C# strings and formatting

The concept

A C# string is an immutable sequence of characters, so methods such as Substring, Replace, and ToUpper return new strings. Split turns a string into an array, string.Join does the reverse, and Trim and PadLeft change the white space around the text. StringBuilder is the right tool for building a long string in a loop, because it does not copy the text on every append.

The == operator on strings compares the characters, and it is case-sensitive. For case-insensitive comparison use string.Equals with StringComparison.OrdinalIgnoreCase. Interpolated strings, written $"...", can include format specifiers after a colon: F2 for two decimals, N0 for a thousands separator, D5 for five digits with leading zeros, and X for hexadecimal. Formatting depends on the culture, so FormattableString.Invariant fixes the output regardless of the computer's settings.

Code example

using System;
using System.Text;

class Program {
  static void Main() {
    string name = "Programonic";
    Console.WriteLine(name.Length + " " + name.Substring(0, 7) + " " + name.IndexOf('m'));
    Console.WriteLine(name.ToUpper() + " " + name.Replace("o", "0"));

    string[] parts = "a,b,,c".Split(',');
    Console.WriteLine(parts.Length + " " + string.Join("-", parts));
    Console.WriteLine("  padded ".Trim().PadLeft(8, '*'));

    Console.WriteLine("apple" == "APPLE");
    Console.WriteLine(string.Equals("apple", "APPLE", StringComparison.OrdinalIgnoreCase));

    double price = 1234.56;
    Console.WriteLine(FormattableString.Invariant($"{price:F2} {price:N0} {42:D5} {255:X}"));

    var builder = new StringBuilder();
    for (int i = 1; i <= 3; i++) {
      builder.Append(i).Append(i < 3 ? "," : "");
    }
    Console.WriteLine(builder);
  }
}

11 Program 6
PROGRAMONIC Pr0gram0nic
4 a-b--c
**padded
False
True
1234.56 1,235 00042 FF
1,2,3

Follow the code

Open the walkthrough to inspect the example one step at a time.

11 Program 6

11 Program 6
PROGRAMONIC Pr0gram0nic

11 Program 6
PROGRAMONIC Pr0gram0nic
4 a-b--c

11 Program 6
PROGRAMONIC Pr0gram0nic
4 a-b--c
**padded

11 Program 6
PROGRAMONIC Pr0gram0nic
4 a-b--c
**padded
False

11 Program 6
PROGRAMONIC Pr0gram0nic
4 a-b--c
**padded
False
True

11 Program 6
PROGRAMONIC Pr0gram0nic
4 a-b--c
**padded
False
True
1234.56 1,235 00042 FF

11 Program 6
PROGRAMONIC Pr0gram0nic
4 a-b--c
**padded
False
True
1234.56 1,235 00042 FF
1,2,3

Behind the scenes

Cost and common mistakes

Most string operations are O(n) in the length of the text.

• Using + in a loop to build a long string creates a new string every time, which is O(n²) overall. Use StringBuilder.
• Without an explicit culture, number formatting follows the computer's regional settings, so the same program can print 1,234.50 or 1.234,50.
• Use the Ordinal comparisons for identifiers and file names, and culture-aware comparisons only for text shown to people.

Try it yourself

How would you print 7 as 007?

Further reading

• C# documentation (external)
• .NET API browser (external)