Exceptions and Error Handling in C#

What is an Exception?

An object representing an error / unexpected condition during program execution. Thrown (raised) and can be caught to recover or fail gracefully.

Common Built-in Exceptions

Type	Scenario
ArgumentException	Bad argument value
ArgumentNullException	Null passed to non-null parameter
InvalidOperationException	Object not in valid state
DivideByZeroException	Division by zero (integral)
FormatException	Parsing invalid text
IndexOutOfRangeException	Index outside bounds
NullReferenceException	Dereferencing null reference

Try / Catch / Finally Pattern

try
{
    var x = int.Parse(input);
    Console.WriteLine(10 / x);
}
catch (FormatException ex)
{
    Console.WriteLine($"Invalid number: {ex.Message}");
}
catch (DivideByZeroException)
{
    Console.WriteLine("Cannot divide by zero.");
}
finally
{
    Console.WriteLine("Always runs (cleanup). ");
}

Throwing Exceptions

if (amount < 0) throw new ArgumentOutOfRangeException(nameof(amount));

Custom Exception

public class NegativeDivisorException : Exception
{
    public NegativeDivisorException(int v) : base($"Divisor cannot be negative: {v}") {}
}

Best Practices

Fail fast for invalid arguments (throw early)
Use specific exception types
Avoid swallowing (empty catch) blocks
Reserve exceptions for exceptional control flow (not normal branching)
Add context message data when rethrowing

Mapping to Sample

See Console Applications/07_Exceptions for applied patterns.