throw / Custom Exceptions

// Throw an exception.
throw new ExceptionType("message");

// Define a custom exception class.
class ClassName : Exception
{
	public ClassName(string message) : base(message) { }
}

using System;

// Throw an exception with throw.
void CheckAge(int age)
{
	if (age < 0 || age > 150) {
		throw new ArgumentOutOfRangeException(nameof(age), "Age must be between 0 and 150.");
	}
	Console.WriteLine($"Age: {age}");
}

try {
	CheckAge(200);
} catch (ArgumentOutOfRangeException e) {
	Console.WriteLine(e.Message);
}

// Define a custom exception class.
class AppException : Exception
{
	public int StatusCode { get; }

	public AppException(string message, int statusCode) : base(message)
	{
		StatusCode = statusCode;
	}
}

// Throw and catch a custom exception.
try {
	throw new AppException("Authentication failed.", 401);
} catch (AppException e) {
	Console.WriteLine($"[{e.StatusCode}] {e.Message}");
}

// Rethrow inside a catch block (preserves the stack trace).
void Process()
{
	try {
		int result = int.Parse("abc");
	} catch (FormatException) {
		Console.WriteLine("Log: Conversion error detected.");
		throw; // Rethrow the exception.
	}
}

try {
	Process();
} catch (FormatException e) {
	Console.WriteLine("Caught at upper level: " + e.Message);
}

A custom exception class is defined by inheriting from Exception (or another existing exception class). Adding application-specific error codes or extra information as properties enables more detailed error handling in the catch block.

When rethrowing, writing throw e; overwrites the stack trace with the current position. Use the argument-less throw; when you want to preserve the original stack trace.

For the exception-catching syntax, see try / catch / finally.