Oppiskele Guard Clauses for Safer Code | Custom Exceptions and Guard Clauses

Guard clauses are a simple yet powerful technique that help you catch problems early in your code. Instead of letting invalid data flow through your methods or piling up deeply nested checks, you can use guard clauses right at the beginning of a method to validate inputs and throw exceptions immediately if something is wrong. This approach leads to cleaner, safer code that is much easier to read and maintain.

Program.cs


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using System;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            try
            {
                RegisterUser("Alice", 25);
                RegisterUser("", 30); // This will trigger a guard clause
            }
            catch (ArgumentException ex)
            {
                Console.WriteLine($"Error: {ex.Message}");
            }
        }

        public static void RegisterUser(string name, int age)
        {
            if (string.IsNullOrWhiteSpace(name))
                throw new ArgumentException("Name cannot be empty.", nameof(name));
            if (age < 0 || age > 120)
                throw new ArgumentException("Age must be between 0 and 120.", nameof(age));
            
            Console.WriteLine($"User '{name}' registered, age {age}.");
        }
    }
}

By placing guard clauses at the start of your methods, you immediately stop bad data before it can cause trouble later. This approach keeps your code flat and easy to follow, instead of creating layers of nested if statements. As a result, your methods remain focused on their main tasks, and error handling becomes much more predictable.

OrderProcessor.cs


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// This file is for illustration only and is not runnable as-is.

public void ProcessOrder(string productCode, int quantity, string customerEmail)
{
    if (string.IsNullOrWhiteSpace(productCode))
        throw new ArgumentException("Product code is required.");
    if (quantity <= 0)
        throw new ArgumentException("Quantity must be greater than zero.");
    if (!customerEmail.Contains("@"))
        throw new ArgumentException("Invalid email address.");

    // Main order processing logic goes here.
}

Definition

A guard clause is a quick check at the start of a method or block that immediately exits or throws an exception if something is wrong. In defensive programming, guard clauses help you stop invalid data or states as soon as possible, making bugs less likely.

1. What is a guard clause?

2. How do guard clauses help prevent bugs?

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