Pass char * argument and get char * return from a C++ DLL to a VB.NET Application

[nsssayom]

I have a C++ DLL function with the following definition-
char * scanImage(char * fileDir)

{
	string dir(fileDir);
	string cvStr = cvScan(dir);
	char * scanReturn;
	scanReturn = (char*)CoTaskMemAlloc((cvStr.size() + 1) * sizeof(char));
	strcpy(scanReturn, cvStr.c_str());
	cout << "DLL Output: " << endl << scanReturn <<endl;
	return scanReturn;
}

Headerfile of the DLL is-

#pragma once
#include <Windows.h>
#include <string>
using namespace std;

#ifdef THEIA_OPENCV_DLL_EXPORTS
#define THEIA_OPENCV_DLL_API __declspec(dllexport)
#else
#define THEIA_OPENCV_DLL_API __declspec(dllimport)
#endif

extern "C"  THEIA_OPENCV_DLL_API char * scanImage(char * fileDir);

Now I want to call this scanImage function from a VB.NET Application.

So far I have been trying these-

Trial 1:

Imports System.Runtime.InteropServices
Public Class Form1

    Public Declare Function scanImage Lib "Theia_OpenCV_Dll.dll" 
    (<MarshalAs(UnmanagedType.LPStr)> fileDir As String) As IntPtr
    
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim out As IntPtr = scanImage("files/stu2/2.bmp")
        Label1.Text = Marshal.PtrToStringAnsi(out)
    End Sub

End Class

This code changed the default text of label1 to blank. But displays nothing. On second click this terminates the program.

Trial 2:

Imports System.Runtime.InteropServices
Public Class Form1

    Public Declare Function scanImage Lib "Theia_OpenCV_Dll.dll" (<MarshalAs(UnmanagedType.LPStr)> fileDir As String) As <MarshalAs(UnmanagedType.LPStr)> System.Text.StringBuilder
    
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim sb As System.Text.StringBuilder = scanImage("files/stu2/2.bmp")
        label1.text = sb.ToString
    End Sub

End Class

This gives me-

Program_name has stopped working

and the program terminates immediately.

I am using VB.NET in Visual Studio 2017.
Now how to solve this problem?

[Niya]

Your Trial 1 code appears to be correct. That being the case, are you sure the C function scanImage is actually working?

[nsssayom]

Your Trial 1 code appears to be correct. That being the case, are you sure the C function scanImage is actually working?

Yeah, the C (Actually C++) function is working. And this problem is solved anyway. And you are right, Trial 1 code was right. There was a little problem with DLL path.