C# string methods cheat sheet — Contains, Split, Replace, Trim, and more

string sentence = "The quick brown fox";

Console.WriteLine(sentence.Contains("brown"));        // True
Console.WriteLine(sentence.StartsWith("The"));        // True
Console.WriteLine(sentence.EndsWith("cat"));          // False

// Case-insensitive comparison
Console.WriteLine(sentence.Contains("BROWN",
    StringComparison.OrdinalIgnoreCase));             // True

// Check for any of several substrings
bool hasAnimal = sentence.Contains("fox") || sentence.Contains("dog");
Console.WriteLine(hasAnimal);                         // True

string csv = "Alice,Bob,Charlie,Diana";

// Split on a single character
string[] names = csv.Split(',');
foreach (string name in names)
    Console.WriteLine(name);
// Alice
// Bob
// Charlie
// Diana

// Split on a string delimiter
string log = "ERROR|404|Page not found";
string[] parts = log.Split('|');
Console.WriteLine(parts[0]); // ERROR
Console.WriteLine(parts[1]); // 404
Console.WriteLine(parts[2]); // Page not found

// Split on multiple delimiters and remove empty entries
string messy = "one,,two, ,three";
string[] clean = messy.Split(
    new[] { ',', ' ' },
    StringSplitOptions.RemoveEmptyEntries
);
// ["one", "two", "three"]

string text = "Hello, World! Hello, C#!";

// Replace a substring
string result = text.Replace("Hello", "Hi");
Console.WriteLine(result);
// Hi, World! Hi, C#!

// Replace a single character
string path = @"C:\Users\Alice\Documents";
string webPath = path.Replace('\\', '/');
Console.WriteLine(webPath);
// C:/Users/Alice/Documents

// Remove a substring by replacing with empty string
string noSpaces = "remove all spaces".Replace(" ", "");
Console.WriteLine(noSpaces); // removeallspaces

string raw = "   hello world   ";

Console.WriteLine(raw.Trim());       // "hello world"
Console.WriteLine(raw.TrimStart());  // "hello world   "
Console.WriteLine(raw.TrimEnd());    // "   hello world"

// Trim specific characters
string url = "///path/to/resource///";
Console.WriteLine(url.Trim('/'));    // "path/to/resource"

// Common use case: clean user input before validation
string userInput = "  alice@example.com  ";
string email = userInput.Trim().ToLower();
Console.WriteLine(email); // alice@example.com

string name = "Alice Smith";

Console.WriteLine(name.ToUpper()); // ALICE SMITH
Console.WriteLine(name.ToLower()); // alice smith

// Culture-aware upper/lower (important for Turkish 'i'→'İ')
string turkish = "istanbul";
Console.WriteLine(turkish.ToUpper());   // ISTANBUL (invariant)
Console.WriteLine(turkish.ToUpper(
    System.Globalization.CultureInfo.GetCultureInfo("tr-TR"))); // İSTANBUL

string text = "Hello, World!";

// Substring(startIndex, length)
Console.WriteLine(text.Substring(7, 5));  // World

// IndexOf returns -1 when not found
int idx = text.IndexOf("World");
if (idx >= 0)
    Console.WriteLine($"Found at index {idx}"); // Found at index 7

// LastIndexOf — search from the end
string path = "/home/user/documents/report.pdf";
int lastSlash = path.LastIndexOf('/');
string fileName = path.Substring(lastSlash + 1);
Console.WriteLine(fileName); // report.pdf

// Modern alternative: Range syntax (C# 8+)
string first5 = text[..5];    // "Hello"
string last6  = text[^6..];   // "World!"

// Right-align numbers in a fixed-width column
int[] numbers = { 1, 42, 999, 10_000 };
foreach (int n in numbers)
    Console.WriteLine(n.ToString().PadLeft(8));

// Output (right-aligned):
//        1
//       42
//      999
//    10000

// Left-align strings
string[] labels = { "Name", "Department", "ID" };
foreach (string label in labels)
    Console.WriteLine(label.PadRight(15) + "|");

// Output:
// Name           |
// Department     |
// ID             |

// Zero-pad a number
string code = "42".PadLeft(6, '0');
Console.WriteLine(code); // 000042

string? a = null;
string  b = "";
string  c = "   ";
string  d = "hello";

Console.WriteLine(string.IsNullOrEmpty(a));          // True
Console.WriteLine(string.IsNullOrEmpty(b));          // True
Console.WriteLine(string.IsNullOrEmpty(c));          // False ← whitespace is not empty
Console.WriteLine(string.IsNullOrWhiteSpace(c));     // True  ← catches whitespace-only
Console.WriteLine(string.IsNullOrWhiteSpace(d));     // False

string[] fruits = { "apple", "banana", "cherry" };

// Join with a separator
string joined = string.Join(", ", fruits);
Console.WriteLine(joined); // apple, banana, cherry

// Join integers
int[] nums = { 1, 2, 3, 4, 5 };
Console.WriteLine(string.Join(" + ", nums)); // 1 + 2 + 3 + 4 + 5

// Join with newline
string multiLine = string.Join(Environment.NewLine, fruits);

string first = "Hello";
string last  = "World";

// These are equivalent for 2–3 strings
string s1 = first + ", " + last + "!";
string s2 = string.Concat(first, ", ", last, "!");

Console.WriteLine(s1); // Hello, World!
Console.WriteLine(s2); // Hello, World!

// For many concatenations in a loop, prefer StringBuilder
// For a fixed set of values, + or string.Concat is fine