var unixFileInfo = new Mono.Unix.UnixFileInfo("test.txt");
    // set file permission to 644
    unixFileInfo.FileAccessPermissions =
        FileAccessPermissions.UserRead | FileAccessPermissions.UserWrite
        | FileAccessPermissions.GroupRead
        | FileAccessPermissions.OtherRead;

using System;
using System.IO;
using System.Runtime.InteropServices;
using static System.Console;

class Program
{
    [DllImport("libc", SetLastError = true)]
    private static extern int chmod(string pathname, int mode);

    // user permissions
    const int S_IRUSR = 0x100;
    const int S_IWUSR = 0x80;
    const int S_IXUSR = 0x40;

    // group permission
    const int S_IRGRP = 0x20;
    const int S_IWGRP = 0x10;
    const int S_IXGRP = 0x8;

    // other permissions
    const int S_IROTH = 0x4;
    const int S_IWOTH = 0x2;
    const int S_IXOTH = 0x1;

    static void Main(string[] args)
    {
        WriteLine("Setting permissions to 0755 on test.sh");
        const int _0755 =
            S_IRUSR | S_IXUSR | S_IWUSR
            | S_IRGRP | S_IXGRP
            | S_IROTH | S_IXOTH;
        WriteLine("Result = " + chmod(Path.GetFullPath("test.sh"), (int)_0755));

        WriteLine("Setting permissions to 0644 on sample.txt");
        const int _0644 =
            S_IRUSR | S_IWUSR
            | S_IRGRP
            | S_IROTH;
        WriteLine("Result = " + chmod(Path.GetFullPath("sample.txt"), _0644));

        WriteLine("Setting permissions to 0600 on secret.txt");
        const int _0600 = S_IRUSR | S_IWUSR;
        WriteLine("Result = " + chmod(Path.GetFullPath("secret.txt"), _0600));
    }
}