Marshalling C dll code into C#

Question

I have the following C-code signature in a dll:

extern __declspec(dllexport) unsigned char *
funct_name (int *w, int *h, char **enc, int len, unsigned char *text, int *lp, int *mp, int *ep)

The C function can modify w, h, enc, lp, mp, and ep (though the latter three can be null and it won't do anything.

I'm using the following in C#

[DllImport("iec16022ecc200.dll", EntryPoint = "iec16022ecc200", ExactSpelling = false, CharSet = CharSet.Ansi, SetLastError = true, CallingConvention=CallingConvention.Cdecl)]
            static extern IntPtr CallEncode(
            [In,Out,MarshalAs(UnmanagedType.LPArray)] Int32[] width,
            [In,Out,MarshalAs(UnmanagedType.LPArray)] Int32[] height,
            [In,Out,MarshalAs(UnmanagedType.LPStr)] ref StringBuilder encoding,
            int barcodeLen,
            [MarshalAs(UnmanagedType.LPStr)] StringBuilder barcode,
            IntPtr lenp,
            IntPtr maxp,
            IntPtr eccp
            );


public void Foo (string textToEncode,out int width, out int height) {
            StringBuilder text = new StringBuilder(textToEncode);
            StringBuilder encoding = new StringBuilder(new string('a', text.Length));

            Int32[] w = new Int32[1];
            Int32[] h = new Int32[1];


            string encodedStr = Marshal.PtrToStringAnsi(CallEncode(w, h, ref encoding, text.Length, text, (IntPtr)0, (IntPtr)0, (IntPtr)0));
            width = w[0];
            height = h[0];
}

I am getting a SystemAccessViolation and I'm not entirely sure when my problem is.

Answer 1

Don't pass a StringBuilder reference to a unmanaged method that takes a char*. StringBuilder content is unicode (wchar).

Instead, replace the StringBuilder parameter with and IntPtr parameter and allocate a appropriate size buffer using Marshal.AllocHGlobal.

Also I don't think passing a StringBuilder to unmanaged code using "ref" is supported by the .Net marshaller.

Answer 2

You mentioned that the 'char **enc' parameter may be modified by the callee, this could be causing the 'SystemAccessViolation' error. A StringBuilder can be dereferenced and modified by the callee, provided it does not exceed the StringBuilders capacity.