Custom exception classes in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom exception class in C#?

A custom exception class is a user-defined class that inherits from <code>Exception</code> or its subclasses to represent specific error conditions in your program.

beginner

How do you define a basic custom exception class in C#?

You create a class that inherits from <code>Exception</code> and provide constructors, for example:<br><pre>public class MyException : Exception { public MyException(string message) : base(message) {} }</pre>

intermediate

Why should you create custom exception classes instead of using general exceptions?

Custom exceptions help you clearly identify and handle specific error cases, making your code easier to read, debug, and maintain.

beginner

Which base class do all custom exceptions in C# usually inherit from?

They usually inherit from the <code>Exception</code> class, which is the base class for all exceptions in C#.

beginner

What is the purpose of calling base(message) in a custom exception constructor?

It passes the error message to the base <code>Exception</code> class so that the message can be accessed later when the exception is caught.

Which keyword is used to create a custom exception class in C#?

A try

B exception

C throw

D class

You use the class keyword to define a new class that inherits from Exception.

What should a custom exception class inherit from?

A System.Exception

B System.Console

C System.String

D System.Object

Custom exceptions should inherit from System.Exception to behave like standard exceptions.

Why do you call base(message) in a custom exception constructor?

A To catch the exception

B To initialize the base Exception with the error message

C To throw the exception immediately

D To log the error message

Calling base(message) passes the message to the base Exception class for proper error handling.

Which of these is a benefit of using custom exceptions?

A They automatically fix bugs

B They prevent exceptions from occurring

C They make error handling more specific and clear

D They replace the need for try-catch blocks

Custom exceptions help you identify and handle specific errors clearly.

Which constructor is commonly included in a custom exception class?

A A constructor that takes a string message

B A constructor that returns void

C A constructor that takes an int

D A constructor that takes no parameters and does nothing

A constructor that takes a string message allows you to pass an error description when throwing the exception.

Explain how to create a custom exception class in C# and why it is useful.

Think about class inheritance and error clarity.

Describe the role of the base Exception class when defining custom exceptions.

Base class is the foundation for all exceptions.

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