C编写的DLL函数，返回值类型为char *，VB.net如何调用这个函数

#include "dll.h"
#include "calculation.h"
#include <string.h>
#include <windows.h>

DLLIMPORT char *SectionSteelAW(char const *RawText, unsigned const CtrlCode) {
    char *FormatedText = NULL;
    char *SecSteType = NULL;
    char *Resault = NULL;
    void *obj = NULL;
    if (formatting(RawText, &FormatedText) == 0)
        return NULL;
    SecSteType = getSecSteType(FormatedText);
    if (SecSteType == NULL)
        return NULL;
    obj = NewObj(SecSteType);
    if (obj == NULL)
        return NULL;
    if (SetData(obj, SecSteType, FormatedText) == 0) {
        FreeObj(&obj, SecSteType);
        return NULL;
    }
    Resault = getResault(obj, SecSteType, CtrlCode);

    FreeObj(&obj, SecSteType), obj = NULL;
    free(FormatedText), FormatedText = NULL;
    //SecSteType来自const字符串数组，不需要释放，只需赋值NULL
    SecSteType = NULL;
    return Resault;
}

BOOL WINAPI DllMain(HINSTANCE hinstDLL,DWORD fdwReason,LPVOID lpvReserved) {
    switch(fdwReason) {
        case DLL_PROCESS_ATTACH: {
            break;
        }
        case DLL_PROCESS_DETACH: {
            break;
        }
        case DLL_THREAD_ATTACH: {
            break;
        }
        case DLL_THREAD_DETACH: {
            break;
        }
    }

    /* Return TRUE on success, FALSE on failure */
    return TRUE;
}

#include <windows.h>
#include <stdio.h>
/*
#define TYPE_AREA                    1
#define TYPE_EXCLUDE_TOPSURFACE        2
#define TYPE_WEIGHT                    4
#define METHOD_ROUGHLY                8
#define METHOD_PRECISELY            16
#define METHOD_LOOKUP                32

DLLIMPORT char *SectionSteelAW(char const *RawText, unsigned const CtrlCode);
*/
typedef char *(*p_SectionSteelAW)(char const *RawText, unsigned const CtrlCode);

int main (int argc, char *argv[]) {
    HMODULE module = NULL;
    p_SectionSteelAW SSAW = NULL;
    char *resault = NULL;
    char RawText[64] = {'\0'};
    unsigned ctrlcode = 0;
   
    module = LoadLibraryA("SectionSteelAW.dll");
    if (module == NULL) {
        MessageBox(0,"Cannot start this program because of missing \"SectionSteelAW.dll\" file!\n","Error",MB_ICONERROR);
        return 0;
    }
    SSAW = (char *(*)(char const *, unsigned const))GetProcAddress(module, "SectionSteelAW");
    if (SSAW == NULL)
        goto clean;
        
    while (scanf("%s %u", RawText, &ctrlcode) != EOF) {
        resault = SSAW(RawText, ctrlcode);
        if (resault != NULL)
            printf("%s\n", resault);
        free(resault);
        resault = NULL;
    }
   
clean:
    FreeLibrary(module);
   
    return 0;
}

Module SectionSteelAW
    Public Const _TESTFLAG = 0                  '测试代码控制符

    Public Const TYPE_AREA = 1                  '计算类型：面积
    Public Const TYPE_EXCLUDE_TOPSURFACE = 2    '计算类型：面积子项，扣除顶面
    Public Const TYPE_WEIGHT = 4                '计算类型：重量
    Public Const METHOD_ROUGHLY = 8             '计算模式：粗略
    Public Const METHOD_PRECISELY = 16          '计算模式：精细
    Public Const METHOD_LOOKUPINTABLE = 32      '计算模式：查表

    Public CTRLCODE As Integer = 0              '控制码
    Public Offset_Rows As Integer = 0           '目标行偏移参数
    Public Offset_Columns As Integer = 1        '目标列偏移参数
    Public Overwrite As Integer = 0             '目标已有数据是否覆盖参数

    Public Declare Function SectionSteelAW Lib "SectionSteelAW.dll" (ByVal RawText As String, ByVal CtrlCode As UInteger) As String

    Public Sub Generate()
        Dim xlApp As Object = Nothing       'Excel对象
        Dim xlWorkbook As Object = Nothing  '工作薄对象
        'Dim xlSheet As Object = Nothing     '工作表对象
        Dim xlRange As Object = Nothing     '单元格区域
        Dim xlCell As Object = Nothing      '单元格

        Dim sRawText As String = Nothing
        Dim sResault As String = Nothing

        Dim starttime As Date
        Dim endtime As Date

        '获取Excel程序、工作薄、工作表、选定区域
        On Error Resume Next
        xlApp = GetObject(, "Excel.Application")
        If Err.Number <> 0 Then
            Err.Clear()
            MsgBox("Please open an Excel application first!", vbOKOnly + vbExclamation, "Waring")
            Exit Sub
        End If
        xlWorkbook = xlApp.ActiveWorkbook
        If xlWorkbook Is Nothing Then
            MsgBox("Please open an Workbook first!", vbOKOnly + vbExclamation, "Waring")
            Exit Sub
        End If
        xlRange = xlApp.Selection
        On Error GoTo 0

        If _TESTFLAG Then starttime = System.DateTime.Now
        'xlApp.screenupdating = False
        For Each xlCell In xlRange
            '不覆写且目标不为空时直接跳过
            If (Overwrite = 0) And (xlCell.offset(Offset_Rows, Offset_Columns).value IsNot Nothing) Then Continue For

            sRawText = xlCell.Value
            sResault = SectionSteelAW(sRawText, CTRLCODE)

            '输出到Excel
            If sResault IsNot Nothing Then
                xlCell.offset(Offset_Rows, Offset_Columns).Value = "=" & sResault
                '赋值为Nothing是否能正确释放？？？
                sResault = Nothing
            Else
                xlCell.offset(Offset_Rows, Offset_Columns).Value = Nothing
            End If
        Next
        'xlApp.screenupdating = True
        If _TESTFLAG Then endtime = System.DateTime.Now : MsgBox("执行持续时间：" & (endtime - starttime).ToString)

        '释放对象内存
        xlCell = Nothing : xlRange = Nothing : xlWorkbook = Nothing : xlApp = Nothing
    End Sub
End Module