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How to implement a C++ callback in C#

I'm having real troubles trying to use a c++ callback in C# and any help from you would be really appreciated.
The first thing that the code do is to create an event:
uEvent = CreateEvent(NULL, true, false, "EventName");
After that, I've got that c++ function that implements a callback in c++:
int RegisterCallback(TCallbackType CallbackType, void *pLLTProfileCallback, void *pUserData)
The CallbackType is used to specify a stdcall. Then, the pLLTProfileCallback would be the name of the function to call, and finally, the user data.
With that function, I'm able to make a callback to another function by typing the following:
RegisterCallback(STD_CALL, (void*)FunctionName, 0)
Where the other function is:
void __stdcall FunctionName(const unsigned char* pucData, unsigned int uiSize, void* pUserData)
And then, I've got the code to wait for the event:
WaitForSingleObject(uEvent, 1000)
My problem comes when I try to do that in C#. The first thing I do is import the RegisterCallback Function from the dll:
[DllImport(DRIVER_DLL_NAME, EntryPoint = "s_RegisterCallback",CallingConvention = CallingConvention.StdCall)]
internal static extern int RegisterCallback(uint pLLT, TCallbackType CallbackType, IntPtr pLLTProfileCallback, IntPtr pUserData);
But then, I don't know how to proceed. Could you please help me?

Delegates in C# which is equivalent to C++ callback.
Please refer to MSDN Introduction & Syntax and usage for same

Shared memory between C++ DLL and C# code

I am currently working on a project with really short deadline, so I don't have much time to understand everything. Also, I am not an expert in C++ development and memory management.
So, what I am trying to do is to create a DLL in with both C and C++ code. Then, I would like to call this DLL in a C# code. Currently, the communication between C++ and C# is OK. The problem comes up when I try to transfer a string from the DLL to the C# code. The error is this one :
System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
at Microsoft.Win32.Win32Native.CoTaskMemFree(IntPtr ptr)
at System.StubHelpers.CSTRMarshaler.ClearNative(IntPtr pNative)
at NMSPRecognitionWrapper.Program.GetResultsExt()
at NMSPRecognitionWrapper.Program.<Main>b__0() in <my dir>\Program.cs:line 54
at NMSPRecognitionWrapper.Program.StartRecognitionExt()
at NMSPRecognitionWrapper.Program.Main(String[] args) in <my dir>\Program.cs:line 60
Also, I can give you some piece of code below (really simplified !). Actually, the C++ expose two methods : StartRecognition() launch operations to get some data from microphone, then process them and store the results. GetResults() return an instance of the results previously stored. The WrapperCallback() allows the C# part to be called when a Result is able for processing. The C# part, when the Callback is called, will ask to get the results using the GetResults() method.
I know the architecture may seem really inappropriate in this presentation, but I don't want to explain the whole project to validate the model, please be sure everything is correct.
To finish, the problem is when the C# callback call the GetResults() method. Trying to access to the resultsForCS seems to be impossible from the C#.
C++ part - header
// NMSPRecognitionLib.h
#pragma once
#include <iostream>
using namespace std;
extern "C" __declspec(dllexport) char* GetResults();
extern "C" static void DoWork();
extern "C" __declspec(dllexport) void StartRecognition();
C++ part - sources
#include "stdafx.h"
#include "NMSPRecognitionLib.h"
static char * resultsForCS;
static SUCCESS ProcessResult(NMSPCONNECTION_OBJECTS *pNmspConnectionObjects, LH_OBJECT hResult)
{
[...]
char* szResult;
[...]
resultsForCS = szResult;
DoWork();
[...]
return Success;
error:
return Failure;
} /* End of ProcessResult */
extern "C" __declspec(dllexport) char* GetResults()
{
return resultsForCS;
}
extern "C"
{
typedef void (*callback_function)();
callback_function gCBF;
__declspec(dllexport) void WrapperCallback(callback_function callback) {
gCBF = callback;
}
static void DoWork() {
gCBF();
}
}
extern "C" __declspec(dllexport) void StartRecognition()
{
char* argv[] = { "path", "params" };
entryPoint(2, argv);
}
C# part
class Program
{
[DllImport("NMSPRecognitionLib.dll", EntryPoint = "GetResults")]
[return: MarshalAs(UnmanagedType.LPStr)]
public static extern string GetResultsExt();
public delegate void message_callback_delegate();
[DllImport("NMSPRecognitionLib.dll", EntryPoint = "WrapperCallback")]
public static extern void WrapperCallbackExt(message_callback_delegate callback);
[DllImport("NMSPRecognitionLib.dll", EntryPoint = "StartRecognition")]
public static extern void StartRecognitionExt();
static void Main(string[] args)
{
WrapperCallbackExt(
delegate()
{
Console.WriteLine(GetResultsExt());
}
);
StartRecognitionExt();
Console.WriteLine("\nPress any key to finish... ");
var nothing = Console.ReadLine();
}
}
I understand that the problem comes because I am using a pointer to store the results (char *), but I actually don't know how to do this in another way. The szResults type is char * too and I can't change this !

