What is C#? What is .NET?

One language, one platform

C# is a programming language. .NET is a platform: a runtime that executes programs plus a huge standard library you call into. You write C#; the platform runs it. People say "C# developer" and ".NET developer" almost interchangeably — but the concepts are distinct, and interviews test the distinction.

using System;

public class Solution
{
    public static void program()
    {
        Console.WriteLine("C# is the language. WriteLine comes from .NET.");
    }
}

Console.WriteLine is not part of the C# language — it's a method the .NET library provides. The C# compiler (Roslyn) checks your code; the .NET runtime (CoreCLR) executes it.

| Piece | What it is | Example | |---|---|---| | C# | the language: syntax, types, rules | string name = "Ada"; | | Roslyn | the C# compiler: your code → IL | catches string x = 5; | | .NET runtime | executes the compiled program | runs Console.WriteLine | | .NET libraries | ready-made functionality | Console, Math, File, List<T> |

Code Journey grades your C# with the real .NET SDK — your code is compiled and executed in a locked-down sandbox, exactly like the platform does in production.

What C# is used for

C# runs web backends (ASP.NET Core), desktop apps, games (Unity is C#-scripted), and cloud services. All of those rest on the fundamentals this course drills: types, control flow, methods, collections, classes. Frameworks come and go; the fundamentals pay the bill for decades.

Compiled, not interpreted

When you submit code here, three things happen:

1. Compile — Roslyn checks syntax and types, producing an assembly.
2. Load — the .NET runtime loads your compiled code.
3. Run — each hidden test executes against it and reports pass/fail.

If your code doesn't compile, you get the compiler's error message — read it top to bottom; it usually names the exact line and problem. That's not an obstacle; it's the fastest feedback loop in software.