0, BinPtr, 4);
Marshal.Copy(types, 0, TypePtr, all);
Marshal.Copy(floats, 0, FloatPtr, controls[0]);
Marshal.Copy(toggles, 0, TogglePtr, controls[1]);
Marshal.Copy(press, 0, PressPtr, controls[2]);
//initialize controls
if (FWrite) InitializeControls(SndrNamePtr,BinPtr,names,TypePtr,FloatPtr,TogglePtr,PressPtr,text);
//update controls
bool changed = UpdateControls(Rtext,Rfloats,Rtoggles,Rpress,BinPtr);
//FF, FT, FS and FP are the outputs in my c# host
if (changed){
for(int j=0; j<controls[0];j++){
FF[j]=Rfloats[j];
}
for(int j=0; j<controls[1];j++){
FT[j]=FloatToBool(Rtoggles[j]);
}
for(int j=0; j<controls[3];j++){
FS[j]=Rtext[j];
}
}
for(int j=0; j<controls[2];j++){
FP[j]=FloatToBool(Rpress[j]);
}
}
finally
{
Marshal.FreeHGlobal(SndrNamePtr);
Marshal.FreeHGlobal(BinPtr);
Marshal.FreeHGlobal(FloatPtr);
Marshal.FreeHGlobal(TogglePtr);
Marshal.FreeHGlobal(PressPtr);
}
}
}
}
public void Dispose()
{
CleanUp();
CloseControls();
}
}
NOTE: the c# code runs without precompiling in a frame-based, c# host environment for graphical programming (vvvv), therefore i've deleted host specific decalarations of inputs (FType,FSenderName) and outputs (FF,FS,FP,FT) to avoid confusion. These will be used to "connect" this code with other functionality. Evaluate will be called every frame by the host.
Now to the actual question(s):
it's working fine so far in 32 bit, but in 64 bit my c# host crashes without any message. after some reading i believe this is due to pointer sizes being different in 32/64bit systems, but i'm not exactly sure what to do/if this actually applies here. I would be very thankful if you could
explain me how (and why) to get this code to run in 64 bit
point out any other mistakes you might spot along the way- i'm completely new to c++ and still a beginner in c#, so i'm pretty confident there's a lot to improve here; especially: memory leaks and passing the values from c++ to c# and vice versa...uiuiui.
I've understood that I shouldn't cast a pointer to an int in 64 bit, so the last thing I've tried is to change from
int Vcontrols = numControls[0];
int Tcontrols = numControls[1];
int Pcontrols = numControls[2];
int Scontrols = numControls[3];
to
int Vcontrols = (INT_PTR)numControls[0];
int Tcontrols = (INT_PTR)numControls[1];
int Pcontrols = (INT_PTR)numControls[2];
int Scontrols = (INT_PTR)numControls[3];
but with no luck, therefore I'm posting my original problem, even if this is a correct improvement(?).
EDIT: thanks to #dkackman for pointing out one unclear point: my cpp code calls functions which come as source code (SpoutControls.h) with the native 32 bit dll. It's not the source for the 32 bit dll itself but declares the functions used to (as far as i can tell) access the same shared memory as the 32 bit dll.
I can also copy paste the code here if this might be the problem?
Also can be found here
thank you.

I am afraid you are out of luck. If your process is 64bit, you won't be able to load that 32bit dll, no matter how much you try.
Can I load a 32 bit DLL into a 64 bit process on Windows?
from https://msdn.microsoft.com/en-us/library/windows/desktop/aa384231(v=vs.85).aspx
On 64-bit Windows, a 64-bit process cannot load a 32-bit dynamic-link
library (DLL).
Without access to its source, your only option would be to convert your host to 32bit or otherwise figure out how to host the 32bit plugin in a 32bit process and use some sort of IPC to communicate with it from a 64bit host process.
So my guess is that this has nothing to do with your wrapper, array passing or interop code.

C# call C++ DLL passing pointer-to-pointer argument

Could you guys please help me solve the following issue?
I have a C++ function dll, and it will be called by another C# application.
One of the functions I needed is as follow:
struct DataStruct
{
unsigned char* data;
int len;
};
DLLAPI int API_ReadFile(const wchar_t* filename, DataStruct** outData);
I wrote the following code in C#:
class CS_DataStruct
{
public byte[] data;
public int len;
}
[DllImport("ReadFile.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Unicode)]
private static extern int API_ReadFile([MarshalAs(UnmanagedType.LPWStr)]string filename, ref CS_DataStruct data);
Unfortunately, the above code is not working... I guess that is due to the C++ func takes a pointer-to-pointer of DataStruct, while I just passed a reference of CS_DataStruct in.
May I know how can I pass a pointer-to-pointer to the C++ func? If it is not possible, is there any workaround? (the C++ API is fixed, so changing API to pointer is not possible)
Edit:
Memory of DataStruct will be allocated by c++ function. Before that, I have no idea how large the data array should be.
(Thanks for the comments below)

