Defensive Design Worksheet

Section 1 · Do Now

Break my program

Console.Write("Enter your age: ");
int age = int.Parse(Console.ReadLine());
Console.WriteLine("In 10 years you'll be " + (age + 10));

List inputs that would break this program, then sort them into two groups.

Inputs that make it crash

Inputs where it runs but gives a wrong or silly result

Section 2 · Guided notes

Defensive design notes

Make notes in your own words as we go through the slides. Use examples.

Anticipating misuse

Input validation

Dealing with invalid data

Authentication

Summary One sentence: measure → consequence. E.g. "[What the program does] means [why that protects it]."

Section 3 · Research

Validation checks and research questions

Sources: BBC Bitesize (OCR J277), Isaac Computer Science, Craig 'n' Dave. Write in your own words; don't copy and paste.

Part A: The six validation checks (everyone)

Check	What it checks	Example of data it rejects	Real form where it's used

Part B: Research questions (on your own)

Research all four questions. For each one, write 2-3 sentences in your own words and include a real example.

A. What is the difference between validation and verification? Give an example of each.

B. How do real websites make logins secure? Think about attempt limits, password rules, 2FA and CAPTCHA.

C. What should a program do with invalid data? Compare crashing, rejecting, and validating with a loop.

D. Why does maintainable code matter? Who reads code after it's written?

One new keyword and its definition

Sources used

One question your research raised

Section 4 · Practical

TryParse, validation and login

TryParse check

In your own words, what's the difference between int.Parse and int.TryParse?

Which validation check does TryParse perform?

Hinge question

while (!int.TryParse(Console.ReadLine(), out age) || age < 0 || age > 120)
{
    Console.Write("Invalid. Enter a whole number from 0 to 120: ");
}
Console.WriteLine("In 10 years you'll be " + (age + 10));

The user types twelve, then -5, then 30.

How many times does "Invalid" appear, and what is the final output?

Plan before you code

Task 1, age validation: which validation checks will you use?

Task 2, login: how will you stop someone guessing passwords forever?

Your code

Paste your finished C# code for Task 1 (age validation)

Paste your finished C# code for Task 2 (login) Include the stretch task if you did it

Section 5 · Maintainability

Messy Code Makeover

Do the Messy Code Makeover task on CodeBash: Year 11 C# Programming → Defensive Design. Switch the editor to C#. Then make notes below.

For each technique, write one sentence: what it is → why it makes code easier to maintain.

Sub programs

Naming conventions

Indentation

Commenting

Which change made the biggest difference to the messy code, and why?

Section 6 · Exit ticket

Exit ticket

1. Name three validation checks and give an input each would reject.

2. Explain one way a program can be protected against misuse. [2 marks]

3. Give two reasons comments make a program easier to maintain.

When you've finished, download your notes and upload the file to Teams.