The string API you'll use daily

Asking questions

string file = "report.PDF";
file.EndsWith(".pdf")              // false — case matters!
file.EndsWith(".pdf", StringComparison.OrdinalIgnoreCase)  // true
file.Contains("port")              // true
file.StartsWith("rep")             // true
string.Empty == file.Trim()        // is it blank?

String comparisons are case-sensitive by default — "PDF" does not end with ".pdf". For user-facing input, say what you mean with a StringComparison argument. Equality (== and Equals) compares contents, not references — strings are the reference type where == does what beginners expect.

Slicing and transforming

string s = "  Code Journey  ";
s.Trim()                       // "Code Journey"      — new string
s.Substring(5)                 // "Journey"           — from index 5 on
s.Substring(5, 3)              // "Jou"               — 3 chars from index 5
s.Replace(" ", "-")            // "  Code-Journey-  "
s.Trim().Replace(" ", "-")     // "Code-Journey"      — chained

Substring(start) takes everything from start; Substring(start, length) takes exactly length chars — and throws if the range leaves the string. Replace swaps every occurrence, not the first.

Split and join

string csv = "rice,beans,oil";
string[] items = csv.Split(',');          // ["rice", "beans", "oil"]
string re = string.Join(" | ", items);    // "rice | beans | oil"

Split cuts a string into an array at each separator; empty fields become empty strings, and you can pass char[] or string[] separators. string.Join is the inverse — glue an array (of strings) with a separator. Together they're the backbone of line- and record-based text processing (CSV-ish files, logs, user records).

The number↔string boundary

int n = 42;
string t = n.ToString();          // "42"
string f = n.ToString("D5");      // "00042" — format codes exist
int back = int.Parse("42");       // throws on garbage
bool ok  = int.TryParse("42", out int value);  // false + value=0 on garbage

Parse throws FormatException on bad input; TryParse reports success as a bool — remember it from Module 3, this is where it lives conceptually.