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String and StringBuilder

Short, interview-ready summary of string APIs and when to use StringBuilder.

Memory hook​

"string = immutable; many operations return new string; StringBuilder = mutable buffer for many concatenations"

string (immutable)​

  • Immutable — every “change” creates a new string
  • Literal: "hello"; Verbatim: @"C:\path" (no escape except "" → ")
  • Interpolation: $"Name: {name}"

Common string APIs​

Length and indexing​

int len = s.Length;
char c = s[0];

Comparison​

bool eq = s1 == s2;
int cmp = string.Compare(s1, s2, StringComparison.Ordinal);  // <0, 0, >0
bool eqIgnoreCase = s1.Equals(s2, StringComparison.OrdinalIgnoreCase);
  • Prefer StringComparison.Ordinal (or OrdinalIgnoreCase) for performance and clarity unless you need culture-specific rules

Search and substring​

bool has = s.Contains("ab");
int idx = s.IndexOf("ab");        // -1 if not found
int last = s.LastIndexOf("ab");
bool starts = s.StartsWith("ab");
bool ends = s.EndsWith("xy");

string sub = s.Substring(2, 3);   // start, length
string sub2 = s[2..5];             // range (C# 8+)

Modify (return new string)​

string lower = s.ToLower();
string upper = s.ToUpper();
string trimmed = s.Trim();        // TrimStart, TrimEnd
string replaced = s.Replace("a", "b");
string removed = s.Remove(2, 3);  // start, count
string inserted = s.Insert(2, "xy");

Split and join​

string[] parts = s.Split(',');
string[] parts2 = s.Split(new[] { ',', ' ' }, StringSplitOptions.RemoveEmptyEntries);
string joined = string.Join(", ", items);

Formatting​

string formatted = string.Format("{0} is {1}", name, age);
string interpolated = $"{name} is {age}";

Null/empty check​

bool empty = string.IsNullOrEmpty(s);
bool white = string.IsNullOrWhiteSpace(s);

StringBuilder (mutable, many concatenations)​

"Use when building a string in a loop or with many concatenations"

var sb = new StringBuilder();
sb.Append("Hello");
sb.AppendLine(" World");
sb.AppendFormat("{0}", value);
sb.AppendJoin(", ", items);

sb.Insert(0, "Prefix ");
sb.Remove(0, 3);
sb.Replace("a", "b");
sb.Clear();

string result = sb.ToString();
int len = sb.Length;
sb.Length = 0;  // clear without new allocation
  • Append / AppendLine — add to end
  • ToString() — build final string
  • Avoid string + string in loops; use StringBuilder instead

"string is immutable; operations like Replace or Trim return a new string. Use common APIs for comparison (Ordinal), search (IndexOf, Contains), split/join, and null checks. Use StringBuilder when building a string with many concatenations to avoid allocations."

Cheat sheet​

string = immutable; literal "", verbatim @", interpolated $
Contains, IndexOf, StartsWith, EndsWith
Substring, Replace, Trim, Split, Join
string.IsNullOrEmpty, IsNullOrWhiteSpace
StringBuilder = Append, AppendLine, ToString(); use in loops