Книга: C# 2008 Programmer

The StringBuilder Class

The StringBuilder Class

Earlier in this chapter you saw how to easily concatenate two strings by using the + operator. That's fine if you are concatenating a small number of strings, but it is not recommended for large numbers of strings. The reason is that String objects in .NET are immutable, which means that once a string variable is initialized, its value cannot be changed. When you concatenate another string to an existing one, you actually discard its old value and create a new string object containing the result of the concatenation. When you repeat this process several times, you incur a performance penalty as new temporary objects are created and old objects discarded.

Consider the following example, where you concatenate all the numbers from 0 to 9999:

int counter = 9999;
string s = string.Empty;
for (int i = 0; i <= counter; i++) {
 s += i.ToString();
}
Console.WriteLine(s);

At first glance, the code looks innocent enough. But let's use the Stopwatch object to time the operation. Modify the code as shown here:

int counter = 9999;
System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
sw.Start();
string s = string.Empty;
for (int i = 0; i <= counter; i++) {
 s += i.ToString();
}
sw.Stop();
Console.WriteLine("Took {0} ms", sw.ElapsedMilliseconds);
Console.WriteLine(s);

On average, it took about 374 ms on my computer to run this operation. Let's now use the StringBuilder class in .NET to perform the string concatenation, using its Append() method:

System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
sw.Start();
StringBuilder sb = new StringBuilder();
for (int i = 0; i <= 9999; i++) {
 sb.Append(i.ToString());
}
sw.Stop();
Console.WriteLine("Took {0} ms", sw.ElapsedMilliseconds);
Console.WriteLine(sb.ToString());

On average, it took about 6 ms on my computer to perform this operation. As you can deduce, the improvement is drastic — 98% ((374-6)/374). If you increase the value of the loop variant (counter), you will find that the improvement is even more dramatic.

The StringBuilder class represents a mutable string of characters. Its behavior is like the String object except that its value can be modified once it has been created.

The StringBuilder class contains some other important methods, which are described in the following table.

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