string.PadLeft() / PadRight()

// Returns a string padded on the left with spaces (or a specified character) to reach totalWidth.
str.PadLeft(int totalWidth)
str.PadLeft(int totalWidth, char paddingChar)

// Returns a string padded on the right with spaces (or a specified character) to reach totalWidth.
str.PadRight(int totalWidth)
str.PadRight(int totalWidth, char paddingChar)

// Returns a string with format placeholders replaced by the given arguments.
string.Format(string format, object arg0, ...)

Program.cs

using System;

// Use PadLeft() to right-align numbers
for (int i = 1; i <= 5; i++) {
    string line = i.ToString().PadLeft(3);
    Console.WriteLine(line);
}

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

This produces the following output:

dotnet run
  1
  2
  3
  4
  5
000042

PadRight() left-aligns columns of text. It is useful for formatting text output as a table.

TableFormat.cs

using System;

// Use PadRight() to left-align text
string[] members = { "Yagami Iori", "Kusanagi Kyo", "Terry Bogard" };
int[] scores = { 95, 88, 72 };
for (int i = 0; i < members.Length; i++) {
    Console.WriteLine(members[i].PadRight(14) + scores[i] + " pts");
}

// Use string.Format() to build a formatted string
string message = string.Format("{0}'s score is {1} points.", "Yagami Iori", 95);
Console.WriteLine(message);

// Numeric format specifiers (percentage, etc.)
double discountRate = 0.1567;
Console.WriteLine(string.Format("Discount rate: {0:P1}", discountRate)); // Discount rate: 15.7%

This produces the following output:

dotnet run
Yagami Iori   95 pts
Kusanagi Kyo  88 pts
Terry Bogard  72 pts
Yagami Iori's score is 95 points.
Discount rate: 15.7%

Numeric format specifier strings such as ToString("D6") provide a concise way to zero-pad or set the number of digits. They can also be used with string interpolation.

FormatString.cs

using System;

// Zero-padding with ToString() format specifiers (alternative to PadLeft)
Console.WriteLine(42.ToString("D6")); // 000042
Console.WriteLine(255.ToString("X4")); // 00FF (hexadecimal, 4 digits)
Console.WriteLine(3.14.ToString("F2")); // 3.14 (2 decimal places)

// Format specifiers also work with string interpolation
double price = 1234.5;
Console.WriteLine($"Price: {price:N0}"); // Price: 1,235
Console.WriteLine($"Rate: {0.756:P1}"); // Rate: 75.6%
Console.WriteLine($"Code: {42:D6}"); // Code: 000042

This produces the following output:

dotnet run
000042
00FF
3.14
Price: 1,235
Rate: 75.6%
Code: 000042

Common Mistake: Original String Longer Than totalWidth

PadLeft() and PadRight() return the original string unchanged — without truncating it — if it is already at or beyond totalWidth. This can break column alignment when values are longer than expected.

pad_sample.cs

using System;

// Original string longer than totalWidth is returned unchanged
string longStr = "ABCDEFGHIJ"; // 10 characters
Console.WriteLine(longStr.PadLeft(5)); // ABCDEFGHIJ (not truncated)
Console.WriteLine(longStr.PadRight(5)); // ABCDEFGHIJ (not truncated)

// To handle overflow, truncate with Substring() first
string truncated = longStr.Substring(0, Math.Min(5, longStr.Length));
Console.WriteLine(truncated); // ABCDE

This produces the following output:

dotnet run
ABCDEFGHIJ
ABCDEFGHIJ
ABCDE

In modern C#, string interpolation ($"{variable}") is often preferred over string.Format(). Format specifiers such as :P1 and :D6 can also be used with string interpolation in the form $"{value:format}".

To zero-pad a number with PadLeft(), you can also use a numeric format string such as number.ToString("D6").

For splitting and joining strings, see Split() / string.Join().