How to port C++ code and DLL to C#

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Hello,
I have a C++ project which I want to convert into a C# project. I've converted the DLL calls and codes but when I run the C# version sometimes it runs and sometimes it gives me System.AccessViolationException and jumps out of the program or sometimes it just exits the program without any exception.

What I have tried:

I've converted this function like this:

C++

# define VIXHZ_EXPORT __declspec(dllimport)

VIXHZ_EXPORT long _VixHz_LoginDevice(char *szIp, int nPort, char* szUserName, char* szPassword, base::string& strOutInfo);

base::string strInfo;
long m_login_id = _VixHz_LoginDevice("IP", 80, "Username", "Password", strInfo);

To:
C#

[DllImport(dll)]
public static extern long _VixHz_LoginDevice(string szIp, int nPort, string szUserName, string szPassword, ref string strOutInfo);
        
string output = "";
long m_login_id = _VixHz_LoginDevice("IP", 80, "Username", "Password", ref output);

Posted 31-May-21 22:17pm

mohammad hasani

Updated 31-May-21 23:05pm

v3

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KarstenK · Answer 1 · 2021-05-31T23:02:00

Solution 1

You must take with string between C+ and C#. Here is working code C# interoperation article which concreates on a simple and working interface.

When using C++ strings in C# than you must use these bstrings else you will fail.

Posted 31-May-21 23:02pm

KarstenK

Chris Copeland · Answer 2 · 2021-05-31T23:05:00

I don't believe that C# has the capacity to automatically understand what a string type is in C++, it's natively unable to convert a C# string into a C++ std::string or otherwise.

Instead of using base::string& as the last parameter consider using char* instead, and then assign the value within the method. Alternatively you can use a StringBuilder so that the C++ method can write to the value.

For example:

C#

// C++
VIXHZ_EXPORT long _VixHz_LoginDevice(char *szIp, int nPort, char* szUserName, char* szPassword, char* strOutInfo, int strOutInfoLength);

// C#
[DllImport(dll)]
public static extern long _VixHz_LoginDevice(string szIp, int nPort, string szUserName, string szPassword, StringBuilder strOutInfo, int strOutInfoLength);

// Call
StringBuilder strOut = new StringBuilder(32);
_VixHz_LoginDevice("1.2.3.4", 1234, "username", "password", strOutInfo, strOutInfo.Capacity);

You can omit the length parameter if you know exactly what the maximum length is expecting to be and the C++ isn't going to exceed it. An example with the StringBuilder can be found here:

Default Marshaling for Strings | Microsoft Docs[^]