Exceptions: try, catch, finally, throw

Files go missing, users type letters into number fields, networks drop. When .NET hits a problem it can’t handle on the spot, it throws an exception: an object describing what went wrong. If nothing catches it, the program crashes. Exception handling lets you respond sensibly instead.

1string input = "abc";2try3{4    int a = int.Parse(input);5    int result = 100 / a;6    Console.WriteLine($"Result: {result}");7}8catch (FormatException)9{10    Console.WriteLine("Please type a number.");11}12catch (DivideByZeroException ex)13{14    Console.WriteLine($"Error: {ex.Message}");15}16finally17{18    Console.WriteLine("Done.");19}20Console.WriteLine("Program continues");

try / catch / finally

• try: the code that might fail. As soon as an exception is thrown, the rest of the block is skipped.
• catch (Type ex): runs only if the exception matches that type (or a subclass). Checked top to bottom; the first match wins.
• finally: always runs, success or failure. Used to release resources (close files, connections).

Common exception types

The exception object

1try2{3    string text = File.ReadAllText("model.ifc");4}5catch (FileNotFoundException ex)6{7    Console.WriteLine($"Missing file: {ex.FileName}");8    Console.WriteLine(ex.Message);        // human-readable description9    // ex.StackTrace shows where it happened: log it for developers10}

Missing file: D:\Work\model.ifc
Could not find file 'D:\Work\model.ifc'.

Throwing your own

1public class Beam2{3    public Beam(double lengthM)4    {5        if (lengthM <= 0)6            throw new ArgumentOutOfRangeException(nameof(lengthM), lengthM, "Length must be positive.");7        LengthM = lengthM;8    }9    public double LengthM { get; }10}11 12// Custom exception for your own domain errors13public class ClashDetectedException : Exception14{15    public ClashDetectedException(string a, string b) : base($"{a} clashes with {b}") { }16}

Throw when a method can’t do what its name promises: invalid arguments, impossible state. Prefer the built-in types (ArgumentException, InvalidOperationException) and create custom ones (always ending in Exception) for domain-specific errors that callers might want to catch separately.

Re-throwing correctly

1try2{3    SaveProject();4}5catch (IOException ex)6{7    Log(ex);          // record it…8    throw;            // …and let it continue upwards with its ORIGINAL stack trace9    // throw ex;      // ✗ resets the stack trace: you lose where it really happened10}

Exception filters

1catch (HttpRequestException ex) when (ex.StatusCode == HttpStatusCode.NotFound)2{3    Console.WriteLine("Not found on the server");4}

Good practice

• Prevent before you catch: use TryParse, check for null, check Count. Exceptions are for exceptional situations, and they are slow.
• Catch only what you can handle meaningfully. Never write an empty catch { } that silently hides errors.
• Catch Exception (everything) only at the top level of the app, to log the error and show a friendly message.
• Use using (next module) or finally to release files and connections.

1using System.Globalization;2 3static double SafeAverage(string[] inputs)4{5    double sum = 0;6    int count = 0;7    foreach (string s in inputs)8    {9        try10        {11            sum += double.Parse(s, CultureInfo.InvariantCulture);12            count++;13        }14        catch (FormatException)15        {16            Console.WriteLine($"Skipping '{s}'");17        }18    }19    if (count == 0) throw new InvalidOperationException("No valid numbers.");20    return sum / count;21}22 23Console.WriteLine(SafeAverage(new[] { "10", "x", "20" }));

Skipping 'x'
15