Overview

Exceptions in programming are events that disrupt the normal flow of a program's execution. Understanding common types of exceptions and how to differentiate between them is crucial for robust exception handling and error resolution. This knowledge allows developers to write more reliable and maintainable code, anticipating and mitigating potential issues before they occur.

Key Concepts

1. Syntax vs. Runtime Exceptions: Differentiating between errors found by the compiler vs. those that occur during execution.
2. Checked vs. Unchecked Exceptions: Understanding the distinction between exceptions that are checked by the compiler and those that are not.
3. Custom Exceptions: The creation and utilization of user-defined exceptions for more granular error handling.

Common Interview Questions

Basic Level

1. What is the difference between an exception and an error in programming languages?
2. Can you explain the use of try, catch, and finally blocks in C#?

Intermediate Level

3. How do checked and unchecked exceptions differ in C#?

Advanced Level

4. How would you design a custom exception class in C#, and in what scenarios would it be useful?

Detailed Answers

1. What is the difference between an exception and an error in programming languages?

Answer: In programming languages like C#, an exception is a problem that arises during the execution of a program, which can be caught and handled by the code. Errors, on the other hand, are more severe issues that are not meant to be caught or handled by the program, often indicating problems that a program should not attempt to recover from.

Key Points:
- Exceptions are conditions that a program can handle.
- Errors are issues that are beyond the control of the program.
- Handling exceptions is a common practice, whereas errors often result in program termination.

Example:

try
{
    // Attempt to access an array element that does not exist
    int[] numbers = {1, 2, 3};
    Console.WriteLine(numbers[5]);
}
catch (IndexOutOfRangeException e)
{
    // Handle the exception
    Console.WriteLine("An exception caught: " + e.Message);
}
finally
{
    // Cleanup code, executed whether an exception is caught or not
    Console.WriteLine("Finally block executed.");
}

2. Can you explain the use of try, catch, and finally blocks in C#?

Answer: In C#, try blocks are used to encapsulate code that may potentially cause an exception. catch blocks are used to catch and handle exceptions if they occur. The finally block is optional and contains code that is executed after the try/catch block, regardless of whether an exception was thrown or not, making it ideal for cleanup code.

Key Points:
- try block: Wraps code that might throw an exception.
- catch block: Catches and handles exceptions.
- finally block: Executes code after try/catch, regardless of the outcome.

Example:

try
{
    int divisor = 0;
    int result = 10 / divisor;
}
catch (DivideByZeroException e)
{
    Console.WriteLine("Attempting to divide by zero. " + e.Message);
}
finally
{
    Console.WriteLine("Finally block executed.");
}

3. How do checked and unchecked exceptions differ in C#?

Answer: C# does not have checked exceptions in the way Java does; all exceptions are unchecked. The term usually refers to how C# handles arithmetic overflow/underflow conditions for integer operations. By default, these are unchecked and do not throw an exception. However, you can use the checked keyword to enforce exception throwing in case of overflow/underflow, providing a mechanism to catch and handle such conditions.

Key Points:
- All exceptions are unchecked in C#; the term primarily applies to arithmetic operations.
- By default, arithmetic operations do not throw exceptions on overflow/underflow.
- The checked keyword can be used to enforce exception throwing in overflow/underflow scenarios.

Example:

try
{
    checked
    {
        int max = int.MaxValue;
        int result = max + 1; // This will cause an OverflowException
    }
}
catch (OverflowException e)
{
    Console.WriteLine("Overflow occurred: " + e.Message);
}

4. How would you design a custom exception class in C#, and in what scenarios would it be useful?

Answer: A custom exception class in C# is useful when you need to throw and handle errors specific to your application's domain that are not adequately represented by the standard exceptions. To create one, you inherit from the System.Exception class or any of its subclasses and can add custom properties or methods as needed.

Key Points:
- Custom exceptions provide a way to represent application-specific errors.
- They inherit from System.Exception.
- They can include additional properties and methods to convey more error information.

Example:

public class UserNotFoundException : Exception
{
    public string Username { get; }

    public UserNotFoundException(string username)
        : base($"User '{username}' not found.")
    {
        Username = username;
    }
}

// Usage
try
{
    throw new UserNotFoundException("johndoe");
}
catch (UserNotFoundException e)
{
    Console.WriteLine(e.Message);
}

This guide covers the basics of exceptions in C#, differentiating between syntax/runtime, checked/unchecked exceptions, and how to implement custom exceptions for specific error handling scenarios.