Yes, the return type is the problem. The pinvoke marshaller must do something to release the memory that was allocated for the string. The contract is that memory allocations that need to be released by the caller must be allocated from the COM heap. CoTaskMemAlloc() in native code, also exposed in .NET as Marshal.AllocCoTaskMem().
This rarely comes to a good end, most native code allocates with malloc() or ::operator new, allocating from a heap that's created by the C runtime library. The wrong heap. So inevitably the CoTaskMemFree() call will fail. Ignored silently in Windows XP and earlier, a kaboom on Vista and up.
You must stop the pinvoke marshaller from trying to release the memory. Do so by declaring the return value as IntPtr. And use Marshal.PtrToStringAnsi() to recover the string.
You still have a Big Problem, the kind of problem that bedevils any native code that tries to use this function as well. You still have a string buffer that needs to be released. You cannot do that from C#, you can't pinvoke the correct version of free() or ::operator delete. A memory leak is inevitable. The only thing you can hope for is that the native code takes care of it, somehow. If it doesn't then you must use C++/CLI to interop with it. With the additional requirement that the native code needs to be rebuilt with the same compiler so that it uses the same shared CRT. Code that's difficult to use correctly from native code is also hard to pinvoke. That's a design flaw, always allow the caller to pass a buffer to be filled in so there's never a question who owns the memory.

Looking at:
at Microsoft.Win32.Win32Native.CoTaskMemFree(IntPtr ptr)
at System.StubHelpers.CSTRMarshaler.ClearNative(IntPtr pNative)
at NMSPRecognitionWrapper.Program.GetResultsExt()
I can see that your callback is called, but the runtime tries to free some memory. I think it assumes your pointer would be to com memory. Try converting the string yourself, it is easy!
[DllImport("NMSPRecognitionLib.dll", EntryPoint = "GetResults")]
public static extern IntPtr GetResultsExt();
[...]
string result = Marshal.PtrToStringAnsi(GetResultsExt())
No 100% guarantee, but worth a try.

I have found that it is usually easier to write a wrapper in C++/CLI around the C++ native code. A C++/CLI class can directly call and use native C++, but is accessible from C# (and any .Net language). In my experience, using DLLImport as you do leads to hard to debug and find errors.

PInvoke: Error in returing text from C++ to C#

I am using Pinvoke for Interoperability between Native(C++) code and Managed(C#) code. What i want to achieve is get some text from native code into my managed code as a retrun value i.e My C# Code calls my C++ function which returns some text back to C#. Following is my code for this.
C++ Code:
static std::wstring WSTR_TEMP;
wchar_t* cppReturnSomeText()
{
UnicodeString usText ("Some Text Here");
WSTR_TEMP = ECUtilsICU::UniCodeStringToWString(usText);
return (wchar_t *)(WSTR_TEMP.c_str());
}
C# Code:
[DllImport(MY_DLL_NAME]
[return: MarshalAs(UnmanagedType.LPWStr)]
private static extern string cppReturnSomeText();
public static string GetSomeText()
{
string s = cppReturnSomeText();
return s;
}
Every thing was working fine as expected. Now i simply change my operating system from WinXP(32Bit) to Win7(64Bit). And when i run this code following error occurred:
"Attempted to read or write protected memory. This is often an indication that other memory is corrupt."

