3. Exception handling

1. What is Exception

• Exception: An error that occurs during the execution of a program.

• Exceptions disrupt the normal flow of the program.

• They are objects derived from the System.Exception base class.

• Examples: Division by zero, invalid file access, null reference.

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2. Rules for Handling Exception

1. Every exception in C# is an object of a class derived from System.Exception.

2. Code that may cause an exception should be placed inside a try block.

3. One or more catch blocks can handle exceptions.

4. The finally block (if used) always executes, whether an exception occurs or not.

5. If an exception is not handled, the runtime will terminate the program.

6. Specific exceptions should be caught before general exceptions.

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3. Exception Classes and Its Important Properties

• Base class: System.Exception

• Important Properties:

  • Message → Description of the error.

  • StackTrace → String representation of the call stack.

  • InnerException → Gets the original exception if wrapped.

  • HelpLink → Link to a help file related to the error.

  • Source → Name of the application/object that caused the error.

  • TargetSite → Method where the exception was thrown.

Common Exception Classes:

• DivideByZeroException

• NullReferenceException

• IndexOutOfRangeException

• FileNotFoundException

• InvalidCastException

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4. Understanding & Using try, catch keyword

• try block → contains code that might throw an exception.

• catch block → handles the exception.

Example:

try
{
    int a = 10, b = 0;
    int c = a / b;  // Will throw DivideByZeroException
}
catch (DivideByZeroException ex)
{
    Console.WriteLine("Error: " + ex.Message);
}

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5. Throwing Exceptions, Importance of finally Block

• throw keyword → Used to explicitly throw an exception.

Example:

throw new InvalidOperationException("Invalid operation performed.");

• finally block:

  • Always executes (even if exception occurs).

  • Used for cleanup activities like closing files, releasing database connections, etc.

Example:

try
{
    // risky code
}
catch (Exception ex)
{
    Console.WriteLine(ex.Message);
}
finally
{
    Console.WriteLine("Cleanup code executed.");
}

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6. Writing Custom Exception Classes

• You can create custom exceptions by inheriting from System.Exception.

• Useful for application-specific error handling.

Example:

public class AgeException : Exception
{
    public AgeException(string message) : base(message) { }
}
 
class Program
{
    static void Main()
    {
        int age = 15;
        try
        {
            if (age < 18)
                throw new AgeException("Age must be 18 or above.");
        }
        catch (AgeException ex)
        {
            Console.WriteLine("Custom Exception: " + ex.Message);
        }
    }
}

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