C# — Strings

String basics

using System;

string s1 = "Hello";
string s2 = "World";
string s3 = string.Concat(s1, " ", s2);
string s4 = $"{s1} {s2}"; // Interpolation

Console.WriteLine(s4); // Hello World
Console.WriteLine(s4.Length); // 11

String operations

using System;

string s = "Hello, World!";

Console.WriteLine(s.Substring(0, 5));    // Hello
Console.WriteLine(s.IndexOf("World"));  // 7
Console.WriteLine(s.Contains("World")); // True
Console.WriteLine(s.Replace("World", "C#")); // Hello, C#!
Console.WriteLine(s.ToUpper());         // HELLO, WORLD!
Console.WriteLine(s.ToLower());         // hello, world!
Console.WriteLine(s.Trim());            // Hello, World!
Console.WriteLine(s.StartsWith("Hello")); // True
Console.WriteLine(s.EndsWith("!"));     // True
Console.WriteLine(s[0]);               // H

String comparison

using System;

string a = "Hello";
string b = "hello";

Console.WriteLine(a == b);              // False (case-sensitive)
Console.WriteLine(a.Equals(b, StringComparison.OrdinalIgnoreCase)); // True
Console.WriteLine(string.Compare(a, b, StringComparison.OrdinalIgnoreCase)); // 0

// Reference vs value equality
string s1 = "Hello";
string s2 = "Hello";
Console.WriteLine(s1 == s2);            // True (value equality)
Console.WriteLine(ReferenceEquals(s1, s2)); // May be True (interning)

String methods

using System;

// Split
string csv = "apple,banana,cherry";
string[] fruits = csv.Split(',');
foreach (var f in fruits)
    Console.WriteLine(f);

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

// Pad
Console.WriteLine("42".PadLeft(5, '0'));  // 00042
Console.WriteLine("Hi".PadRight(10, '-')); // Hi--------

// Contains and starts/ends
Console.WriteLine("Hello".Contains("ell")); // True
Console.WriteLine("Hello".StartsWith("He")); // True
Console.WriteLine("Hello".EndsWith("lo"));   // True

StringBuilder

For efficient string manipulation:

using System;
using System.Text;

var sb = new StringBuilder();
sb.Append("Hello");
sb.Append(" ");
sb.Append("World");
sb.Insert(5, ",");
sb.Replace("World", "C#");

string result = sb.ToString();
Console.WriteLine(result); // Hello, C#

// StringBuilder with capacity
var sb2 = new StringBuilder(100); // Pre-allocate
for (int i = 0; i < 50; i++)
{
    sb2.Append(i).Append(" ");
}
Console.WriteLine(sb2.ToString());

String interpolation

using System;

string name = "Alice";
int age = 30;
double score = 95.5;

// Basic interpolation
Console.WriteLine($"{name} is {age} years old");

// Expressions
Console.WriteLine($"Next year: {age + 1}");
Console.WriteLine($"Uppercase: {name.ToUpper()}");

// Format specifiers
Console.WriteLine($"Score: {score:F2}");   // 95.50
Console.WriteLine($"Score: {score:P0}");   // 9550%

// Alignment
Console.WriteLine($"{"Name",-10} {"Score",5}");
Console.WriteLine($"{"Alice",-10} {score,5:F1}");

Raw string literals (C# 11)

using System;

// No escape needed
string path = """C:\Users\Alice\Documents""";
Console.WriteLine(path);

// Multi-line
string json = """
    {
        "name": "Alice",
        "age": 30
    }
    """;
Console.WriteLine(json);

String and char arrays

using System;

string s = "Hello";

// To char array
char[] chars = s.ToCharArray();
Array.Reverse(chars);
Console.WriteLine(new string(chars)); // olleH

// From char array
string fromChars = new string(new[] { 'H', 'i' });
Console.WriteLine(fromChars); // Hi

// Char operations
Console.WriteLine(char.IsLetter('A'));    // True
Console.WriteLine(char.IsDigit('5'));     // True
Console.WriteLine(char.ToUpper('a'));     // A

Common patterns

using System;

// Null/empty checks
string? s = null;
Console.WriteLine(string.IsNullOrEmpty(s));      // True
Console.WriteLine(string.IsNullOrWhiteSpace(s)); // True

// String interpolation for file paths
string name = "report";
string path = $@"C:\Users\Documents\{name}.txt";
Console.WriteLine(path);

// String interpolation for SQL (use parameters in real code!)
string table = "users";
string query = $"SELECT * FROM {table} WHERE active = 1";
Console.WriteLine(query);

Mini Practice

Write C# code that:

1. Splits a CSV string and prints each field
2. Uses StringBuilder to build a large string efficiently
3. Demonstrates string interpolation with alignment
4. Converts a string to a char array and reverses it

Up Next

In the next lesson, you'll learn about If Else — conditional branching in C#.