从C ++ dll使用__declspec（dllexport）签名声明以在C＃中调用时出现问题

Question

I´m trying to call a method that is in a C++ dll declarated as __declspec(dllexport) to use in C#, but I don´t know how to return a string value from C++ and how to declare the signature using DllImport in C#.

C++ code "VNVAPI.dll"

  __declspec(dllexport) char * GetGpuName(int phyGPUid)
  {
      CNvidia * pInstance = CNvidia::GetInstance();
      char  szName[512]={0};
      pInstance->GetGpuName(phyGPUid,szName,512);
      return szName;
  }

C# method signature:

[DllImport("VNVAPI.dll")]
   public static extern  char GetGpuName(int phyGPUid);

Error generated:

A call to PInvoke function 'Core!Core.Hardware.IO.NVAPI::GetGpuName' 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.

Thanks.

Answer 1

As has been pointed out by others you need to specify the C calling convention in your P/Invoke and also use string on the managed side to marshal the null terminated char*.

However you should rejig the C++ routine to take a char* as an input parameter, together with a buffer length parameter. You then write into this buffer in the native code. This avoids the current problem that the data, as you presently have the code, is returned from the stack which, of course, is unwound as the function returns.

The suggestion to use static will make this memory global and so avoid stack unwind problems, at the expense of thread safety. Yes it will likely work for this use case but its a bad habit to get into.

Answer 2

The error message might be confusing.

Without going into detail, try this:

static char  szName[512]={0};

If you still get the error, the you need to specify the calling convention in the DllImport attribute.

Edit:

Also make the return type string for C# method declaration.

Answer 3

The error message suggests checking the calling convention. I suspect that the function is using C calling convention, which would explain the unbalanced stack. I would suggest that you specify the calling convention (as @leppi suggested) in your DllImport attribute. Like this:

[DllImport("VNVAPI.dll", CallingConvention=CallingConvention.Cdecl)]

According to the Strings samples , the return value should be string :

static extern string GetGpuName(int phyGPUid);