为什么StringBuilder比字符串连接慢？

Question

Why is StringBuilder slower when compared to + concatenation? StringBuilder was meant to avoid extra object creation, but why does it penalize performance?

    static void Main(string[] args)
    {
        int max = 1000000;
        for (int times = 0; times < 5; times++)
        {
            Console.WriteLine("\ntime: {0}", (times+1).ToString());
            Stopwatch sw = Stopwatch.StartNew();
            for (int i = 0; i < max; i++)
            {
                string msg = "Your total is ";
                msg += "$500 ";
                msg += DateTime.Now;
            }
            sw.Stop();
            Console.WriteLine("String +\t: {0}ms", ((int)sw.ElapsedMilliseconds).ToString().PadLeft(6));

            sw = Stopwatch.StartNew();
            for (int j = 0; j < max; j++)
            {
                StringBuilder msg = new StringBuilder();
                msg.Append("Your total is ");
                msg.Append("$500 ");
                msg.Append(DateTime.Now);
            }
            sw.Stop();
            Console.WriteLine("StringBuilder\t: {0}ms", ((int)sw.ElapsedMilliseconds).ToString().PadLeft(6));
        }
        Console.Read();
    }

EDIT: Moving out of scope variables as suggested:

Answer 1

Change so that the StringBuilder isn't instantiated all the time, instead .Clear() it:

time: 1
String +    :   3348ms
StringBuilder   :   3151ms

time: 2
String +    :   3346ms
StringBuilder   :   3050ms

etc.

Note that this still tests exactly the same functionality, but tries to reuse resources a bit smarter.

Code: (also live on http://ideone.com/YuaqY )

using System;
using System.Text;
using System.Diagnostics;

public class Program
{
    static void Main(string[] args)
    {
        int max = 1000000;
        for (int times = 0; times < 5; times++)
        {
            {
                Console.WriteLine("\ntime: {0}", (times+1).ToString());
                Stopwatch sw = Stopwatch.StartNew();
                for (int i = 0; i < max; i++)
                {
                    string msg = "Your total is ";
                    msg += "$500 ";
                    msg += DateTime.Now;
                }
                sw.Stop();
                Console.WriteLine("String +\t: {0}ms", ((int)sw.ElapsedMilliseconds).ToString().PadLeft(6));
            }

            {
                Stopwatch sw = Stopwatch.StartNew();
                StringBuilder msg = new StringBuilder();
                for (int j = 0; j < max; j++)
                {
                    msg.Clear();
                    msg.Append("Your total is ");
                    msg.Append("$500 ");
                    msg.Append(DateTime.Now);
                }
                sw.Stop();
                Console.WriteLine("StringBuilder\t: {0}ms", ((int)sw.ElapsedMilliseconds).ToString().PadLeft(6));
            }
        }
        Console.Read();
    }
}

Answer 2

You are creating a new instance of StringBuilder with every iteration, and that incurs some overhead. Since you are not using it for what it's actually meant to do (ie: build large strings which would otherwise require many string concatenation operations), it's not surprising to see worse performance than concatenation.

A more common comparison / usage of StringBuilder is something like:

string msg = "";
for (int i = 0; i < max; i++)
{
    msg += "Your total is ";
    msg += "$500 ";
    msg += DateTime.Now;
}

StringBuilder msg_sb = new StringBuilder();
for (int j = 0; j < max; j++)
{
    msg_sb.Append("Your total is ");
    msg_sb.Append("$500 ");
    msg_sb.Append(DateTime.Now);
}

With this, you'll observe a significant performance difference between StringBuilder and concatenation. And by "significant" I mean orders of magnitude , not the ~ 10% difference you are observing in your examples.

Since StringBuilder doesn't have to build tons of intermediary strings that will just get thrown away, you get much better performance. That's what it's meant for. For smaller strings, you are better off using string concatenation for simplicity and clarity.

Answer 3

The benefits of StringBuilder should be noticeable with longer strings.

Every time you concatenate a string you create a new string object, so the longer the string, the more is needed to copy from the old string to the new string.

Also, creating many temporary objects may have an adverse effect on performance that is not measurable by a StopWatch, because it "pollutes" the managed heap with temporary objects and may cause more garbage collection cycles.

Modify your test to create (much) longer strings and use (many) more concatenations / append operations and the StringBuilder should perform better.

Answer 4

In addition to not using StringBuilder as in the most efficient manner, you're also not using string concatenation as efficiently as possible. If you know how many strings you're concatenating ahead of time, then doing it all on one line should be fastest. The compiler optimizes the operation so that no intermediate strings are generated.

