Run C# 12 online

What's in C# 12

• Primary constructors on all classes and structs — parameters in scope for the whole body.
• Collection expressions: `[1, 2, 3]` builds any collection type.
• `ref readonly` parameters and inline arrays for zero-allocation buffers.

What you get in C# 12

Clicking "Run now" opens the playground with this exact code and a matching test case — passes on first Run.

using System;
class Solution { static void Main() { int n = int.Parse(Console.ReadLine()!.Trim()); Console.WriteLine(n * 2); } }

How ls-judge runs C# 12

Every submission is compiled and executed inside its own Firecracker microVM — the same isolation primitive AWS Lambda uses. No shared container, no shared kernel with other users' code. The C# 12toolchain shipped with the microVM is the real thing (not a browser-side emulation): syscalls behave the way they do in production, and the compile / runtime behaviour matches what you would see on your own machine with that version installed.

Feed stdin from the Tests panel, watch verdicts stream per test case over a WebSocket. Free to use, no sign-up, works from any browser.