I used the following test implementation:
int API_ReadFile(const wchar_t* filename, DataStruct** outData)
{
*outData = new DataStruct();
(*outData)->data = (unsigned char*)_strdup("hello");
(*outData)->len = 5;
return 0;
}
void API_Free(DataStruct** pp)
{
free((*pp)->data);
delete *pp;
*pp = NULL;
}
The C# code to access those functions are as follows:
[StructLayout(LayoutKind.Sequential)]
struct DataStruct
{
public IntPtr data;
public int len;
};
[DllImport("ReadFile.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Unicode)]
unsafe private static extern int API_ReadFile([MarshalAs(UnmanagedType.LPWStr)]string filename, DataStruct** outData);
[DllImport("ReadFile.dll", CallingConvention = CallingConvention.Cdecl)]
unsafe private static extern void API_Free(DataStruct** handle);
unsafe static int ReadFile(string filename, out byte[] buffer)
{
DataStruct* outData;
int result = API_ReadFile(filename, &outData);
buffer = new byte[outData->len];
Marshal.Copy((IntPtr)outData->data, buffer, 0, outData->len);
API_Free(&outData);
return result;
}
static void Main(string[] args)
{
byte[] buffer;
ReadFile("test.txt", out buffer);
foreach (byte ch in buffer)
{
Console.Write("{0} ", ch);
}
Console.Write("\n");
}
The data is now transferred to buffer safely, and there should be no memory leaks. I wish it would help.

It isn't necessary to use unsafe to pass a pointer to an array from a DLL. Here is an example (see the 'results' parameter). The key is to use the ref attribute. It also shows how to pass several other types of data.
As defined in C++/C:
#ifdef __cplusplus
extern "C" {
#endif
#ifdef BUILDING_DLL
#define DLLCALL __declspec(dllexport)
#else
#define DLLCALL __declspec(dllimport)
#endif
static const int DataLength = 10;
static const int StrLen = 16;
static const int MaxResults = 30;
enum Status { on = 0, off = 1 };
struct Result {
char name[StrLen]; //!< Up to StrLen-1 char null-terminated name
float location;
Status status;
};
/**
* Analyze Data
* #param data [in] array of doubles
* #param dataLength [in] number of floats in data
* #param weight [in]
* #param status [in] enum with data status
* #param results [out] array of MaxResults (pre-allocated) DLLResult structs.
* Up to MaxResults results will be returned.
* #param nResults [out] the actual number of results being returned.
*/
void DLLCALL __stdcall analyzeData(
const double *data, int dataLength, float weight, Status status, Result **results, int *nResults);
#ifdef __cplusplus
}
#endif
As used in C#:
private const int DataLength = 10;
private const int StrLen = 16;
private const int MaxThreatPeaks = 30;
public enum Status { on = 0, off = 1 };
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct Result
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = StrLen)] public string name; //!< Up to StrLen-1 char null-terminated name
public float location;
public Status status;
}
[DllImport("dllname.dll", CallingConvention = CallingConvention.StdCall, EntryPoint = "analyzeData#32")] // "#32" is only used in the 32-bit version.
public static extern void analyzeData(
double[] data,
int dataLength,
float weight,
Status status,
[MarshalAs(UnmanagedType.LPArray, SizeConst = MaxResults)] ref Result[] results,
out int nResults
);
Without the extern "C" part, the C++ compiler would mangle the export name in a compiler dependent way. I noticed that the EntryPoint / Exported function name matches the function name exactly in a 64-bit DLL, but has an appended '#32' (the number may vary) when compiled into a 32-bit DLL. Run dumpbin /exports dllname.dll to find the exported name for sure. In some cases you may also need to use the DLLImport parameter ExactSpelling = true. Note that this function is declared __stdcall. If it were not specified, it would be __cdecl and you'd need CallingConvention.Cdecl.
Here is how it might be used in C#:
Status status = Status.on;
double[] data = { -0.034, -0.05, -0.039, -0.034, -0.057, -0.084, -0.105, -0.146, -0.174, -0.167};
Result[] results = new Result[MaxResults];
int nResults = -1; // just to see that it changes (input value is ignored)
analyzeData(data, DataLength, 1.0f, status, ref results, out nResults);

If you do call native code, make sure your structs are alligned in the memory. CLR does not guarantee alignment unless you push it.
Try
[StructLayout(LayoutKind.Explicit)]
struct DataStruct
{
string data;
int len;
};
More info:
http://www.developerfusion.com/article/84519/mastering-structs-in-c/

Return C++ array to C#

I can't seem to figure out how to return an array from an exported C++ DLL to my C# program. The only thing I've found from googling was using Marshal.Copy() to copy the array into a buffer but that doesn't give me the values I'm trying to return, I don't know what it's giving me.
Here's what I've been trying:
Exported function:
extern "C" __declspec(dllexport) int* Test()
{
int arr[] = {1,2,3,4,5};
return arr;
}
C# portion:
[DllImport("Dump.dll")]
public extern static int[] test();
static void Main(string[] args)
{
Console.WriteLine(test()[0]);
Console.ReadKey();
}
I know the return type int[] is probably wrong because of the managed/unmanaged differences, I just have no idea where to go from here. I can't seem to find an answer for anything but returning character arrays to strings, not integer arrays.
I figured the reason the values I'm getting with Marshal.Copy are not the ones I'm returning is because the 'arr' array in the exported function gets deleted but I'm not 100% sure, if anyone can clear this up that would be great.

