Strings: indexing and immutability

Strings are sequences of char

string lang = "C#";
char first = lang[0];      // 'C'
char last  = lang[^1];     // '#'  — ^1 is "one from the end"
int  len   = lang.Length;  // 2

Indexing starts at 0; [i] returns a char (single quotes: 'C'), not a string. lang[^1] uses the hat operator — count from the end. An out-of-range index throws IndexOutOfRangeException immediately; there is no silent clamping.

foreach (char c in text) walks every character without an index — use it when you don't need the position; use a for loop when you do.

Immutable: strings never change

string name = "code";
name.ToUpper();        // produces "CODE" ... and throws it away
string upper = name.ToUpper();   // name is STILL "code"

Every string "modification" method (ToUpper, Replace, Trim, Substring, ...) returns a new string and leaves the original untouched. Forgetting to capture the result is the single most common string bug:

line = line.Trim();    // correct: rebind the variable

Why immutable? One string object can be safely shared everywhere — no method can reach into your string and change it behind your back. The cost: heavy rebuild-in-a-loop code allocates a new string each pass (that's the StringBuilder discussion, later).

Building strings: interpolation wins

var user = "minh"; var score = 97;
Console.WriteLine($"{user} scored {score} points");     // interpolation
Console.WriteLine(user + " scored " + score + " points"); // concatenation

String interpolation ($"...") reads in output order and converts each expression with its ToString. Prefer it to + chains — and note the $ prefix is what enables the {} holes.