Marshalling my C++ .dll in C# with "const char*" and "const int"

Question

Having in C++:

extern "C" MY_API int connect(const char* pcHost,const int nPort, ConnectionId_T *pConId);

Being:

pcHost an IP
nPort a port
pConId a constant (-1)

My try is:

[DllImport("my.dll", CharSet = CharSet.Ansi)]
public static extern int connect(StringBuilder pcHost, int nPort, ConnectionId_T pConId);

But when I try to connect:

StringBuilder ip = new System.Text.StringBuilder();
ip.Append("1.1.1.1");
int nPort = 1111;
ConnectionId_T nConId = MY_NCONID // This is defined previously as -1 (public const int MY_NCONID = -1)
connect(ip, nPort, nConId))

I get an AccesViolationException on the connect line. Do I've my marshal wrong?

PS: ConectionId_T is defined as using ConnectionId_T = System.Int32;

Thanks in advance.

@codroipo I also tried `[MarshalAs(UnmanagedType.LPStr)]` and I get the same error. — Avión, May 26 '16 at 10:40
you will also have to add attribute to your method defination `[DllImport("yourdll.dll", CharSet = CharSet.Ansi)]` — Mujahid Daud Khan, May 26 '16 at 10:41
@MujahidDaudKhan That's already included on the original post. — Avión, May 26 '16 at 10:42

score -1 · Answer 1 · answered May 26 '16 at 11:40

PS: ConectionId_T is defined as using ConnectionId_T = System.Int32;

Based on this, if I assume ConnectionId_T is nothing but int in C++ world, then you need to change the function import as:

[DllImport("my.dll", CharSet = CharSet.Ansi)]
public static extern int connect(StringBuilder pcHost, int nPort, IntPtr pConId);

Or as:

[DllImport("my.dll", CharSet = CharSet.Ansi)]
public static extern int connect(StringBuilder pcHost, int nPort, ref int pConId);

Mujahid Daud Khan · Accepted Answer · 2016-05-26T11:15:09.100

-2

[DllImport("my.dll", CharSet = CharSet.Ansi,CallingConvention=CallingConvention.Cdecl)]
public static extern int connect(IntPtr pcHost, int nPort, ref ConnectionId_T pConId);

and use Intptr pcHost = Marshal.StringToHGlobalAnsi(pcHostString))

edited May 26 '16 at 11:15

answered May 26 '16 at 10:46

Mujahid Daud Khan

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Why are you changing `pConId` to an `int` when it's originally a `ConnectionId_T`? – Avión May 26 '16 at 10:48
Does it makes any difference? – Mujahid Daud Khan May 26 '16 at 10:52
How can I convert the string of the IP to a `IntPtr ` so I can make it work? – Avión May 26 '16 at 10:53
by calling this method as explained above [Marshal.StringToHGlobalAnsi(string s)](https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshal.stringtohglobalansi(v=vs.100).aspx#Anchor_0) – Mujahid Daud Khan May 26 '16 at 10:56
It worked by adding `out` before `pConId` as the `.dll` returns the value on that variable. Thanks! – Avión May 26 '16 at 11:11
i think that it would also work with `ref int pConId ` – Mujahid Daud Khan May 26 '16 at 11:13

Marshalling my C++ .dll in C# with "const char*" and "const int"

2 Answers2