using C function in C#

Question

i have a dll, built with mingw
one of the header files contains this:

extern "C" {
  int get_mac_address(char * mac); //the function returns a mac address in the char * mac
}

I use this dll in another c++ app, built using Visual C++ (2008SP1), not managed, but plain c++ (simply include the header, and call the function)

But now I have to use it in a C# application

The problem is that i can't figure out how exactly (i'm new in .net programming)

this is what i've tried

public class Hwdinfo {
    [DllImport("mydll.dll")]
    public static extern void get_mac_address(string s);
}

When i call the function, nothing happens

(the mydll.dll file is located in the bin folder of the c# app, and it gives me no errors or warnings whatsoever)

What are you expecting to happen? You aren't returning anything. If you are wanting to get the parameter back out you may need to pass it as ref string s or out string s. — Stephan, May 13 '10 at 15:25

score 10 · Answer 1 · answered May 13 '10 at 15:24

10

I think you need to define the extern as:

public class Hwdinfo { 
    [DllImport("mydll.dll")] 
    public static extern int get_mac_address(out string s); 
}

You should match both the return argument type on the function (int) as well as mark the string parameter as an out parameter so that your C# code is generated to expect to receive a value from the called function, rather than just passing one in.

Remember, strings in C# are treated as immutable, this behavior extends to external calls as well.

answered May 13 '10 at 15:24

LBushkin

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+1 I think the same. =) Damn! 40 seconds faster! Hehehe... I was clicking "Post Answer" when your answer came in! =) I know, I wrote too much before writing my answer! =P – Will Marcouiller May 13 '10 at 15:26
@Will Marcouiller: Sometimes it's just the luck of the draw as to who sees and responds to a question first. – LBushkin May 13 '10 at 15:28
Sure! I was trying to be kidding, after all! =) Anyway! Hehehe... Have a nice day! – Will Marcouiller May 13 '10 at 15:30

Will Marcouiller · Answer 2 · 2010-05-13T15:31:18.120

3

If you expect your MAC address to come through your string parameter, I guess you had better to make it a reference.

public class Hwdinfo { 
    [DllImport("mydll.dll")] 
    public static extern int get_mac_address(out string s); 
}

Or something like so.

edited May 13 '10 at 15:31

answered May 13 '10 at 15:25

Will Marcouiller

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score 3 · Accepted Answer · answered May 13 '10 at 15:32

To use string output parameters with DllImport, the type should be StringBuilder.


public class Hwdinfo {
    [DllImport("mydll.dll")]
    public static extern int get_mac_address(StringBuilder s);
}

Here's an MSDN Article about using Win32 dlls and C#:
http://msdn.microsoft.com/en-us/magazine/cc164123.aspx

score 1 · Answer 4 · answered May 13 '10 at 15:25

1

You can find lots of examples here: http://pinvoke.net/

I suspect that you your best hints would come from something like: http://pinvoke.net/default.aspx/shell32.SHGetSpecialFolderPath

answered May 13 '10 at 15:25

John Fisher

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score 1 · Answer 5 · answered May 13 '10 at 15:26

1

Strings in .NET are immutable so try:

public class Hwdinfo { 
    [DllImport("mydll.dll")] 
    public static extern int get_mac_address(char[] s); 
}

answered May 13 '10 at 15:26

Daniel Renshaw

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The .NET Marshaller can deal with this using StringBuilder, as in the accepted answer – Sebastian May 02 '13 at 07:36

score 1 · Answer 6 · edited May 23 '17 at 12:07

1

C# PInvoke out strings declaration

This suggests you might try using a StringBuilder as your parameter instead of a string. If that doesn't work then an out parameter would be my next choice.

edited May 23 '17 at 12:07

Community

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answered May 13 '10 at 15:31

Stephan

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using C function in C#

6 Answers6