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.