C# StringBuilder

A StringBuilder is a text buffer that builds a string piece by piece. Append adds text to the end of the buffer.

Append Builds a Line

Three Append calls build one line piece by piece. ToString prints the finished line.

using System;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        StringBuilder sb = new StringBuilder();
        sb.Append("Report");
        sb.Append(" for ");
        sb.Append("Monday");
        Console.WriteLine(sb.ToString());
    }
}

Insert Adds the Comma

The pieces first spell Hello World with a space. Insert places a comma at index 5.

using System;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        StringBuilder sb = new StringBuilder();
        sb.Append("Hello");
        sb.Append(" ");
        sb.Append("World");
        sb.Insert(5, ",");
        Console.WriteLine(sb.ToString());
    }
}

The Buffer Grows in Place

Length reports the character count in the buffer. The count grows after the second Append call.

using System;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        StringBuilder sb = new StringBuilder();
        sb.Append("Hello");
        Console.WriteLine(sb.Length);
        sb.Append(" World");
        Console.WriteLine(sb.ToString());
        Console.WriteLine(sb.Length);
    }
}

Example

using System;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        StringBuilder sb = new StringBuilder();
        sb.Append("Row ");
        sb.Append(7);
        sb.Append(" Seat ");
        sb.Append(12);
        Console.WriteLine(sb.ToString());
    }
}

Key points

The System.Text namespace holds the StringBuilder type.
A string never changes after it is created.
A StringBuilder edits its buffer in place.
Append adds text to the end of the buffer.
Insert places text at a given index in the buffer.
ToString reads the finished text out of the buffer.

In this example, the StringBuilder starts empty. Append adds the word Row and a space, then the number 7. Two more Append calls add the seat label and the number 12. ToString prints the finished line, Row 7 Seat 12.

Note

A StringBuilder suits code that builds long text in many small steps.

Try it Yourself

Edit the code, then press Run.

Running is simulated

using System;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        StringBuilder sb = new StringBuilder();
        sb.Append("Row ");
        sb.Append(7);
        sb.Append(" Seat ");
        sb.Append(12);
        Console.WriteLine(sb.ToString());
    }
}

Exercise

Which StringBuilder method adds text to the end of the buffer?

Insert ToString Append Length