Custom Exceptions

Part of the Logic & Flow section of Coddy's C# journey — lesson 29 of 66.

Custom exceptions allow you to create specific exception types for your application's unique error scenarios.

To create a custom exception, define a class that inherits from Exception. Inheritance means your new class automatically gains all the behavior of the parent class — in this case, Exception — while letting you add or customize functionality. The syntax class MyException : Exception means MyException extends Exception.

public class InvalidAgeException : Exception
{
    public InvalidAgeException() : base("Age is not valid.")
    {
    }
    
    public InvalidAgeException(string message) : base(message)
    {
    }
}

: Exception — inherits from the built-in Exception class.
: base(...) — calls the parent Exception constructor to set the error message.
The first constructor provides a default message when none is supplied.
The second constructor accepts a custom message so callers can describe the error in detail.

Now you can throw your custom exception when needed:

int age = -5;
if (age < 0)
{
    throw new InvalidAgeException("Age cannot be negative.");
}

When this code executes with a negative age, it will throw your custom exception with the message "Age cannot be negative."

Note: In the exercises for this lesson, validation functions return true when the input is valid and throw a custom exception when it is not. Because an unhandled exception stops execution immediately, the test cases only need to verify the true return path — the exception itself is the observable result for invalid inputs.

Challenge

Medium

Create a custom exception class named InvalidTemperatureException that inherits from Exception. It should have:

A default constructor that sets a message "Temperature is not valid."
A constructor that accepts a custom message.

Then implement a method named checkTemperature that:

Takes an integer parameter representing a temperature in Celsius
Throws an InvalidTemperatureException with the message "Temperature below absolute zero!" if the temperature is less than -273
Returns true if the temperature is valid

Cheat sheet

To create a custom exception, define a class that inherits from Exception:

public class InvalidAgeException : Exception
{
    public InvalidAgeException() : base("Age is not valid.")
    {
    }
    
    public InvalidAgeException(string message) : base(message)
    {
    }
}

Throw your custom exception when needed:

int age = -5;
if (age < 0)
{
    throw new InvalidAgeException("Age cannot be negative.");
}

Try it yourself

using System;

class CheckTemperature
{
    // Create your custom exception class here
    
    public static bool checkTemperature(int celsius)
    {
        // Write your code here
    }
    
}