Custom exception classes in C Sharp (C#)

Introduction

Custom exception classes let you create your own error types to better explain problems in your program.

When you want to clearly show a specific problem in your code.

When built-in errors are too general and don't explain the issue well.

When you want to catch and handle a special error differently.

When you want to add extra information to an error.

When you want your program to be easier to understand and fix.

Syntax

C Sharp (C#)

public class MyCustomException : Exception
{
    public MyCustomException() { }
    public MyCustomException(string message) : base(message) { }
    public MyCustomException(string message, Exception inner) : base(message, inner) { }
}

Your custom exception class should inherit from the built-in Exception class.

You can add constructors to pass messages or inner exceptions.

Examples

A simple custom exception that takes a message.

C Sharp (C#)

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

Custom exception with extra information about the missing file.

C Sharp (C#)

public class FileMissingException : Exception
{
    public string FileName { get; }
    public FileMissingException(string fileName) : base($"File {fileName} is missing.")
    {
        FileName = fileName;
    }
}

Sample Program

This program defines a custom exception called AgeException. It checks if age is less than 18 and throws the custom error. The error is caught and its message is printed.

C Sharp (C#)

using System;

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

class Program
{
    static void CheckAge(int age)
    {
        if (age < 18)
        {
            throw new AgeException("Age must be at least 18.");
        }
        Console.WriteLine("Age is valid.");
    }

    static void Main()
    {
        try
        {
            CheckAge(15);
        }
        catch (AgeException ex)
        {
            Console.WriteLine($"Custom error caught: {ex.Message}");
        }
    }
}

Important Notes

Always inherit from Exception or a subclass of it when creating custom exceptions.

Use meaningful names for your custom exceptions to make your code easier to read.

You can add extra properties or methods to your custom exception if you want to store more details.

Summary

Custom exceptions help explain specific problems clearly.

Create them by inheriting from Exception and adding constructors.

Use them to catch and handle special errors in your program.

Practice

1. What is the main reason to create a custom exception class in C#?

easy

A. To automatically fix errors when they occur

B. To make the program run faster

C. To avoid using try-catch blocks

D. To represent specific error conditions clearly in your program

Solution

1. Step 1: Understand the purpose of exceptions

   Exceptions represent errors or unexpected situations in a program.

2. Step 2: Identify why custom exceptions are used

   Custom exceptions help describe specific problems clearly, making error handling easier and more meaningful.

3. Final Answer:

   To represent specific error conditions clearly in your program -> Option D

4. Quick Check:

   Custom exceptions clarify errors = A [OK]

Hint: Custom exceptions explain specific errors clearly [OK]

Common Mistakes:

• Thinking custom exceptions improve speed
• Believing they remove need for try-catch
• Assuming they fix errors automatically

2. Which of the following is the correct way to declare a custom exception class named MyException in C#?

easy

A. class MyException : Exception { public MyException(string message) : base(message) {} }

B. class MyException { public MyException(string message) {} }

C. class MyException : int { public MyException(string message) : base(message) {} }

D. class MyException : Exception { public void MyException(string message) {} }

Solution

1. Step 1: Check inheritance from Exception

   Custom exceptions must inherit from Exception to behave like exceptions.

2. Step 2: Verify constructor calls base constructor

   The constructor should call base(message) to pass the error message properly.

3. Final Answer:

   class MyException : Exception { public MyException(string message) : base(message) {} } -> Option A

4. Quick Check:

   Inherit Exception + call base constructor = A [OK]

Hint: Inherit Exception and call base constructor for message [OK]

Common Mistakes:

• Not inheriting from Exception
• Using wrong base class like int
• Defining constructor as void method

3. What will be the output of the following C# code?

class MyException : Exception { public MyException(string message) : base(message) {} }
try {
  throw new MyException("Error happened");
} catch (MyException ex) {
  Console.WriteLine(ex.Message);
}

medium

A. Exception caught

B. Error happened

C. MyException

D. No output

Solution

1. Step 1: Understand the throw statement

   The code throws a MyException with message "Error happened".

2. Step 2: Catch block prints exception message

   The catch block catches MyException and prints ex.Message, which is "Error happened".

3. Final Answer:

   Error happened -> Option B

4. Quick Check:

   Throw and catch prints message = C [OK]

Hint: Catch prints exception message property [OK]

Common Mistakes:

• Expecting class name instead of message
• Thinking catch block won't run
• Assuming no output from exception

4. Identify the error in this custom exception class declaration:

class MyError : Exception {
  public MyError(string msg) {
    base(msg);
  }
}

medium

A. The class must not inherit from Exception

B. The constructor should be named differently from the class

C. The constructor should call base(msg) using a colon, not inside the body

D. The base class Exception does not accept a string parameter

Solution

1. Step 1: Check constructor syntax for base call

   In C#, calling the base class constructor must be done with a colon after the constructor signature, not inside the body.

2. Step 2: Identify correct syntax

   The correct syntax is public MyError(string msg) : base(msg) {}, not calling base(msg); inside the constructor body.

3. Final Answer:

   The constructor should call base(msg) using a colon, not inside the body -> Option C

4. Quick Check:

   Base constructor call uses colon syntax = B [OK]

Hint: Call base constructor with colon, not inside method body [OK]

Common Mistakes:

• Calling base constructor inside body instead of colon
• Not inheriting from Exception
• Misnaming constructor

5. You want to create a custom exception InvalidAgeException that should be thrown when a user's age is less than 0 or greater than 120. Which of the following code snippets correctly defines and uses this exception?

hard

A. class InvalidAgeException : Exception { public InvalidAgeException(string msg) : base(msg) {} }void CheckAge(int age) { if(age < 0 || age > 120) throw new InvalidAgeException("Age is invalid"); }

B. class InvalidAgeException { public InvalidAgeException(string msg) {} }void CheckAge(int age) { if(age < 0 || age > 120) throw new InvalidAgeException("Age is invalid"); }

C. class InvalidAgeException : Exception { public void InvalidAgeException(string msg) {} }void CheckAge(int age) { if(age < 0 || age > 120) throw new InvalidAgeException("Age is invalid"); }

D. class InvalidAgeException : Exception { public InvalidAgeException() {} }void CheckAge(int age) { if(age < 0 || age > 120) throw new InvalidAgeException(); }

Solution

1. Step 1: Verify custom exception class definition

   The class inherits from Exception and has a constructor calling base(msg) to pass the message.

2. Step 2: Check usage in method

   The method throws the exception with a message when age is invalid, which matches the requirement.

3. Final Answer:

   Correct class inheritance, constructor, and usage with message -> Option A

4. Quick Check:

   Inherit Exception + throw with message = D [OK]

Hint: Inherit Exception, add constructor, throw with message [OK]

Common Mistakes:

• Not inheriting from Exception
• Defining constructor as void method
• Missing message in exception constructor