1

My problem is , i have some functions in a DLL some of these functions are for example :

#include <string>
using namespace std;
extern "C" __declspec(dllexport) string __cdecl encryption(string s)
{
 return s;
}

Whenever i try to call this function from C# , here is the code im using :

[DllImport("Packer.dll", EntryPoint = "encryption")]
static extern string encryption(string s);

i get an error : A call to PInvoke function 'Packer' 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.

im guessing i get this error because i dont have the right declarations for the function can anyone guide me how to fix that , thanks in advance

sth
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method
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4 Answers4

7

std::string can not be used with PInvoke, because C++ does not have an ABI for its objects which is required to properly clean stack, copy objects, etc. This is one of the greatest pains of C++.

You have to use char* pointers and plain C APIs. Simply put, PInvoke does not work with C++.

hamstergene
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    It's more complicated than that -- if his P/Invoke signature's return type is `string` then the CLR will try to free the memory returned by the function as though it were a `BSTR`. Merely changing the native return type is not a solution. – ildjarn Oct 05 '11 at 19:55
  • Indeed, the proper C way of doing it is to pass a pre-allocated buffer along with its size into the function and let it write in the buffer for you to read afterwards. Don't let C do any memory allocation for you, it's bad, bad news. – Blindy Oct 05 '11 at 19:58
  • @ildjarn that's right, and Jerry Coffin's point about stdcall shows another issue, but still, the biggest problem here is an attempt to return a C++ object. – hamstergene Oct 05 '11 at 19:58
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    It is not enough to just return **any** kind of pre-allocated memory from P/Invoke-d function. This has to be done in a very specific way (`CoTaskMemAlloc`), so it is compatible with how the P/Invoke will try to deallocate that memory. – Branko Dimitrijevic Oct 05 '11 at 20:07
  • @Branko Actually, if you marshal `char*` as IntPtr then use `Marshal.PtrToStringAnsi()` to create `System.String`, you will achieve “just returning a pointer to buffer” without having to call `CoTaskMemAlloc` :) – hamstergene Oct 05 '11 at 20:14
  • Or even easier, marshal a `StringBuffer` and let the framework work its magic! – Blindy Oct 05 '11 at 20:37
  • P/Invoke does work with C++ DLL, it is a platform invoke service, It is not limited to C Style function. – xInterop Apr 20 '13 at 02:20
  • @dave PInvoke just calls exported functions, it ships nothing to help you with C++. Try catching a C++ exception with it, for example. – hamstergene Apr 20 '13 at 08:30
  • If you export a C++ class in C++ DLL, the non-pure-virtual methods of class will be exported, all the exported methods can be called, even the un-exported virtual methods of C++ class can be called from C#. You can catch the C++ exception when PInvoking a C++ method, the only downside of catching the SEH expcetion from C# is that the .NET run-time only reports it as a generic SEHException without any details about the C++ exception thrown using throw keywords. If you use RaiseException, that information would be reported back to .NET. – xInterop Apr 20 '13 at 08:55
5

As I'm sure you already know, the C++ std::string is actually a template. Only when the template is instantiated (as std::basic_string<char> in this case), the exact layout of the objects of that type and signatures of the methods are determined by the C++ compiler.

Unfortunately, only the C++ compiler in question has the access to all the relevant information (such as template source code) to make these kinds of decisions. That's why non-C features such as templates are generally not "transferable" even between different C++ compilers, let alone C++ and C#.

Also, C++ names are typically "mangled" in a C++ compiler-specific manner.

You'll have to change the signature of your native method, so it is becomes a "pure" C function:

  • Ensure there is no C++ name mangling by declaring the function as extern "C" (you are already doing that).
  • Use char* parameter instead of std::string.
  • Return char* result instead of std::string, but be very careful how you do it.
  • Ensure your DllImportAttribute.CallingConvention matches the __cdecl, __stdcall or __fastcall in your C.
Community
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Branko Dimitrijevic
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  • I already tried that but i get this error : Attempted to read or write protected memory. This is often an indication that other memory is corrupt. – method Oct 05 '11 at 20:09
  • sure , the C# declaration : static extern string encryption(string s); C++ : extern "C" __declspec(dllexport) char* encryption(char* s) { return "test" ; } – method Oct 05 '11 at 20:14
  • @user959615 `return "test"` is the problem. Please follow the link in my answer to see why. – Branko Dimitrijevic Oct 05 '11 at 20:16
  • @user959615 Also, `DllImportAttribute.CallingConvention` defaults to `Winapi`, which in turn defaults to `StdCall` convention, so you have a mismatch there as well. – Branko Dimitrijevic Oct 05 '11 at 20:21
  • I'm sorry it seems like I'm not getting this right but thanks all for your help I'm going to read more about P/Invoke and c++ , Again thanks all for your help – method Oct 05 '11 at 20:26
4

The problem here is, you're using the STL string class which C# doesn't know how to marshal. You have two options here:

  1. refactor your C++ code to work with char * buffers. Or write a wrapper or an overload or something that uses char * instead of string.
  2. Write a C++/CLI wrapper around your C++ functions that uses System::String and calls the STL string versions internally.
Blindy
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2

If memory serves, if you don't specify otherwise P/Invoke assumes the calling convention is __stdcall. If so, changing your __cdecl to __stdcall should fix the first problem. As @Adam Rosenfield points out, you probably also need to pass and return a char const *, not a string. C# and C++ almost certainly have somewhat different ideas of what constitutes a string.

Jerry Coffin
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  • Perhaps it would be better to change the calling convention of the C# declaration instead. – Jeff Mercado Oct 05 '11 at 19:51
  • I get this error : Unable to find an entry point named 'encryption' in DLL 'Packer.dll'. Edit : Im going to try as Jeff mercado said Edit 2 : tried doing it from C# declaration but i got this A call to PInvoke function 'Packer' 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. – method Oct 05 '11 at 19:51
  • @user959615: yes, by default stdcall will add a suffix indicating the size of the arguments, so it would turn into `encryption@4`. You can eliminate that with a linker [module definition file](http://msdn.microsoft.com/en-us/library/hyx1zcd3.aspx). – Jerry Coffin Oct 05 '11 at 19:59
  • @JeffMercado: Yes, either should work of course -- they just have to match. – Jerry Coffin Oct 05 '11 at 19:59