Getting exception while using C++ dll in VB.Net - c++

Question

I have created unmanaged dll and used in VB.Net.
Both code snippiest is as follows.
VB.Net
Imports System.Text
Imports System.Runtime.InteropServices
Module Module1
Sub Main()
Dim c As cls = New cls()
c.Start()
End Sub
End Module
Public Class cls
Declare Sub Only Lib "dllproj2.dll" Alias "Only" (b As StringBuilder)
Public Sub Start()
Dim s As StringBuilder = New StringBuilder(15)
Only(s) '**Actual call to dll code **
Dim s1 As String = s.ToString.ToLower
Dim len As Integer = s.ToString.Length.ToString()
Console.Write(s.ToString())
End Sub
End Class
C++ dll
#include<stdio.h>
#include<stdlib.h>
#include<cstring>
extern "C"{
void Only(char *a)
{
char arr[10];
printf("Reached");
sprintf(arr,"This %d",33);
printf("\n%s\n",arr);
memcpy(a,arr,10);
}
}
Now as soon as I access line Only(s) I get exception shown in image.
I am not able to understand cause of exception. Output of code is fine, but while running it using Visual Studio 2012 Express it is giving above error.
It is sample code, which we also used in production, I afraid it may cause problem in future.
Kindly suggest is there any way to get rid of exception.
Thanks.

Answer 1

You have to declare bas: UnmanagedType.LPStr
Declare Sub Only Lib "dllproj2.dll" Alias "Only" (<InAttribute(), MarshalAs(UnmanagedType.LPStr)> b As StringBuilder)