C# StringBuilder Capacity Test

You are interested in how StringBuilder in .NET allocates its capacity. When the StringBuilder runs out of space to Append characters, it reallocates and copies the data, leading to slowdowns.

This C# article reveals how StringBuilder manages its capacity.

Analysis

The graph shows that StringBuilder doubles its capacity, meaning it reallocates its buffers so it can store twice as many characters each time. The internal algorithm in StringBuilder resizes its buffer when you try to add the 17th character, and then again when you try to add the 33rd character.

Program that tests StringBuilder capacity [C#]

using System.IO;
using System.Text;

class Program
{
    static void Main()
    {
	using (var writer = new StreamWriter("data.txt"))
	{
	    StringBuilder builder = new StringBuilder();
	    for (int i = 0; i <= 256; i++)
	    {
		writer.Write(builder.Capacity);
		writer.Write(",");
		writer.Write(builder.Length);
		writer.WriteLine();
		builder.Append("1"); // <-- Add one character
	    }
	}
    }
}

Output

16, 0
16, 1
16, 2
16, 3
16, 4
16, 5
16, 6
16, 7
16, 8
16, 9
16,10
16,11
16,12
16,13
16,14
16,15
16,16
32,17
32,18
32,19
32,20
32,21
32,22
32,23
32,24
32,25
32,26
32,27
32,28
32,29
32,30
32,31
32,32
64,33

What the program does. This is a simple console program in C# that loops through 257 integers, 0 to 256 inclusive. It writes the Capacity and Length of the StringBuilder, and then appends one character. The output shows, in the first column, the Capacity; and in the second column, the buffer Length.

Tangent on the StringBuilder program here. To append a single character, it is more efficient to use a char instead of a single-character string. See my work with StringBuilder data types.

An easier way to find this out. As a busy .NET developer, you would want to open the StringBuilder Append method in the IL Disassembler. Then, you could examine the internal instructions the class uses to manage its buffer.

Summary

In this article, we saw a visualization and demonstration of how StringBuilder doubles its buffer size. In many programs, the StringBuilder will reallocate itself several times at the very smallest sizes, and then fewer times as it grows larger. This is fortunate, as reallocating a huge StringBuilder would be more costly.