7. Strings, StringBuilder and immutability

string is a reference type that behaves like a value: it is immutable — every "change" creates a new string.

string s = "a";
s += "b";            // allocates a new string "ab"; "a" becomes garbage

var sb = new StringBuilder();
for (int i = 0; i < 1000; i++) sb.Append(i).Append(',');
string csv = sb.ToString();   // one final allocation

• Use StringBuilder when concatenating in loops; for a few concatenations + or interpolation is fine.
• String interning: identical literals share one instance (string.Intern).
• Compare with string.Equals(a, b, StringComparison.OrdinalIgnoreCase) rather than ToLower().
• string.IsNullOrWhiteSpace(x) covers null, "" and " ".