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.

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

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 , 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.

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.

Want to push further? This example brings together a class with properties , an enum , a computed property , and a — 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

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 {' '} , what is (int)Level.High ?

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

Practice quiz

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