Exception Handling

Things go wrong. Files get deleted, networks fail, users enter letters where numbers belong. Exceptions are C#'s way of handling these surprises gracefully instead of crashing.

Try-Catch

Wrap risky code in a try block. If an exception happens, execution jumps to the catch block. The program keeps running instead of blowing up.

try
{
    int x = int.Parse("not-a-number");
    Console.WriteLine("This won't print");
}
catch (FormatException ex)
{
    Console.WriteLine("Oops: " + ex.Message);
}

Console.WriteLine("Program keeps going");
    

Multiple Catch Blocks

Different exceptions need different handling. Stack multiple catch blocks — C# tries them in order and runs the first one that matches.

try
{
    string input = Console.ReadLine();
    int num = int.Parse(input);
    int result = 100 / num;
    Console.WriteLine(result);
}
catch (DivideByZeroException)
{
    Console.WriteLine("Can't divide by zero");
}
catch (FormatException)
{
    Console.WriteLine("That's not a number");
}
catch (Exception ex)
{
    Console.WriteLine("Something else: " + ex.Message);
}
    

Always put the most specific exceptions first, and Exception (the base type) last as a safety net.

Finally

The finally block runs no matter what — whether the try succeeded or a catch ran. Use it for cleanup: closing files, releasing connections, that sort of thing.

FileStream file = null;
try
{
    file = File.Open("data.txt", FileMode.Open);
}
catch (FileNotFoundException)
{
    Console.WriteLine("File not found");
}
finally
{
    file?.Close();
    Console.WriteLine("Cleanup done");
}
    

Throwing Exceptions with throw

When something is wrong in your own code, throw an exception to let the caller know.

static void SetAge(int age)
{
    if (age < 0)
        throw new ArgumentException("Age can't be negative");
    Console.WriteLine("Age set to " + age);
}

try
{
    SetAge(-5);
}
catch (ArgumentException ex)
{
    Console.WriteLine(ex.Message);
}
    

Common Exception Types

• FormatException — bad format for parsing
• DivideByZeroException — dividing by zero
• NullReferenceException — calling a method on null
• IndexOutOfRangeException — accessing past array bounds
• FileNotFoundException — file doesn't exist
• ArgumentException — invalid argument passed