16.10 Class StringBuilder

The string class provides many capabilities for processing strings. However a string’s contents can never change. Operations that seem to concatenate strings are in fact creating new strings—the += operator creates a new string and assigns its reference to the variable on the left of the += operator.

The next several sections discuss the features of class StringBuilder (namespace System.Text), used to create and manipulate dynamic string information—i.e., mutable strings. Every StringBuilder can store a certain number of characters that’s specified by its capacity. Exceeding the capacity of a StringBuilder causes the capacity to expand to accommodate the additional characters. As we’ll see, members of class StringBuilder, such as methods Append and AppendFormat, can be used for concatenation like the operators + and += for class string—without creating any new string objects. StringBuilder is particularly useful for manipulating in place a large number of strings, as it’s much more efficient than creating individual immutable strings.

Performance Tip 16.2

Objects of class string are immutable (i.e., constant strings), whereas objects of class StringBuilder are mutable. C# can perform certain optimizations involving strings (such as the sharing of one string among multiple references), because it knows these objects will not change.

StringBuilder Constructors

Class StringBuilder provides six overloaded constructors. Class StringBuilderConstructor (Fig. 16.9) demonstrates three of these overloaded constructors. Line 10 employs the no-parameter StringBuilder constructor to create a StringBuilder that contains no characters and has an implementation-specific default initial capacity. Line 11 uses the StringBuilder constructor that takes an int argument to create a StringBuilder that contains no characters and has the initial capacity specified in the int argument (i.e., 10). Line 12 uses the StringBuilder constructor that takes a string argument to create a StringBuilder containing the characters of the string argument—the initial capacity might differ from the string’s size. Lines 14–16 implicitly use StringBuilder method ToString to obtain string representations of the StringBuilders’ contents.

Fig. 16.9 Demonstrating StringBuilder class constructors.

Alternate View 

 1    // Fig. 16.9: StringBuilderConstructor.cs
 2    // Demonstrating StringBuilder class constructors.
 3    using System;
 4    using System.Text;
 5
 6    class StringBuilderConstructor
 7    {
 8       static void Main()
 9       {
10          var buffer1 = new StringBuilder();       
11          var buffer2 = new StringBuilder(10);     
12          var buffer3 = new StringBuilder("hello");
13
14          Console.WriteLine($"buffer1 = "{buffer1}"");
15          Console.WriteLine($"buffer2 = "{buffer2}"");
16          Console.WriteLine($"buffer3 = "{buffer3}"");
17       }
18    }


buffer1 = ""
buffer2 = ""
buffer3 = "hello"
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