String 和 StringBuilder 的性能注意事项 - C#

Question

All,

For the string string s = "abcd" , does string w = s.SubString(2) return a new allocated String object ie string w = new String ("cd") internally or a String literal?

For StringBuilder, when appending string values and if the size of the StringBuilder needs to be increased, are all the contents copied over to a new memory location or simply the pointers to each of the earlier String value are reassigned to the new location?

Answer 1

String is immutable, so any operation that "changes" the string, will in effect return a new string . This includes SubString and all other operations on String , including those that does not change the length (such as ToLower() or similar).

StringBuilder contains internally a linked list of chunks of characters. When it needs to grow, a new chunk is allocated and inserted at the end of the list, and data is copied here. In other words, the whole StringBuilder buffer will not be copied on an append, only the data you are appending. I double-checked this against the Framework 4 reference sources.

Answer 2

For the string string s = "abcd", does string w = s.SubString(2) return a new allocated String object? Yes

For StringBuilder, when appending string values and if the size of the StringBuilder needs to be increased, are all the contents copied over to a new memory location? Yes

Any change in String small or large results in a new String

Answer 3

Difference between the String and StringBuilder is an important concept which makes the difference when an application has to deal with the editing of a high number of Strings.

String

The String object is a collection of UTF-16 code units represented by a System.Char object which belong to the System namespace. Since the value of this objects are read-only, the entire object String has defined as immutable. The maximum size of a String object in memory is 2 GB, or about 1 billion characters.

Immutable Being immutable means that every time a methods of the System.String is used, a new sting object is created in memory and this cause a new allocation of space for the new object.

Example: By using the string concatenation operator += appears that the value of the string variable named test change. In fact, it create a new String object, which has a different value and address from the original and assign it to the test variable.

string test;
test += "red"; // a new object string is created
test += "coding"; // a new object string is created
test += "planet"; // a new object string is created

StringBuilder

The StringBuilder is a dynamic object which belong to the System.Text namespace and allow to modify the number of characters in the string that it encapsulates, this characteristic is called mutability.

Mutability To be able to append, remove, replace or insert characters, A StringBuilder maintains a buffer to accommodate expansions to the string. If new data is appended to the buffer if room is available; otherwise, a new, larger buffer is allocated, data from the original buffer is copied to the new buffer, and the new data is then appended to the new buffer.

StringBuilder sb = new StringBuilder("");
sb.Append("red");
sb.Append("blue");
sb.Append("green ");
string colors = sb.ToString();

Performances

In order to help you better understand the performance difference between String and StringBuilder, I created the following example:

Stopwatch timer = new Stopwatch();
string str = string.Empty;
timer.Start();
for (int i = 0; i < 10000; i++) {
    str += i.ToString();
}
timer.Stop();
Console.WriteLine("String : {0}", timer.Elapsed);

timer.Restart();

StringBuilder sbr = new StringBuilder(string.Empty);
for (int i = 0; i < 10000; i++) {
    sbr.Append(i.ToString());
}
timer.Stop();
Console.WriteLine("StringBuilder : {0}", timer.Elapsed);

The output is

Output
String : 00:00:00.0706661
StringBuilder : 00:00:00.0012373

Answer 4

Strings are immutable objects so every time you had to make changes you create a new instance of that string. The substring method does not change the value of the original string.

Regards.

Answer 5

If you are going to make large numbers of edits to a string it better to do this via StringBuilder.

From MSDN :

You can use the StringBuilder class instead of the String class for operations that make multiple changes to the value of a string. Unlike instances of the String class, StringBuilder objects are mutable; when you concatenate, append, or delete substrings from a string, the operations are performed on a single string.