Re: am having a problem with pinvoke and StringBuilder[ ]

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"LK" <lkant2000@xxxxxxxxx> wrote in message news:u0RgHEhqIHA.524@xxxxxxxxxxxxxxxxxxxxxxx
Hi,

From a C# program I need to access an existing DLL that enumerates the
boards detected via a hardware probe of the system. The function that
does the enumeration expects an int * where it stores the number of boards
detected and a char *** where it stores the name of each board detected.

Since the String class is immutable, I have used StringBuilder in my code
but this results in a NullPointerException. Just for testing, I replaced
StriingBuilder[] with String[] and the code worked just fine (i.e, no
NullPointerExceptions occured and all messages populated in
the String[] in managed code was correctly printed by the DLL).

I am wondering what I am doing wrong in my code. For reference, I am
enclosing my C# code and my sample DLL code

thank you for your help.
Laxmikant Rashinkar (LK)

Here is the C# code
---------------------
using System;
using System.Text;
using System.Runtime.InteropServices;

class CallDll
{
[DllImport("TestLib.dll")]
public static extern void AccessCheck();

[DllImport("TestLib.dll")]
public static extern int EnumerateBoards(ref int numBoards, ref StringBuilder[] boards);

public static void Main()
{
int numBoards = 0;
int maxBoards = 5;
int i;

StringBuilder[] boards = new StringBuilder[maxBoards];
for(i=0; i<maxBoards; i++)
boards[i] = new StringBuilder(100);

AccessCheck();
EnumerateBoards(ref numBoards, ref boards);
Console.WriteLine("Found " + numBoards + " boards\n");
for(i=0; i<numBoards; i++)
Console.WriteLine("" + boards[i] + "\n");
}
}

Here is the DLL code
----------------------
#include <stdio.h>
#include <string.h>

extern "C" __declspec(dllexport) void AccessCheck()
{
printf("Hello from TestLib.dll\n");
}

extern "C" __declspec(dllexport) int EnumerateBoards(int *numBoards, char ***boards)
{
char **cpp = *boards;

strcpy(cpp[0], "VGA");
strcpy(cpp[1], "NIC");
*numBoards = 2;
return 0;
}

Here is the output from my program:
------------------------------------
Hello from TestLib.dll

Unhandled Exception: System.NullReferenceException: Object reference not set
to an instance of an object.
at CallDll.EnumerateBoards(Int32& numBoards, StringBuilder[]& boards)
at CallDll.Main()

And here is how I compile my test code
----------------------------------------
@echo off
cl TestLib.cpp -LD -FeTestLib.dll
csc CallDll.cs
CallDll




Why the triple (***) indirection, a stringBuilder is passed as a pointer and a StringBuilder[] is passed as a pointer to a pointer?

...
extern "C" __declspec(dllexport) int EnumerateBoards(int *numBoards, char **boards)
{
char **cpp = boards;
...

Remove the ref from the DllImport declaration and the function call in your C# code and it should work.

....
public static extern int EnumerateBoards(ref int numBoards, StringBuilder[] boards);

EnumerateBoards(ref numBoards, boards);


Willy.


.

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