C#

GDirections direccion;

                       var RutasDireccion = GMapProviders.GoogleMap.GetDirections(out direccion, inicio, final, false, false, false, false, false);

                       GMapRoute RutaObtenida = new GMapRoute(direccion.Route, "Ruta ubicación"); //Here the error: Object reference not set to an instance of an object.'

                       GMapOverlay CapaRutas = new GMapOverlay("Capa de la ruta");
                       CapaRutas.Routes.Add(RutaObtenida);
                       gMapa.Overlays.Add(CapaRutas);
                       gMapa.Zoom = gMapa.Zoom + 1;
                       gMapa.Zoom = gMapa.Zoom - 1;

                       contadorIndicadoresRuta = 0;
                       trazarRuta = false;
                       button3.Enabled = true;
                       break;

TcpClient client = new TcpClient();
client.Connect(ipaddress,port);

if (c.DataType.Value == CellValues.SharedString)

An error occurred!!!: The object reference has not been specified to an instance of an object.

using (SpreadsheetDocument myDoc = SpreadsheetDocument.Open(input, true))
                {

                    WorkbookPart workbookPart = myDoc.WorkbookPart;
                    WorksheetPart worksheetPart = workbookPart.WorksheetParts.First();
                    SheetData sheetData = worksheetPart.Worksheet.Elements<SheetData>().First();
 
                    foreach (Row r in sheetData.Elements<Row>())
                    {

                        foreach (Cell c in r.Elements<Cell>())
                        {


                            string value = c.InnerText;


                            if (c.DataType.Value == CellValues.SharedString)
                            {

                                var stringTable = workbookPart.GetPartsOfType<SharedStringTablePart>()
                                        .FirstOrDefault();
                                if (stringTable != null)
                                {
                                    value = stringTable.SharedStringTable.ElementAt(int.Parse(value)).InnerText;
                                    GenerateBarcodeFromExcel(value, output);
                                }

                            }
                            else
                            {
                                GenerateBarcodeFromExcel(c.CellValue.Text, output);
                            }
                               
                        }
                    }

                    myDoc.Close();

What are Good Things to Know in a C# Interview?

Off the top of my head, I have come up with this list:

   * SOLID mythodology.
   * Dependency Injection
   * Levels of Try, Catch, throw, and finally statements and how they work
   * The tenants of Object Oriented Programming such as polymorphism, enacpsulation, etc.
   * Unit Testing (are interfaces used somehow)
   * The difference between an abstract class and interface
   * garbage collection and when to override or overload it
   * difference between override or overload
   * Is there inheritance in C# ?
   * What is an interface used for?

I am posting this here to see of anyone can add to this list.  I am asking about general topics, not specific interview questions.

var mail = new MimeMessage();
                mail.From.Add(new MailboxAddress("From Name", "DoNotReply@someDomain.com"));
                mail.To.Add(new MailboxAddress("To Name","customerAddress@someDomain.com"));
                mail.Subject = "كشف حساب من تاريخ " + dateTimePicker1.Text + " حتى تاريخ " + dateTimePicker2.Text;
                var text_part = new TextPart(MimeKit.Text.TextFormat.Plain);
                string body = @"<table border='1'><table style='background-color:#E5E4E2;'><tr><tr style='background-color:#1e90ff;color:#ffffff;'><td>تراكمي</td><td>دائن</td><td>مدين</td><td>البيان</td><td>المرجع</td><td>التاريخ</td></tr>";

                foreach (ListViewItem lstitem in listView1.Items)
                {
                    body += @"<tr><td>" + lstitem.SubItems[1].Text + "</td><td>" + lstitem.SubItems[2].Text + "</td><td>" + lstitem.SubItems[3].Text + "</td><td>" + lstitem.SubItems[4].Text + "</td><td>" + lstitem.SubItems[5].Text + "</td><td>" + lstitem.SubItems[6].Text + "</td></tr>";
                }

                body += @"</table></style></style>";

                body += @"<br /><br /> المبلغ المطلوب " + sum1s.Text;

                body += @"<br /><br /> Thank You";
                mail.Body = new TextPart("html") { Text = body };

                byte[] bytes = Encoding.UTF8.GetBytes(mail.ToString());
                string raw_message = Convert.ToBase64String(bytes)
                  .Replace('+', '-')
                  .Replace('/', '_')
                  .Replace("=", "");

                UserCredential credential;
                //read your credentials file
                using (FileStream stream = new FileStream(Application.StartupPath + @"/credentials.json", FileMode.Open, FileAccess.Read))
                {
                    string path = Environment.GetFolderPath(Environment.SpecialFolder.Personal);
                    path = Path.Combine(path, ".credentials/gmail-dotnet-quickstart.json");
                    credential = GoogleWebAuthorizationBroker.AuthorizeAsync(GoogleClientSecrets.Load(stream).Secrets, Program.Scopes, "user", CancellationToken.None, new FileDataStore(path, true)).Result;
                }
                //call your gmail service
                var service = new GmailService(new BaseClientService.Initializer() { HttpClientInitializer = credential, ApplicationName = Program.ApplicationName });
                var msg = new Google.Apis.Gmail.v1.Data.Message();
                msg.Raw = raw_message;
                service.Users.Messages.Send(msg, "me").Execute();
                MessageBox.Show("تم ارسال التقرير بنجاح", "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);