StringBuilder: AppendFormat and Capacity

Question

In this code I try to appendFormat a message with a length bigger than the string builder's capacity:

    static void Main(string[] args)
    {
        StringBuilder sb = new StringBuilder(10);
        sb.AppendFormat("1234567890123"); // 13 characters
        Console.WriteLine(sb.Capacity);
    }

Do you know what should be the output (answer at the bottom)?

Okay, let's try to change this code and init StringBuilder with capacity, still less than the string length, for example 12:

    static void Main(string[] args)
    {
        StringBuilder sb = new StringBuilder(12);
        sb.AppendFormat("1234567890123"); // 13 characters
        Console.WriteLine(sb.Capacity);
    }

So, my question is: does AppendFormat really double the start capacity of StringBuilder if string couldn't be appened? If the appended string's length should be 24 characters, then the Capacity will become 48?

Output code: 20 & 24

possible duplicate of [How does the StringBuilder decide how large its capacity should be?](http://stackoverflow.com/questions/4495855/how-does-the-stringbuilder-decide-how-large-its-capacity-should-be) — Ondrej Tucny, Aug 19 '13 at 21:13
If you want to prevent the capactiy from expanding, you want to set StringBuilder.MaxCapacity. — User, Aug 19 '13 at 21:16
@Peter, yeah, i know it. Just wanted to understand, how does capacity changing in AppendFormat method — Maxim Zhukov, Aug 19 '13 at 21:17

score 3 · Accepted Answer · answered Aug 19 '13 at 21:13

3

does AppendFormat really double the start capacity of StringBuilder if the string couldn't be appended?

Yes -- see here.

Whenever the append operation causes the length of the StringBuilder object to exceed its capacity, its existing capacity is doubled and the Append operation succeeds.

answered Aug 19 '13 at 21:13

McGarnagle

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Yeah, `capacity = capacity << 1; // This means "capacity *= 2;"` that's the answer – Maxim Zhukov Aug 19 '13 at 21:14

StringBuilder: AppendFormat and Capacity

1 Answers1