Passing string from C# to C .dll - getting extra characters

Question

I am working on a C# project that will use a .dll (written in C) When passing a char array to the .dll function runInterpretation I am getting extra characters added to my string.

C# code:

[DllImport(@"c:\projectName\main.dll", CallingConvention = CallingConvention.Cdecl, EntryPoint = "runInterpretation")]

public static extern int runInterpretation(char[] inputStr);

static void Main(string[] args)
{
    string inputString = "DR1234,2014/07/27 15:20:10,1,0,3,0,0,2,5,30,10,10,0,55,205,21500,86400,110,0,";

    int tmp = runInterpretation(inputString.ToCharArray());

}

Which calls a C .dll:

#include <stdio.h>                                                                      
#include <time.h>                                                                       
#include <string.h>                                                                     

#include "fixedtext.h"                                                              
#include "englarrini.h"
#include "numarrini.h"                                                              
#include "argcvalue.c"                                                              
#include "dgavalid.c"                                                                   
#include "ratio.c"                                                                      
#include "interp.c"                                                                     
#include "validdisplay.c"                                                           

#ifdef DEVX
#define DATAFILEPATH "/home/drmcc/log/"                             
#elif DEVY
#define DATAFILEPATH "/home/drmcc/log/"                             
#elif PC
#define DATAFILEPATH ".\\data\\"                                            
#else
#define DATAFILEPATH ".\\data\\"                                            
#endif

__declspec(dllexport) int runInterpretation(char *inputString[])
{
    printf("args %s\n", inputString);

    return 1;
}

The end result when running my C# project is as follows (random extra chars after the last ',')

I would like to know why the extra characters are being added and how might I get rid of them.

Thank you

Answer 1

I don't think you want what you have for your C dll's argument. If you're just wanting one string passed in, then it should be:

__declspec(dllexport) int runInterpretation(char *inputString)

or better:

__declspec(dllexport) int runInterpretation(wchar_t *inputString)

From there, you just need to tell C# how to marshal your string. The easiest way to do that is to use the MarshalAsAttribute. If you go with the char * interpretation, use

public static extern int runInterpretation(
[MarshalAs(UnmanagedType.LPStr)]
String inputStr);

or the Unicode version:

public static extern int runInterpretation(
[MarshalAs(UnmanagedType.LPWStr)]
String inputStr);

It's also worth noting that if you change your C parameter to Unicode, you don't have to use the Marshalling hint at all in C# as it will marshal correctly by default.

Answer 2

In C, strings are expected to be null terminated. The "extra characters" are whatever happens to be in memory after the array until a null character is found. ToCharArray() won't append a null character because that wouldn't make sense in the context of C#. I can think of two ways to fix this:

• Assuming you can change the API of the C DLL, add a second length parameter, and use that to make sure you don't go past the end of the array
• Append a null character to the string prior to calling ToCharArray() . See https://stackoverflow.com/a/2794356/3857 for an example