Checkpoint: C# Basics

You've learned data types, casting, Console input, strings, List , Dictionary , properties and enums — let's combine them. This checkpoint isn't new theory; it's where the pieces click together into real, working programs. You'll warm up, recap what you know, build a complete program from scratch, and test yourself with a short quiz.

Learn Checkpoint: C# Basics in our free C# course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick recall.

Part of the free C# course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You've Learned

Everything in this checkpoint draws on the eight lessons you just completed. Here's the toolkit you now carry into every C# program:

Topic

You can now…

Key tool

Data Types

Pick the right type for any value

int, double, decimal, bool

Type Casting

Convert and validate safely

TryParse, (int)

User Input

Read and re-prompt for input

Console.ReadLine

Strings

Format, split, join, build text

$"...", StringBuilder

List<T>

Store ordered, growable data

Add, RemoveAll, Sort

Dictionary

Look up and tally by key

TryGetValue, GetValueOrDefault

Properties

Wrap data with validation

{ get; set; }

Enums

Model fixed sets of options

enum, switch

1. Warm-Up

Before the main challenge, run this short program that already weaves together five skills: splitting strings, parsing with TryParse , collecting into a List<int> , sorting, and interpolated output. Trace each line and predict the result before you run it.

2. Build Challenge: Word Frequency Report

Now the main event. Build a complete word frequency report : split a sentence into words, tally each word in a Dictionary , print every word with its count, and announce the most frequent word. This is a genuine real-world task — search engines, analytics, and spam filters all start here. Attempt it yourself in the editor before revealing the solution.

🎯 Multi-Step Build Challenge

The starter below has the brief, the expected output, and a scaffold in comments. Combine strings + Dictionary + loops + interpolation to make it work.

Here's one clean way to do it. Compare it with your attempt — notice how the count and the "best so far" are tracked in the same loop, so we never scan the dictionary twice.

🔎 Stretch: model real data

Want to push further? This example brings together a class with properties , an enum , a computed property , and a List<Task> — exactly how a real task list app stores its data. Read it, run it, then try adding a new task or a fourth Priority level.

⏱ Timed Quiz

📝 Checkpoint Quiz

Answer each in your head (or out loud) before revealing. Six questions covering the whole basics track.

Q1. Which type should hold a price like 19.99 that must be exact?

decimal (written 19.99m ). double has binary rounding drift and is wrong for money.

Q2. The user types "abc" . What does int.TryParse("abc", out int n) return, and what is n ?

It returns false , and n is set to its default of 0 . No exception is thrown — that's why TryParse is safe for input.

Q3. You read a key that isn't in a Dictionary with the indexer. What happens, and what should you use instead?

The indexer throws KeyNotFoundException . Use TryGetValue (or GetValueOrDefault / ContainsKey ) to read safely.

Q4. Why can't you safely remove items from a List inside a foreach over that same list?

Changing the collection mid-iteration throws InvalidOperationException . Use RemoveAll(predicate) , or loop backwards with a for loop using indexes.

Q5. What's the advantage of an auto-property over a public field?

A property keeps a stable public surface: you can later add validation in the setter, make the setter private, or compute the value — all without breaking calling code. A public field locks you out of that.

Q6. Given enum Level { Low = 1, Medium, High } , what is (int)Level.High ?

3 — Low is explicitly 1, so Medium auto-increments to 2 and High to 3.

Pro Tips for the Road Ahead

• 💡 Reach for the right collection: List for ordered data, Dictionary for lookups by key.
• 💡 Validate at the boundary: parse and check input the moment it arrives, before it spreads through your code.
• 💡 Let types do the work: enums and properties catch whole classes of bugs at compile time.
• 💡 Read your output: if a program runs but prints the wrong thing, the bug is usually a type or a loop boundary — exactly what you practised here.

Watch-Outs You've Now Met

• Integer division: 7 / 2 is 3 . Cast to double for a real average.
• Concatenating input: "5" + 3 is "53" — parse before doing maths.
• Missing keys / out-of-range indexes: guard with TryGetValue and Count checks.
• Forgetting using s: List / Dictionary need System.Collections.Generic ; Sum / Average need System.Linq .

Frequently Asked Questions

🎉 Checkpoint Complete

• ✅ You combined input, parsing, collections, strings and formatting into one working program
• ✅ You built a real word-frequency report with the Dictionary tally pattern
• ✅ You modelled data with classes, properties and enums in a single list
• ✅ You passed the quiz on the core gotchas: division, concatenation, missing keys, mutation
• ✅ Next up: Nullable Types & Null Operators — handle "no value" safely and elegantly

Practice quiz

Continue this course

• Previous: Enums
• Next: Nullable Types & Null Operators