Working with strings

Build, inspect and transform text with string methods, interpolation and StringBuilder.

Strings are sequences of characters

A string holds text. Under the hood it's a sequence of char values that you can inspect:

string word = "C# rocks";
 
Console.WriteLine(word.Length);   // 8
Console.WriteLine(word[0]);       // 'C' — indexes start at 0
Console.WriteLine(word[^1]);      // 's' — ^1 means "first from the end"
Console.WriteLine(word[3..]);     // "rocks" — a range from index 3 to the end

Strings are immutable

Once created, a string never changes. Methods that seem to modify a string actually return a new one:

string name = "ada";
name.ToUpper();                 // creates "ADA"… and throws it away
Console.WriteLine(name);        // ada
 
name = name.ToUpper();          // keep the result
Console.WriteLine(name);        // ADA

Watch out

Forgetting to use the returned value is a very common bug: text.Trim(); on its own line does nothing useful.

The methods you'll use constantly

Method	Example	Result
`ToUpper()` / `ToLower()`	`"Hi".ToUpper()`	`"HI"`
`Trim()`	`" hi ".Trim()`	`"hi"`
`Contains(x)`	`"hello".Contains("ell")`	`true`
`StartsWith(x)` / `EndsWith(x)`	`"file.cs".EndsWith(".cs")`	`true`
`IndexOf(x)`	`"hello".IndexOf('l')`	`2` (or `-1` if missing)
`Substring(start, length)`	`"hello".Substring(1, 3)`	`"ell"`
`Replace(a, b)`	`"a-b-c".Replace("-", "")`	`"abc"`
`Split(sep)`	`"a,b,c".Split(',')`	`["a", "b", "c"]`
`string.Join(sep, items)`	`string.Join("-", parts)`	`"a-b-c"`
`string.IsNullOrWhiteSpace(s)`	`string.IsNullOrWhiteSpace(" ")`	`true`

And for single characters, the char type has helpers: char.ToUpper(c), char.IsDigit(c), char.IsLetter(c).

To repeat a character, use new string('*', 5) → "*****".

Building strings

Concatenation with + works, but interpolation is clearer:

string first = "Ada", last = "Lovelace";
int born = 1815;
 
string a = "Name: " + first + " " + last + " (" + born + ")";
string b = $"Name: {first} {last} ({born})";      // same result, easier to read

Format specifiers

Inside { }, add a colon and a format:

double price = 1234.5;
Console.WriteLine($"{price:F2}");    // 1234.50   — 2 decimals
Console.WriteLine($"{price:N0}");    // 1,235     — thousands separator, no decimals
Console.WriteLine($"{0.256:P1}");    // 25.6 %    — percentage
Console.WriteLine($"{42,6}|");       //     42|   — right-aligned in 6 characters
Console.WriteLine($"{DateTime.Now:yyyy-MM-dd}");

Special characters and multi-line text

string tab = "Name:\tAda";              // \t tab, \n new line, \" quote, \\ backslash
string path = @"C:\Users\Ada\file.txt"; // verbatim: backslashes are literal
 
string json = """
    {
      "name": "Ada",
      "born": 1815
    }
    """;                                 // raw string literal: no escaping at all

Comparing strings

== compares the text, which is what you want most of the time. For case-insensitive comparison:

bool same = string.Equals("Hello", "HELLO", StringComparison.OrdinalIgnoreCase);  // true

StringBuilder for building in loops

Because strings are immutable, += in a loop creates a new string every time. For many appends, use StringBuilder:

using System.Text;
 
var sb = new StringBuilder();
for (int i = 1; i <= 5; i++)
{
    sb.Append(i).Append(' ');
}
Console.WriteLine(sb.ToString());   // "1 2 3 4 5 "

Try it

Program.csCtrl + Enter to run

Loading editor…

C# runtime not loaded

Key takeaways

Strings are immutable: methods return new strings — keep the result.
Index with s[0], s[^1], and ranges like s[1..4].
Prefer interpolation $"..." and use format specifiers like :F2 and :N0.
Use StringBuilder when building text piece by piece in a loop.

You will learn