Strings, chars, and interpolation

char vs string

char grade = 'A';          // single quotes: ONE UTF-16 code unit
string name = "Ada";       // double quotes: a sequence of chars

They're different types: "A" (string) and 'A' (char) are not interchangeable. A string is immutable — methods like ToUpper() return a new string; the original never changes:

string shout = name.ToUpper();   // "ADA" — name is still "Ada"

Interpolation: the modern default

string user = "Lan";
int points = 42;
Console.WriteLine($"User {user} has {points} points.");
// Any expression goes inside the braces:
Console.WriteLine($"Double: {points * 2}");

$" switches interpolation on. Inside {}, the compiler type-checks the expression. Formatting follows the value after : — {price:F2} for two decimals, {n:D5} to pad an integer:

Console.WriteLine($"[{score:D3}]");   // [007]

Prefer interpolation over + concatenation: it reads in order, survives edits, and type-checks. You'll meet StringBuilder much later, when loop-built strings become a real performance topic — immutability means a + b + c allocates, but for beginner-scale strings clarity wins.

Useful string members now

name.Length, name.Contains("da"), name.StartsWith("A"), name.ToUpper(), name.Trim(). Comparisons: == compares string contents in C# (unlike Java) — a == b is true when the text matches.