Memory access error when calling a C ++ function from managed C # code

I have a managed C # program that calls a function in an unexplored C ++ dll. I am getting an error. I debug tirelessly but can't seem to find a solution. Please advise me to pass the correct data format from C # to the C ++ callable function.

Here's what I have:

Mistake:

" at Frame_Processor.ProcessFrames(Frame_Processor* , SByte* , SByte* , Int32 , SByte* ) "   
     ***-- this is the  unmanged C++ function that is called.***

" at Frame_Processor_Wrapped.ProcessFrames(String framesPath, String datasetPath, Int32 frameCount, String keyName)" 
    ***-- this is the manged function that called it.***

      

        
        
        
      

    

it seems that the format of the arguments is the problem. SByte * / string. Please advise.

NEW EDIT:

C # (managed code)

static void Main(string[] args)
        {
            //test
            string id = "abcd";
            string in = "C:\\input\\";
            string out = "C:\\output";
            int count = 10;

            Frame_Processor_Wrapped fp = new Frame_Processor_Wrapped();
            fp.ProcessFrames(in, out, count, id);
        }

      

        
        
        
      

    

C ++ / CLI Code (with marshaling) - AKA Frame_Processor_Wrapped

 void Frame_Processor_Wrapped::ProcessFrames(System::String^ in, System::String^ out, int count, System::String^ id)
        {               
            StringConversion ^convert = gcnew StringConversion();

            //convert System.String to char*
            char* in_char= convert->myStringToChar(in);             
            char* out_char= convert->myStringToChar(out);
            char* id_char= convert->myStringToChar(id);

            frameProcessor->ProcessFrames(in_char, out_char, count, id_char);                               
        }

      

        
        
        
      

    

*

/* Conversion Class*/   
StringConversion::StringConversion() {}

        char* StringConversion::myStringToChar(System::String^ str)     {
            System::IntPtr ptr = Marshal::StringToHGlobalAnsi(str);         return
     static_cast<char*>(ptr.ToPointer());   }

      

        
        
        
      

    

*

C ++ DLL (unchanged)

void Frame_Processor::ProcessFrames(char* in, char* out, int count, char* id)
    {
        //use the parameters here for something (w,x,y,z varaibles)
        w = out;
        x = in;
        y = count;
        z = id;

        //do something
        RunProcessFrames();
    }

      

        
        
        
      

    

I am getting a runtime error for memory access.