I guess the problem is that you are allocating memory with your C++ runtime's allocator but then the C# marshaller attempts to deallocate it. That cannot work. You need to allocate and deallocate with the same allocator.
The best way I know to solve your problem is to marshal with a BSTR. That uses the COM allocator which is happy to be shared between native and managed modules.
#include <comutil.h>
BSTR cppReturnSomeText()
{
UnicodeString usText("Some Text Here");
std::wstring result = ECUtilsICU::UniCodeStringToWString(usText);
return ::SysAllocString(result.c_str());
}
On the C# side you do this:
[DllImport(MY_DLL_NAME, CallingConvention=CallingConvention.Cdecl)]
[return: MarshalAs(UnmanagedType.BStr)]
private static extern string cppReturnSomeText();
One other benefit of this is that your code is now thread-safe because you no longer need a global variable.

P/Invoke problem (unbalanced stack)

I am trying to make a wrapper for a native c++ .dll using P/Invoke.
The source code for the .dll has the following entry point specified:
// .h-file
CHROMAPRINT_API ChromaprintContext *chromaprint_new(int algorithm);
And the method implementation:
// .cpp-file
ChromaprintContext *chromaprint_new(int algorithm)
{
ChromaprintContextPrivate *ctx = new ChromaprintContextPrivate();
ctx->algorithm = algorithm;
ctx->fingerprinter = new Fingerprinter(CreateFingerprinterConfiguration(algorithm));
return (ChromaprintContext *)ctx;
}
The ChromaprintContextPrivate type is a structure:
>// .cpp-file
struct ChromaprintContextPrivate {
int algorithm;
Fingerprinter *fingerprinter;
vector<int32_t> fingerprint;
};
My C# wrapper code:
// .cs-file
[System.Runtime.InteropServices.DllImportAttribute(
"libchromaprint.dll",
EntryPoint = "chromaprint_new")]
private static extern System.IntPtr chromaprint_new(int algorithm);
public static IntPtr Chromaprint_New(ChromaprintAlgorithm algorithm)
{
// Hardcoded parameter for testing
return chromaprint_new(0); // (int)algorithm
}
Calling IntPtr ptr = Chromaprint_New(0); raises the following MDA exception:
A call to PInvoke function 'MyProject.ChromaprintWrapper!'MyProject.ChromaprintWrapper.LibChromaPrint::chromaprint_new' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature.
So I understand what the problem is (the number of entries on the stack isn't what is expected). I'm assuming the method parameter int algorithm is ok. I'm not sure about the return type though. Should it be a structure instead of a pointer?
I got the C# code above by running the .h-file through P/Invoke Interop Assistant. Is the return type wrong? what should it be?
What is the C# representation of vector<int32_t> fingerprint;?
(See ChromaprintContextPrivate structure above.)

You most likely need to specify the calling convention.
Try the following:
[System.Runtime.InteropServices.DllImportAttribute("libchromaprint.dll",
EntryPoint = "chromaprint_new",
CallingConvention=CallingConvention.Cdecl)]
By default, this uses Winapi (which is effectively StdCall) to make it easier to call into the Windows API, but this is not typically the default on most C++ libraries.

It's because of how the parameters are actually passed, the calling convention.
Try
[DllImport("libchromaprint.dll", CallingConvention = CallingConvention.Cdecl)]
private static extern IntPtr chromaprint_new(int algorithm);

You need to declare cdecl calling convention on the dllimport attribute.

Error in my calling Function

this is my code
public delegate void NotifyFunc(enumType notificationType, IntPtr data);
[DllImport("VssSdkd")]
public static extern void startVssSdkClientEcho(string IpAddress,
long port, NotifyFunc notifyFunc, eProtocolType proType, bool Req);
am call the function like
CallBack calbck = new CallBack(TestDllImport.Callbackas);
startVssSdkClientEcho("192.168.10.240", 12050, calbck, TestDllImport.eProtocolType.eProtocolType_VssSdk, false);
here my receving callback function is
static public void Callbackas(eNotificationType type, IntPtr datas) {}
here first time am receving data and then it's giving Error msg like :
run time check failure #0 - the value if ESP was not properly saved across a function call.
this is usually a result of calling a function declared with one calling convention
with a function pointer declared with a different calling convention
please help me to identify my error... thanks in advance

Is this your code you are calling (sorry cursory search could only find your other post referring to startVssSdkClientEcho). Is it decalred cdecl (i.e. it has no __declspec at all) or is it stdcall. If it is cdecl you need to add CallingConvention attribute to the DllImport
i.e.
[DllImport("VssSdkd", CallingConvention=CallingConvention.Cdecl)]
Of course it might also be the callback calling convention at fault which is another issue.