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)

Answer 1

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.

Answer 2

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.

Answer 3

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

Answer 4

Strings in .NET are immutable so try:

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

Answer 5

http://stackoverflow.com/questions/1994477/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.

Answer 6

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