StringBuilder Class

The StringBuilder class represents a mutable string of characters. It provides an efficient way to modify strings without creating new string instances.

Namespace: System.Text

Assembly: System.Runtime.dll

Constructors

StringBuilder()

StringBuilder(string)

StringBuilder(int capacity)

StringBuilder(string, int capacity)

Initializes a new instance of the StringBuilder class.

var sb = new StringBuilder();

Initializes with the specified string.

var sb = new StringBuilder("Hello");

Initializes with a specified capacity.

var sb = new StringBuilder(capacity: 256);

Initializes with a string and capacity.

var sb = new StringBuilder("Hello", capacity: 128);

Key Methods

Append(string) – Adds the specified string to the end of the current instance.
AppendLine() – Appends the default line terminator.
Insert(int, string) – Inserts a string at the specified index.
Remove(int, int) – Deletes a range of characters.
Replace(string, string) – Replaces all occurrences of a specified string.
ToString() – Returns a string representation of the current instance.

var sb = new StringBuilder();
// Build a multi‑line string
sb.AppendLine("First line")
  .AppendLine("Second line")
  .Append("End");
string result = sb.ToString();

Important Properties

Capacity – Gets or sets the number of characters that the current StringBuilder can hold without resizing.
Length – Gets or sets the number of characters currently in the StringBuilder.
MaxCapacity – Gets the maximum capacity of the current instance.
Chars[int] – Gets or sets the character at the specified index.

var sb = new StringBuilder("ABC");
sb[1] = 'X'; // becomes "AXC"
sb.Length = 2; // truncates to "AX"

Practical Example – Building CSV

using System;
using System.Text;
class Program {
    static void Main() {
        var sb = new StringBuilder();
        string[] headers = { "Id", "Name", "Email" };
        sb.AppendLine(string.Join(",", headers));
        for (int i = 1; i <= 5; i++) {
            sb.AppendLine(${i},{"User"+i},{"user"+i+"@example.com"}});
        }
        Console.WriteLine(sb.ToString());
    }
}