Online C# playground
Write and run C# right here — it executes in your browser, so there's nothing to install and nothing to sign up for.
- real CS error codes, explained
- 18 practice problems, auto-judged
- zero setup — runs in your browser
Program input (stdin) — click to add input for Console.ReadLine()
Runs locally in your browser. Infinite loops are stopped by a step watchdog instead of freezing the tab. The engine covers the beginner core — see what's supported.
Auto-judged practice
Eighteen problems from read-and-print to a matrix diagonal — write your solution and the browser grades it test by test.
Real compiler errors, decoded
CS0029, CS0103, CS0246… see the error happen live in your browser, understand the cause, copy the fix.
Beginner traps explained
Why 7 / 2 is 3 in C#, what var really does — plain-English guides with runnable snippets.
Easter eggs 🥚
Every egg here is real C# you can run — the point is the surprise, not a joke.
- In the playground: type
Console.WriteLine("chuliu");and run it. - Also:
Console.WriteLine("42");— and thenConsole.WriteLine(6 * 7);to see why. - In the example dropdown: five entries marked 🥚 — an interview trap, a float that refuses to be 0.3, an int odometer, a string that ignores
ToUpper(), and the shortest infinite loop that gets caught by the watchdog. - The classic:
int a = 010;prints10, because a leading zero means octal in C# (and it is a trap in every interview).
Nothing here is hidden from the source code — which is the actual egg: the language itself is the toy.
Why another C# site?
Every C# tutorial shows you code in a screenshot. Here, every snippet on every page actually runs — press the button under any example and see real output, or real compiler errors with the same codes Visual Studio gives you (CS0029, CS0103…). The interpreter is ~60 KB of JavaScript that ships with the page: no server round-trips, no accounts, no waiting.
Where to go next: start with the Hello World lesson, browse the error decoder when the compiler confuses you, or jump into the problem set. The editor remembers your draft between visits.