Reading input from the console
Console.ReadLine returns text — always. What that means for everything downstream.
ReadLine: text, always text
Console.Write("Your name: ");
string name = Console.ReadLine() ?? "";
Console.WriteLine($"Hello, {name}!");
Console.ReadLine() reads one line and hands it to you as a string — even if the user typed digits. "42" (text) and 42 (number) are different universes: you cannot do arithmetic on text. The ?? "" covers the edge case where input ends before a line arrives (ReadLine then returns null).
Every interactive program you've used is this loop underneath: read text → interpret → respond.
The shape of input handling
Console.Write("Radius: ");
string input = Console.ReadLine() ?? "";
double radius = double.Parse(input); // crashes on "abc"!
double area = Math.PI * radius * radius;
Console.WriteLine($"Area: {area:F2}");
double.Parse converts text to a number — and throws FormatException if the text isn't a valid number. A user who types ten or 3,14 or just presses Enter crashes this program. That's the problem the next two lessons solve properly (TryParse + validation).
A note on this course's sandbox
Challenges here grade the logic (parse, validate, respond) through a method that takes the raw input string — the same code shape ReadLine programs need, but deterministic and testable. When you take this code to a real console, the ReadLine wrapper around it is trivial.