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.