7.2 Throwing Exceptions - Throw

Let’s go through throwing exceptions using throw in C# — an important part of error handling.

---

📌 1. What is throw?

• The throw keyword is used to manually raise an exception

• Signals that an error or unexpected condition has occurred

• Can be caught using try-catch

---

📌 2. Basic Syntax

throw new ExceptionType("Error message");

• ExceptionType → any class derived from System.Exception

• "Error message" → descriptive message about the error

---

📌 3. Example: Throwing a Standard Exception

void ValidateAge(int age)
{
    if (age < 0)
        throw new ArgumentException("Age cannot be negative");
}
 
try
{
    ValidateAge(-5);
}
catch (ArgumentException ex)
{
    Console.WriteLine("Caught an exception: " + ex.Message);
}

Output:

Caught an exception: Age cannot be negative

---

📌 4. Rethrowing an Exception

• Sometimes you want to catch an exception, do something, then rethrow

try
{
    int result = 10 / 0;
}
catch (DivideByZeroException ex)
{
    Console.WriteLine("Logging error: " + ex.Message);
    throw;  // Rethrow same exception
}

• throw; preserves the original stack trace

• Don’t use throw ex; if you want to keep the original stack trace intact

---

📌 5. Throwing Custom Exceptions

• You can define your own exception class by inheriting from Exception

public class NegativeAgeException : Exception
{
    public NegativeAgeException(string message) : base(message) { }
}
 
void ValidateAge(int age)
{
    if (age < 0)
        throw new NegativeAgeException("Age cannot be negative!");
}
 
try
{
    ValidateAge(-1);
}
catch (NegativeAgeException ex)
{
    Console.WriteLine(ex.Message);
}

Output:

Age cannot be negative!

---

📌 6. Summary Table

Concept	Description
throw new Exception()	Manually raise a standard exception
throw	Rethrow caught exception preserving stack trace
Custom Exception	Create your own exception class by inheriting from Exception
Purpose	Signal errors, enforce validation, control flow in exceptional situations

---

✅ Tips:

• Use specific exception types instead of generic Exception

• Include clear error messages

• Use throw to stop execution when an invalid state is detected