meet it

Make an exception named after your own problem.

Built-in exceptions like FormatException describe computer troubles. Your app has its own, like a sold-out cake. A is a class you write and name after the problem. It inherits from Exception, so C# treats it like any other exception.

: base(message) hands the message to the Exception constructor, which stores it in Message. Now code can catch (OutOfStockException e) and handle a sold-out cake differently from every other problem. End each exception's name with Exception, so readers know what it is.

using System;

int left = 0;
try
{
    if (left == 0)
    {
        throw new OutOfStockException("We have no eclairs left.");
    }
    Console.WriteLine("One eclair, coming up!");
}
catch (OutOfStockException e)
{
    Console.WriteLine($"Sorry! {e.Message}");
}

class OutOfStockException : Exception
{
    public OutOfStockException(string message) : base(message) { }
}

A catch for your own exception ignores every other kind of problem.

guess first

The alarm goes off. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

using System;

try
{
    Console.WriteLine("Checking the doors...");
    throw new AlarmException("Back door open!");
}
catch (AlarmException e)
{
    Console.WriteLine(e.Message);
}
catch (Exception)
{
    Console.WriteLine("Something else broke.");
}

class AlarmException : Exception
{
    public AlarmException(string message) : base(message) { }
}

your turn

Your turn. The show is sold out.

Put your name in the message on line 8. On line 17, build FullyBookedException on Exception. On line 19, pass the message to the base constructor. Press run.

Your code

output

press run to see what happens

fix it

The treasure chest lost its message. Fix it!

Broken code to fix

output

press run to see what happens

The message printed is C#'s own, not yours. Where does message go once the constructor has it?
The constructor receives the message but never hands it on to the Exception constructor.
Change line 19 to public WrongCodeException(string message) : base(message) { }, then press run.

nailed it

your app's problems have names. :D

A custom exception inherits from Exception.
: base(message) passes the message on.
Catch it by name to handle it on its own.

new words in your collection

custom exception

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