Bartłomiej Sobieszek Bartłomiej Sobieszek - 3 months ago 13
C# Question

How to use StringBuilder wisely

I am little confused about using

StringBuilder
class, first:


A
String
object concatenation operation always creates a new object from the existing
string
and the new data. A
StringBuilder
object maintains a buffer to accommodate the concatenation of new data. New data is appended to the end of the buffer if room is available; otherwise, a new, larger buffer is allocated, data from the original buffer is copied to the new buffer, then the new data is appended to the new buffer.


But where is the point of creating
StringBuilder
instance to avoid creating new one of
String
? It sounds like trading "one for one".

static void Main(string[] args)
{
String foo = "123";
using (StringBuilder sb = new StringBuilder(foo)) // also sb isn't disposable, so there will be error
{
sb.Append("456");
foo = sb.ToString();
}

Console.WriteLine(foo);
Console.ReadKey();
}


Why I shouldn't just use

+=


Edit: Ok, I know now how to reuse one instance of
StringBuilder
(still don't know if this is right with code standards), but this isn't worth to use with just one
string
, isn't it?

Answer

Modifying immutable structures like strings must be done by copying the structure and by that consuming more memory and slowing the application's run time (plus GC time etc...).

StringBuilder comes to solve this problem by using the same mutable object for manipulations.

However:

when concatenating a string in compile time as the following:

string myString = "123";
myString += "234";
myString += "345";

it will actually compile to something like that:

string myString = string.Concat("123", "234", "345");

this function is faster than working with StringBuilder for the number of strings entering the function is known.

so for compile-time-known string concatenations you should prefer string.Concat().

as for unknown number of string like in the following case:

string myString = "123";
if (Console.ReadLine() == "a")
{
    myString += "234";
}
myString += "345";

Now the compiler can't use the string.Concat() function, however, StringBuilder appears to be more efficient in time and memory consumption only when the concatenation is done with 6-7 or more strings.

Bad practice usage:

StringBuilder myString = new StringBuilder("123");
myString.Append("234");
myString.Append("345");

Fine practice usage:

StringBuilder myString = new StringBuilder("123");
if (Console.ReadLine() == "a")
{
    myString.Append("234");
}
myString.Append("345");

Best practice usage:

StringBuilder myString = new StringBuilder("123");
while (Console.ReadLine() == "a")
{
    myString.Append("234"); //Average loop times 4~ or more
}
myString.Append("345");