Access Violation Exception in c#

Ken H

hi friend,

When I call the C++ dll function, the following error occurred:

Error Message=Attempts to read from or write to protected memory. This is often an indication that other memory is corrupt.

C++ codes:

string test(string s[]){
string t="";

for(int i=0;i<2;i++){
t+=s[i];
}

return t;
}

C# codes:

class Program

{

[DllImport("safeTest.dll", CallingConvention = CallingConvention.Cdecl)]

public static extern string test(string[] s);
static void Main(string[] args)
{

string[] city = { "London ", "San Francisco " };
string temp = test(city);
Console.WriteLine(temp);
Console.ReadKey();

}

}

Thanks.

3 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes12y ago
Accepted answer
The problem here is that the C++ type std:string is not compatible with P/Invoke.

You need to use C-style strings instead.

Also rather than return a string, it's better to pass the function a pointer to a buffer which it can fill with the result. Bearing in mind that strings are immutable in C#, this can be done using a StringBuilder.

The following code is working OK for me:

// C++ code

#include
#include

extern "C"
{
_declspec(dllexport) void test(char* buffer, const char** s, int len)
{
strcpy(buffer, "");

for(int i=0;i
{
strcat(buffer, s[i]);
}
}
}

// C# code

using System;
using System.Runtime.InteropServices;
using System.Text;

class Program
{
[DllImport("safeTest.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern string test(StringBuilder buffer, string[] s, int len);

static void Main(string[] args)
{
string[] city = { "London ", "San Francisco " };
// work out capacity needed for buffer
int capacity = 0;
for(int i = 0; i < city.Length; i++) capacity += city[i].Length;
StringBuilder buffer = new StringBuilder(capacity + 1);
test(buffer, city, city.Length);
Console.WriteLine(buffer);
Console.ReadKey();
}
}

The output is:

London San Francisco
Vulpes12y ago
Oops, yes you're right - forgot to change the return type :)

It still worked OK as we were not using it.
Ken H12y ago
But this statement needs to be corrected:

public static extern string test(StringBuilder buffer, string[] s, int len); // error

public static extern void test(StringBuilder buffer,string[] s,int len); // right

Thank for you,Vulpes. :)