C# StringBuilder Capacity

This C# article reveals how StringBuilder manages its capacity.

StringBuilder capacity.

StringBuilder in .NET allocates its capacity. When it runs out of space to Append characters, it reallocates and copies the data, leading to slowdowns. It uses a clever algorithm for this.


The graph shows that StringBuilder doubles its capacity. It reallocates its buffers so it can store twice as many characters each time. The algorithm resizes the buffer when you add the 17th character and the 33rd character.StringBuilder Append
C# program that tests StringBuilder capacity 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.

Note: The output shows in the first column the Capacity. And in the second column, we see the buffer Length.

Tip: To append a single character, it is more efficient to use a char instead of a single-character string.

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.


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 smallest sizes, and then fewer times as it grows larger.

Note: This is fortunate, as reallocating a huge StringBuilder would be more costly.

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