I added a couple more test cases. One is basically the same as what sehe suggested, and the other generates the string in one line:

sw = Stopwatch.StartNew();
builder = new StringBuilder();
for (int j = 0; j < max; j++)
{
    builder.Clear();
    builder.Append("Your total is ");
    builder.Append("$500 ");
    builder.Append(DateTime.Now);
}
sw.Stop();
Console.WriteLine("StringBuilder (clearing)\t: {0}ms", ((int)sw.ElapsedMilliseconds).ToString().PadLeft(6));

sw = Stopwatch.StartNew();
for (int i = 0; i < max; i++)
{
    msg = "Your total is " + "$500" + DateTime.Now;
}
sw.Stop();
Console.WriteLine("String + (one line)\t: {0}ms", ((int)sw.ElapsedMilliseconds).ToString().PadLeft(6));

And here is an example of the output I see on my machine:

time: 1
String +    :   3707ms
StringBuilder   :   3910ms
StringBuilder (clearing)    :   3683ms
String + (one line) :   3645ms

time: 2
String +    :   3703ms
StringBuilder   :   3926ms
StringBuilder (clearing)    :   3666ms
String + (one line) :   3625ms

In general: - StringBuilder does better if you're building a large string in a lot of steps, or you don't know how many strings will be concatenated together.
- Mashing them all together in a single expression is better whenever it's a reasonable option option.

Answer 5

Note that

string msg = "Your total is ";
msg += "$500 ";
msg += DateTime.Now;

compiles down to

string msg = String.Concat("Your total is ", "$500 ");
msg = String.Concat(msg, DateTime.Now.ToString());

This totals two concats and one ToString per iteration. Also, a single String.Concat is really fast, because it knows how large the resulting string will be, so it only allocates the resulting string once, and then quickly copies the source strings into it. This means that in practice

String.Concat(x, y);

will always outperform

StringBuilder builder = new StringBuilder();
builder.Append(x);
builder.Append(y);

because StringBuilder cannot take such shortcuts (you could call a thirs Append, or a Remove, that's not possible with String.Concat).

The way a StringBuilder works is by allocating an initial buffer and set the string length to 0. With each Append, it has to check the buffer, possibly allocate more buffer space (usually copying the old buffer to the new buffer), copy the string and increment the string length of the builder. String.Concat does not need to do all this extra work.

So for simple string concatenations, x + y (ie, String.Concat) will always outperform StringBuilder.

Now, you'll start to get benefits from StringBuilder once you start concatenating lots of strings into a single buffer, or you're doing lots of manipulations on the buffer, where you'd need to keep creating new strings when not using a StringBuilder. This is because StringBuilder only occasionally allocates new memory, in chunks, but String.Concat, String.SubString, etc. (nearly) always allocate new memory. (Something like "".SubString(0,0) or String.Concat("", "") won't allocate memory, but those are degenerate cases.)

Answer 6

I think its better to compare effeciancy between String and StringBuilder rather then time.

what msdn says: A String is called immutable because its value cannot be modified once it has been created. Methods that appear to modify a String actually return a new String containing the modification. If it is necessary to modify the actual contents of a string-like object, use the System.Text.StringBuilder class.

string msg = "Your total is "; // a new string object
msg += "$500 "; // a new string object
msg += DateTime.Now; // a new string object

see which one is better.

Answer 7

Here is an example that demonstrates a situation in which StringBuilder will execute more quickly than string concatenation:

static void Main(string[] args)
{
    const int sLen = 30, Loops = 10000;
    DateTime sTime, eTime;
    int i;
    string sSource = new String('X', sLen);
    string sDest = "";
    // 
    // Time StringBuilder.
    // 
    for (int times = 0; times < 5; times++)
    {
        sTime = DateTime.Now;
        System.Text.StringBuilder sb = new System.Text.StringBuilder((int)(sLen * Loops * 1.1));
        Console.WriteLine("Result # " + (times + 1).ToString());
        for (i = 0; i < Loops; i++)
        {
            sb.Append(sSource);
        }
        sDest = sb.ToString();
        eTime = DateTime.Now;
        Console.WriteLine("String Builder took :" + (eTime - sTime).TotalSeconds + " seconds.");
        // 
        // Time string concatenation.
        // 
        sTime = DateTime.Now;
        for (i = 0; i < Loops; i++)
        {
            sDest += sSource;
            //Console.WriteLine(i);
        }
        eTime = DateTime.Now;
        Console.WriteLine("Concatenation took : " + (eTime - sTime).TotalSeconds + " seconds.");
        Console.WriteLine("\n");
    }
    // 
    // Make the console window stay open
    // so that you can see the results when running from the IDE.
    // 
}

Result # 1 String Builder took :0 seconds. Concatenation took : 8.7659616 seconds.

Result # 2 String Builder took :0 seconds. Concatenation took : 8.7659616 seconds.

Result # 3 String Builder took :0 seconds. Concatenation took : 8.9378432 seconds.

Result # 4 String Builder took :0 seconds. Concatenation took : 8.7972128 seconds.

Result # 5 String Builder took :0 seconds. Concatenation took : 8.8753408 seconds.

StringBulder is much faster than + concatenation..