I have implemented the solution Sriram has proposed. In case someone wants it here it is.
In C++ you create a DLL with this code:
extern "C" __declspec(dllexport) int* test()
{
int len = 5;
int * arr=new int[len+1];
arr[0]=len;
arr[1]=1;
arr[2]=2;
arr[3]=3;
arr[4]=4;
arr[5]=5;
return arr;
}
extern "C" __declspec(dllexport) int ReleaseMemory(int* pArray)
{
delete[] pArray;
return 0;
}
The DLL will be called InteropTestApp.
Then you create a console application in C#.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.InteropServices;
namespace DLLCall
{
class Program
{
[DllImport("C:\\Devs\\C++\\Projects\\Interop\\InteropTestApp\\Debug\\InteropTestApp.dll")]
public static extern IntPtr test();
[DllImport("C:\\Devs\\C++\\Projects\\Interop\\InteropTestApp\\Debug\\InteropTestApp.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern int ReleaseMemory(IntPtr ptr);
static void Main(string[] args)
{
IntPtr ptr = test();
int arrayLength = Marshal.ReadInt32(ptr);
// points to arr[1], which is first value
IntPtr start = IntPtr.Add(ptr, 4);
int[] result = new int[arrayLength];
Marshal.Copy(start, result, 0, arrayLength);
ReleaseMemory(ptr);
Console.ReadKey();
}
}
}
result now contains the values 1,2,3,4,5.
Hope that helps.

More or less the same as the answer above, but this worked for me.
For an array returned from a c method like this:
EXPORT char** methodname(void);
I did not succeed in getting this to work with the MarshalAs attribute, but this did the trick: (condensed code)
[DllImport(libName, EntryPoint = "methodname", CallingConvention = CallingConvention.Cdecl)]
public static extern IntPtr methodname();
var listPtr = methodname();
var list = new List<string>();
IntPtr itemPtr = IntPtr.Zero;
var offset = 0;
while ((itemPtr = Marshal.ReadIntPtr(listPtr, offset)) != IntPtr.Zero)
{
Console.WriteLine(Marshal.PtrToStringAnsi(itemPtr));
offset += 4;
}

Handling Array returned from c++ dll to C#

I have this in my dll created in c++
extern "C" __declspec(dllexport)
char* __stdcall hh()
{
char a[2];
a[0]='a';
a[1]='b';
return(a);
}
And this is how I am trying to handle code in c#
[DllImport(#"mydll.dll",CharSet = CharSet.Ansi,CallingConvention = CallingConvention.StdCall)]
public static extern IntPtr hh();
static void Main(string[] args)
{
IntPtr a = hh();
//How to proceed here???
}
}
Help in proceeding further.

There is no way to handle such arrays. char a[2] is allocated on the stack in your C++ function and is destroyed as soon as you return from it. You should either pass an array from C# and fill it in the C++ code or allocate array in the heap and provide some means for freeing it.
When you have it correct the handling will depend on how you return the data from C++ code. If it's still IntPtr you could use Marshal.ReadByte methods to read characters from memory and use Encoding methods to convert those bytes into string if necessary.
const int bufferSize = 2; // suppose it's some well-known value.
IntPtr p = ...; // get this pointer somehow.
for (var i = 0; i != bufferSize; ++i)
{
var b = Marshal.ReadByte(p, i);
Console.WriteLine(b);
}

I got a solution as follows::
OUR C++ code goes as follows
extern "C" __declspec(dllexport)
char** __stdcall hh()
{
static char* myArray[3] = {"A1", "BB2", "CC3",};
return myArray;
}
And C# goes as follows
[DllImport(#"ourdll.dll",CharSet = CharSet.Ansi,CallingConvention = CallingConvention.StdCall)]
public static extern IntPtr hh();
static void Main(string[] args)
{
IntPtr a = hh();
int j = 0;
string[] s=new string[100];
do
{
s[j] = Marshal.PtrToStringAnsi(Marshal.ReadIntPtr(a,4*j));
j++;
}
while(s[j-1] != null);
}
The only problem now faced is that how can we know size of the array
so that in this statement
string[] s=new string[100];
we neednot waste our memory.

The answer would be
string stra = Marshal.PtrToStringAnsi(a);
But you also have the problem that the dll returns garbage per your code as char* is a local c style string.
Would be ok if you would return something like:
const char* str = "Hello from DLL.";

Try to use not empty StringBuilder as the return value.