CA2101 Warning when making extern calls

Question

I'm using the WinPcap libraries and have set up all my native method calls. Upon building I get the CA2101: Specify marshaling for P/Invoke string arguments Code Analysis warning. My extern function is defined like this:

[DllImport("wpcap", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
internal static extern int pcap_compile(IntPtr /* pcap_t* */ adaptHandle,
                                        IntPtr /*bpf_program **/fp,
                                        string /*char * */str,
                                        int optimize,
                                        uint netmask);

If I change the CharSet to CharSet.Unicode, I resolve the Code Analysis warning but my function no longer works. How can I resolve the warning and keep my code working?

Answer 1

This warning occurs because truncating Unicode text to an ASCII string can cause security issues.

If you cannot use Unicode strings, set BestFitMapping = false, ThrowOnUnmappableChar = true on the attribute to prevent this security issue. For more information, see the documentation.