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c# DLLImport calling c++ method with char* as parameter

I got an external DLL (c++) with the follwing method:
void _stdcall Set_Config(char* config)
I use the following c# code to call the method:
[DllImport(DllName,CharSet=CharSet.Auto)]
public static extern void Set_Config(String config);
But when i execute my c# code i get either an acces violation exception or an System.Runtime.InteropServices.SEHException.
(My dll is 32 bit, and my c# compiler compiles to 32 bit)
I also tried to replace String config with Stringbuilder, but the same result.
Can someone help me with this problem or give some example code how i have to call the c++ method?

CharSet.Auto will encode as UTF-16. But your function accepts char* and so the text is encoded as 8 bit text, presumably ANSI.
[DllImport(DllName, CharSet = CharSet.Ansi)]
public static extern void Set_Config(string config);
Calling the function is trivial.
I am assuming that the string is being passed into the function. On the other hand, it is odd that the parameter is char* rather than const char*. Either the developer doesn't know how to use const, or perhaps the function really does send data back to the caller.
If the data flows the other way, then you are in trouble. You'd need to pass a StringBuilder but you've no way to tell the DLL how large a buffer is available. If the data is flowing the other way then the code looks like this:
[DllImport(DllName, CharSet = CharSet.Ansi)]
public static extern void Set_Config(StringBuilder config);
Call the function like this:
StringBuilder config = new StringBuilder(256); // cross your fingers
Set_Config(config);
Either way, you need to be more clear as to what this function is actually doing. You cannot hope to call this function until you know whether to pass data in, or receive data out.

You have to pass an IntPtr which is a raw pointer to your string. (From my memories, Marshal.StringToBSTR)
public static extern void Set_Config(IntPtr config);

How To hook unmanaged function inside managed function?

Here is my C++ Code:
This function is exported in my DLL.
EXPORT set_hook(fnc_public_hook hook){
public_hook = hook;
}
Now if I hook it from unmanaged code like following everything works fine.
set_hook(my_fnc);
Now before every event in my DLL my_fnc is called, so I can do some pre process on data.
The problem is that I don't know how to do this in .NET.
How to get .NET function pointer ? And how to use set_hook() to call my .NET function before every event ?

using System.Runtime.InteropServices;
Somewhere, create a delegate to your function you would like called
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
internal delegate uint YourFunctionDelegate();
Set your delegate to point to your function, ie in your constructor,
YourFunctionDelegate = myDotNetFunc;
In NativeMethods class inside your namespace,
[DllImport("YourDLL.dll", EntryPoint = "set_hook", CallingConvention = CallingConvention.Cdecl)]
private static extern int setHook([MarshalAs(UnmanagedType.FunctionPtr)]YourFunctionDelegate myDelegate);
Finally, you can call your c++ function with
NativeMethods.setHook(YourFunctionDelegate);
Make sure you know which calling convention is appropriate.
EDIT: Dont get garbage collected!
http://msdn.microsoft.com/en-us/library/ms182294%28v=vs.80%29.aspx
Preventing unmanaged function pointer garbage collection

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.

C++ dll not returning to c#

I am working on a project that puts C# interacting with a previously created DLL in C++.
The code below shows how am I exporting the function I need:
extern "C" __declspec(dllexport) int iterateAndTest(int testSize, char* testHash){
CUDADLL dll;
int ret = dll.iterateAndTest(testSize, testHash);
return ret;
}
The code below shows how I declare the function in C#:
[DllImport("C:\\Users\\BrunoBraga\\Documents\\Visual Studio 2012\\Projects\\CUDADLL\\Debug\\CUDADLL.dll")]
public static extern int iterateAndTest(int testSize, string testHash);
The problem is: the dll function is not returning anything. In fact, the dll once called, never returns to C# and the program simply ends.
I am not sure if I am giving the right code, but i suppose there is something about the dlls I am missing.
Sorry for the delay on response. THere were two problems:
1-When i created the object, i should have used new insted of just CUDADLL dll. I thought i could do this since without the new i was still able to access the inner fields.;
2-The program should run in admin, otherwise, some crashes would ocur;
Thanks again guys.

In Your c++ code insert __stdcall
extern "C" __declspec(dllexport) __stdcall int iterateAndTest(int testSize, char* testHash);
in C#
[DllImport("yourdll.dll", EntryPoint="iterateAndTest", CallingConvention=CallingConvention.StdCall)]
public static extern int iterateAndTest(int testSize, string testHash);
use cout in your C++ function, if the function is called output will be displayed in output window if you are using Visual Studio.
try placing your c++ dll in your output directory
Possible Errors: dll not found or entry point exception will be thrown.

How can I write a signature on C# for a wrapped C++ method having a pointer to a function it its arguments?

I'm writing a wrapper for a dll. The dll has a method whose signature resembles the following:
unsigned long aMethod(void *anyParameter,
void (*anotherMethod)(const char *, void *))
I've searching at google for a tutorial to give me insight on how to write the signature on C# so the framework can do the marshalling process.
How can it be written?
Do you know about any tutorial, book or documentation on this subject?

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
delegate void AnotherMethodDelegate(string s, IntPtr anyParameter);
[DllImport("dllname",
CallingConvention = CallingConvention.Cdecl,
CharSet = CharSet.Ansi)]
uint aMethod(IntPtr anyParameter, AnotherMethodDelegate anotherMethod);