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Föreläsning 13

Felhantering och filhantering 😉

Felhantering

Fånga kastade fel

Ofångat fel

  1. static void Main(string[] args) {
  2. FirstMethod();
  3. SecondMethod();
  4. }
  6. static void FirstMethod() {
  7. Console.WriteLine("FirstMethod before throw");
  8. throw new ArgumentException("FirstMethod error");
  9. Console.WriteLine("FirstMethod after throw");
  10. }
  12. static void SecondMethod() {
  13. Console.WriteLine("SecondMethod");
  14. }

Fångat fel

  1. static void Main(string[] args) {
  2. try {
  3. FirstMethod();
  4. }
  5. catch(ArgumentException) {
  7. }
  8. SecondMethod();
  9. }
  11. static void FirstMethod() {
  12. Console.WriteLine("FirstMethod before throw");
  13. throw new ArgumentException("FirstMethod error");
  14. Console.WriteLine("FirstMethod after throw");
  15. }
  17. static void SecondMethod() {
  18. Console.WriteLine("SecondMethod");
  19. }

Fångat fel med meddelande

  1. static void Main(string[] args) {
  2. try {
  3. FirstMethod();
  4. }
  5. catch(ArgumentException e) {
  6. Console.WriteLine(e.Message);
  7. }
  8. SecondMethod();
  9. }
  11. static void FirstMethod() {
  12. Console.WriteLine("FirstMethod before throw");
  13. throw new ArgumentException("FirstMethod error");
  14. Console.WriteLine("FirstMethod after throw");
  15. }

Vad händer här?

  1. int? number = null;
  2. while (number == null) {
  3. Console.Write("Please enter a number: ");
  4. number = int.Parse(Console.ReadLine());
  5. }
  6. Console.WriteLine("You entered: " + number);

Hantera felinmatning

  1. int? number = null;
  2. while (number == null) {
  3. try {
  4. Console.Write("Please enter a number: ");
  5. number = int.Parse(Console.ReadLine());
  6. }
  7. catch(FormatException) {
  8. Console.WriteLine("You did not enter a number");
  9. }
  10. }
  11. Console.WriteLine("You entered: " + number);

Hantera alla tänkbara fel

  1. int? number = null;
  2. while (number == null) {
  3. try {
  4. Console.Write("Please enter a number: ");
  5. number = int.Parse(Console.ReadLine());
  6. }
  7. catch {
  8. Console.WriteLine("You did not enter a number");
  9. }
  10. }
  11. Console.WriteLine("You entered: " + number);

Fel flera nivåer in

  1. static void Main(string[] args) {
  2. try {
  3. FirstMethod();
  4. }
  5. catch(ArgumentException) {
  6. Console.WriteLine("Error caught");
  7. }
  8. Console.WriteLine("After catch");
  9. }
  11. static void FirstMethod() {
  12. Console.WriteLine("FirstMethod before SecondMethod");
  13. SecondMethod();
  14. Console.WriteLine("FirstMethod after SecondMethod");
  15. }
  17. static void SecondMethod() {
  18. Console.WriteLine("SecondMethod before error");
  19. throw new ArgumentException("SecondMethod error");
  20. Console.WriteLine("SecondMethod after error");
  21. }

Tvetydiga fel

  1. try {
  2. DisplayMessage(font: "Arial", message: "Hello!");
  3. }
  4. catch(ArgumentException) {
  5. // Which argument was wrong?
  6. }

Vad vi skulle vilja ha

  1. try {
  2. DisplayMessage(font: "Arial", message: "Hello!");
  3. }
  4. catch(MissingFontException) {
  5. ...
  6. }
  7. catch(TooLongMessageException) {
  8. ...
  9. }

Egna felklasser

  1. class MissingFontException : Exception {
  2. ...
  3. }
  5. class TooLongMessageException : Exception {
  6. ...
  7. }
  9. static void DisplayMessage(string font, string message) {
  10. ...
  11. throw new MissingFontException();
  12. ...
  13. throw new TooLongMessageException();
  14. ...
  15. }

Filhantering

Spara data till och läs data från hårddisken

Exempel.txt

  1. This is some text
  2. This is some text on a different line
  3. Some numbers: 1 2 3 4 5 6 7 8 9 0

Exempel.txt med synliga nyrader

  1. This is some text\n
  2. This is some text on a different line\n
  3. Some numbers: 1 2 3 4 5 6 7 8 9 0

Vad är datatypen på en fil?

Läsa en fil

  1. using System.IO;
  2. ...
  4. static void Main(string[] args) {
  5. string path = @"C:\Windows\System32\drivers\etc";
  6. string text = File.ReadAllText(path);
  7. string lines[] = File.ReadAllLines(path);
  8. }

Läsa en fil via relativ sökväg

  1. using System.IO;
  2. ...
  4. static void Main(string[] args) {
  5. string path = "Test.txt";
  6. string text = File.ReadAllText(path);
  7. }

"Be om ursäkt"

  1. using System.IO;
  2. ...
  4. static void Main(string[] args) {
  5. string path = "Test.txt";
  6. try {
  7. string text = File.ReadAllText(path);
  8. }
  9. catch (FileNotFoundException) {
  10. Console.WriteLine("Please create Test.txt");
  11. }
  12. }

"Be om tillåtelse"

  1. using System.IO;
  2. ...
  4. static void Main(string[] args) {
  5. string path = "Test.txt";
  6. if (File.Exists(path))
  7. string text = File.ReadAllText(path);
  8. }
  9. else {
  10. Console.WriteLine("Please create Test.txt");
  11. }
  12. }

Läsa och skriva

  1. using System.IO;
  2. ...
  4. static void Main(string[] args) {
  5. string path = "Test.txt";
  6. File.WriteAllText(path, "These are the contents");
  7. string text = File.ReadAllText(path);
  8. Console.WriteLine(text);
  9. }

Exempel på användningsområde

  1. using System.IO;
  2. ...
  4. static void Main(string[] args) {
  5. Console.Write("Enter discount code: ");
  6. string code = Console.ReadLine();
  7. string[] validCodes = File.ReadAllLines("Codes.txt");
  8. if (validCodes.Contains(code)) {
  9. Console.WriteLine("You will receive a discount!");
  10. }
  11. else {
  12. Console.WriteLine(code + " is not valid.");
  13. }
  14. }

Mer avancerat exempel

  1. using System.IO;
  2. ...
  4. static void Main(string[] args) {
  5. string[] lines = File.ReadAllLines("People.txt");
  6. List<Person> people = new List<Person>();
  7. foreach (string line in lines) {
  8. string[] parts = line.Split(',');
  9. string name = parts[0];
  10. int age = int.Parse(parts[1]);
  11. people.Add(new Person {
  12. Name = name,
  13. Age = age
  14. });
  15. }
  16. }