Exception Handling in C#

# CHAPTER 19

Exception Handling #

1. Introduction #

2. Learning Objectives #

• Understand the try, catch, and finally blocks.

• Catch specific types of exceptions.

• Use the throw keyword.

• Ensure resources are cleaned up using finally.

3. The try and catch Blocks #

• try: A block of code you suspect might cause an error.

• catch: The block of code that executes *only if* an error occurs inside the try block.

using System;

class Program
{
    static void Main()
    {
        try
        {
            Console.Write("Enter a number: ");
            // If the user types "Hello", Parse() will throw a FormatException!
            int num = int.Parse(Console.ReadLine()); 
            
            Console.WriteLine($"You entered: {num}");
        }
        catch (Exception ex) // Catches the error and stores data in 'ex'
        {
            // The program does not crash! It jumps here.
            Console.WriteLine("ERROR: You did not enter a valid number.");
            Console.WriteLine($"System Details: {ex.Message}");
        }
        
        Console.WriteLine("Program continues normally...");
    }
}

4. Catching Specific Exceptions #

try
{
    int[] numbers = { 10, 20, 30 };
    Console.WriteLine(numbers[5]); // Throws IndexOutOfRangeException
}
catch (IndexOutOfRangeException ex)
{
    Console.WriteLine("Error: Tried to access an invalid array index.");
}
catch (DivideByZeroException ex)
{
    Console.WriteLine("Error: Cannot divide by zero.");
}
catch (Exception ex) // The generic catch-all MUST be last
{
    Console.WriteLine($"Unknown error: {ex.Message}");
}

5. The finally Block #

try
{
    Console.WriteLine("Opening file...");
    // Simulate error
    throw new Exception("File corrupted!"); 
}
catch (Exception ex)
{
    Console.WriteLine("Caught an error.");
}
finally
{
    // This will print even though an error occurred!
    Console.WriteLine("Closing file..."); 
}

6. The throw Keyword #

static void CheckAge(int age)
{
    if (age < 0)
    {
        // Manually trigger an error
        throw new ArgumentException("Age cannot be negative.");
    }
    Console.WriteLine($"Age is {age}");
}

7. Mini Project: Safe Calculator #

using System;

namespace SafeCalculator
{
    class Program
    {
        static void Main()
        {
            try
            {
                Console.Write("Enter numerator: ");
                int num1 = int.Parse(Console.ReadLine());

                Console.Write("Enter denominator: ");
                int num2 = int.Parse(Console.ReadLine());

                int result = num1 / num2;
                Console.WriteLine($"Result: {result}");
            }
            catch (FormatException)
            {
                Console.WriteLine("Error: Please enter numbers only.");
            }
            catch (DivideByZeroException)
            {
                Console.WriteLine("Error: Mathematics forbids division by zero.");
            }
            catch (Exception ex)
            {
                Console.WriteLine($"An unexpected error occurred: {ex.Message}");
            }
        }
    }
}

8. Common Mistakes #

• Empty Catch Blocks: Writing catch (Exception) { } and doing nothing inside it is called "swallowing" the exception. It hides bugs and makes debugging impossible. At least log the error to the console!

• Using Try-Catch for Control Flow: Do not use exceptions to control normal program logic. Exceptions are computationally expensive. Use an if statement to check if a string is null, don't just let it throw a NullReferenceException.

9. Best Practices #

• Catch specific exceptions (like FileNotFoundException) instead of always catching the generic Exception.

• Use the ex.Message or ex.StackTrace properties to log detailed information for debugging.

10. Exercises #

1. 1. Write a program that accesses the 10th element of a 3-element array. Wrap it in a try-catch block that specifically catches IndexOutOfRangeException.

1. 2. Add a finally block that prints "Execution finished."

11. MCQs with Answers #

12. Interview Questions #

• Q: Explain the exact execution flow of try, catch, and finally.

• Q: Why should you avoid catching the generic Exception base class if possible?

• Q: What is the difference between throw; and throw ex; inside a catch block? (Answer: throw; preserves the original stack trace, while throw ex; resets the stack trace to the catch block, making debugging harder).

13. Summary #

14. Next Chapter Recommendation #