C# calling an unmanaged C driver(dll)

Question

I'm really not sure what I am doing wrong. I am passing a string to my dll written in C from C# as lots of examples on the net state..for some reason the string is coming out as NULL in the C dll. Any idea of what I am doing wrong?

C:

 extern __declspec(dllexport) void Cmd(long CmdType,long DataPar, const char *DataStr);

 void Cmd(long CmdType,long DataPar,const char *DataStr)
 {
     // DataStr is NULL here even when passing a string with data in it
 }

C#:

    [DllImport(@"pjsua-i386-Win32-vc8-Debug.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]

    public static extern void Cmd(long CmdType, long DataPar,[MarshalAs(UnmanagedType.LPStr)]String s);

    Cmd(1,0,"TEST");

I've also tried other things like IntPtr and marshalling across the string but all turns out the same with NULL. And also a bunch of other things.

Answer 1

In Microsoft C++ and C, on 32 and 64-bit builds a long is only 32 bits.

However, in C# a long is always 64 bits.

So what I think is happening is that some of the bytes of the two 64 bit longs that are pushed onto the stack by the C# call are being popped off as the Data* in the C++.

Because the second parameter you're pushing is 0, it so happens that bytes with value 0 are being used for the pointer, hence it is null.

Change the declaration to int instead of long to solve your problem.

Answer 2

Did you try the below option:

    [DllImport(@"pjsua-i386-Win32-vc8-Debug.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
    public static extern void Cmd(int CmdType, int DataPar,IntPtr s);

    Cmd(1,0,Marshal.StringToHGlobalAnsi(str));