.NET Resource Management - Effective C# - page 97

last technique involves building the final value for immutable types. The
System.String class is immutable: After you construct a string, the con-
tents of that string cannot be modified. Whenever you write code that
appears to modify the contents of a string, you are actually creating a new
string object and leaving the old string object as garbage. This seemingly
innocent practice:
string msg = "Hello, " ;
msg += thisUser.Name;
msg += ". Today is " ;
msg += System. DateTime .Now.ToString();
is just as inefficient as if you had written this:
string msg = "Hello, " ;
// Not legal, for illustration only:
string tmp1 = new String (msg + thisUser.Name);
msg = tmp1; // "Hello " is garbage.
string tmp2 = new String (msg + ". Today is " );
msg = tmp2; // "Hello <user>" is garbage.
string tmp3 = new String (msg + DateTime .Now.ToString());
msg = tmp3; // "Hello <user>. Today is " is garbage.
The strings tmp1, tmp2, and tmp3, and the originally constructed msg
("Hello"), are all garbage. The += method on the string class creates a new
string object and returns that string. It does not modify the existing string
by concatenating the characters to the original storage. For simple con-
structs such as the previous one, you should use the string.Format()
method:
string msg = string .Format( "Hello, {0}. Today is {1}" ,
thisUser.Name, DateTime .Now.ToString());
For more complicated string operations, you can use the StringBuilder
class:
StringBuilder msg = new StringBuilder( "Hello, " );
msg.Append(thisUser.Name);
msg.Append( ". Today is " );
msg.Append( DateTime .Now.ToString());
string finalMsg = msg.ToString();
StringBuilder is the mutable string class used to build an immutable string
object. It provides facilities for mutable strings that let you create and