Passing a structure having strings to C++

Question

Hi all,

I am trying to pass a structure that contains a string to C++. I am having trouble in doing so. There seems to be a read/write memory error when I execute the code. The following is the code listing

C++

============
| Driver.h |
============

struct FluidDescription
{
	char *componentName;

	int phaseCode;
};

struct InputData
{
	FluidDescription ccmp;
};

#if defined (EXPORT)
#define DLL_EXPORT __declspec(dllexport)

#else
#define DLL_EXPORT __declspec(dllimport)
#endif

extern "C"
{
	int DLL_EXPORT WriteInputFile(InputData *input);
}

==============
| Driver.cpp |
==============

#include <iostream>
#include "Driver.h"

using namespace std;

int WriteInputFile(InputData *input)
{
	cout << input->ccmp.phaseCode << endl;
	cout << input->ccmp.componentName << endl;

	return 0;
}

VB.net

Imports System.Runtime.InteropServices

Public Class Form1
    <DllImport("StructPassTest.dll", CallingConvention:=CallingConvention.Cdecl)> _
    Private Shared Function WriteInputFile(ByRef input As InputData) As Integer

    End Function
    <StructLayout(LayoutKind.Sequential)> _
    Public Structure InputData
        Public ccmp As FluidDescription
    End Structure
    <StructLayout(LayoutKind.Sequential)> _
    Public Structure FluidDescription
        Public phaseCode As Integer
        Public componentName As String
    End Structure
    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        Dim input As New InputData
        Dim rc As Integer
        
        input.ccmp.componentName = "Test"
        input.ccmp.phaseCode = 1
        rc = WriteInputFile(input)
        MsgBox(rc)

        
    End Sub
End Class

The program fails when it enters the function WriteInputFile. On searching the internet, i figured that I had to use the Marshaling feature to convert the string within the structure to unmanaged type.

In order to do that it was suggested IntPtr be used. So I changed the struct FluidDescription as follows

<StructLayout(LayoutKind.Sequential)> _
    Public Structure FluidDescription
        Public phaseCode As Integer
        Public componentName As IntPtr
    End Structure

and I tried my best to somehow convert the string into a IntPtr and had no luck in doing so.

I would appreciate if someone can post a snippet as to how I can populate the variable componentName with a value and pass it to C++

Should I also convert the structure InputData to a IntPtr before passing it to the function WriteInputFile?

Thanks in advance,

Kaushik

Answer 1

First step is correcting the order of the fields in the structure FluidDescription.

In addition, try this:

<MarshalAs(UnmanagedType.LPStr)> Public componentName As String

BTW, there's a specialized Interop forum: http://social.msdn.microsoft.com/Forums/en-US/vbinterop/threads

Please change the thread type from discussion to question.

---

Armin

Answer 2

Hi Armin,

Changing the order solved the problem for me. I didn't have to use the

<MarshalAs(UnmanagedType.LPStr)> Public componentName As String

in my structure.

I am curious as to why I was able to get it to work without using IntPtr that some of the forums were recommending?

-Kaushik

Answer 3

Hi Iamreal,

please check this link MarshalAsAttribute Class:http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshalasattribute(v=vs.100).aspx

---

No code, No fact.

Answer 4

I am curious as to why I was able to get it to work without using IntPtr that some of the forums were recommending?

Because you're "da maaaan"! Oh, or maybe "da womaaan". ;-)

---

Armin