String output: format or concat in C#?

Question

Let's say that you want to output or concat strings. Which of the following styles do you prefer?

• var p = new { FirstName = "Bill", LastName = "Gates" };

• Console.WriteLine("{0} {1}", p.FirstName, p.LastName);

• Console.WriteLine(p.FirstName + " " + p.LastName);

Do you rather use format or do you simply concat strings? What is your favorite? Is one of these hurting your eyes?

Do you have any rational arguments to use one and not the other?

I'd go for the second one.

Answer 1

Try this code.

It's a slightly modified version of your code.
1. I removed Console.WriteLine as it's probably few orders of magnitude slower than what I'm trying to measure.
2. I'm starting the Stopwatch before the loop and stopping it right after, this way I'm not losing precision if the function takes for example 26.4 ticks to execute.
3. The way you divided the result by some iterations was wrong. See what happens if you have 1000 milliseconds and 100 milliseconds. In both situations, you will get 0 ms after dividing it by 1000000.

Stopwatch s = new Stopwatch();

var p = new { FirstName = "Bill", LastName = "Gates" };

int n = 1000000;
long fElapsedMilliseconds = 0, fElapsedTicks = 0, cElapsedMilliseconds = 0, cElapsedTicks = 0;

string result;
s.Start();
for (var i = 0; i < n; i++)
    result = (p.FirstName + " " + p.LastName);
s.Stop();
cElapsedMilliseconds = s.ElapsedMilliseconds;
cElapsedTicks = s.ElapsedTicks;
s.Reset();
s.Start();
for (var i = 0; i < n; i++)
    result = string.Format("{0} {1}", p.FirstName, p.LastName);
s.Stop();
fElapsedMilliseconds = s.ElapsedMilliseconds;
fElapsedTicks = s.ElapsedTicks;
s.Reset();


Console.Clear();
Console.WriteLine(n.ToString()+" x result = string.Format(\"{0} {1}\", p.FirstName, p.LastName); took: " + (fElapsedMilliseconds) + "ms - " + (fElapsedTicks) + " ticks");
Console.WriteLine(n.ToString() + " x result = (p.FirstName + \" \" + p.LastName); took: " + (cElapsedMilliseconds) + "ms - " + (cElapsedTicks) + " ticks");
Thread.Sleep(4000);

Those are my results:

1000000 x result = string.Format("{0} {1}", p.FirstName, p.LastName); took: 618ms - 2213706 ticks
1000000 x result = (p.FirstName + " " + p.LastName); took: 166ms - 595610 ticks

Answer 2

I'm amazed that so many people immediately want to find the code that executes the fastest. If ONE MILLION iterations STILL take less than a second to process, is this going to be in ANY WAY noticeable to the end user? Not very likely.

Premature optimization = FAIL.

I'd go with the String.Format option, only because it makes the most sense from an architectural standpoint. I don't care about the performance until it becomes an issue (and if it did, I'd ask myself: Do I need to concatenate a million names at once? Surely they won't all fit on the screen...)

Consider if your customer later wants to change it so that they can configure whether to display "Firstname Lastname" or "Lastname, Firstname." With the Format option, this is easy - just swap out the format string. With the concat, you'll need extra code. Sure that doesn't sound like a big deal in this particular example but extrapolate.

Answer 3

Oh dear - after reading one of the other replies I tried reversing the order of the operations - so performing the concatenation first, then the String.Format...

Bill Gates
Console.WriteLine(p.FirstName + " " + p.LastName); took: 8ms - 30488 ticks
Bill Gates
Console.WriteLine("{0} {1}", p.FirstName, p.LastName); took: 0ms - 182 ticks

So the order of the operations makes a HUGE difference, or rather the very first operation is ALWAYS much slower.

Here is the results of a run where operations are completed more than once. I have tried changing the orders but things generally follow the same rules, once the first result is ignored:

Bill Gates
Console.WriteLine(FirstName + " " + LastName); took: 5ms - 20335 ticks
Bill Gates
Console.WriteLine(FirstName + " " + LastName); took: 0ms - 156 ticks
Bill Gates
Console.WriteLine(FirstName + " " + LastName); took: 0ms - 122 ticks
Bill Gates
Console.WriteLine("{0} {1}", FirstName, LastName); took: 0ms - 181 ticks
Bill Gates
Console.WriteLine("{0} {1}", FirstName, LastName); took: 0ms - 122 ticks
Bill Gates
String.Concat(FirstName, " ", LastName); took: 0ms - 142 ticks
Bill Gates
String.Concat(FirstName, " ", LastName); took: 0ms - 117 ticks

As you can see subsequent runs of the same method (I refactored the code into 3 methods) are incrementally faster. The fastest appears to be the Console.WriteLine(String.Concat(...)) method, followed by normal concatenation, and then the formatted operations.

The initial delay in startup is likely the initialisation of Console Stream, as placing a Console.Writeline("Start!") before the first operation brings all times back into line.

Answer 4

Strings are immutable, this means the same tiny piece of memory is used over and over in your code. Adding the same two strings together and creating the same new string over and over again doesn't impact memory. .Net is smart enough just to use the same memory reference. Therefore your code doesn't truly test the difference between the two concat methods.

Try this on for size:

Stopwatch s = new Stopwatch();

int n = 1000000;
long fElapsedMilliseconds = 0, fElapsedTicks = 0, cElapsedMilliseconds = 0, cElapsedTicks = 0, sbElapsedMilliseconds = 0, sbElapsedTicks = 0;

Random random = new Random(DateTime.Now.Millisecond);

string result;
s.Start();
for (var i = 0; i < n; i++)
    result = (random.Next().ToString() + " " + random.Next().ToString());
s.Stop();
cElapsedMilliseconds = s.ElapsedMilliseconds;
cElapsedTicks = s.ElapsedTicks;
s.Reset();

s.Start();
for (var i = 0; i < n; i++)
    result = string.Format("{0} {1}", random.Next().ToString(), random.Next().ToString());
s.Stop();
fElapsedMilliseconds = s.ElapsedMilliseconds;
fElapsedTicks = s.ElapsedTicks;
s.Reset();

StringBuilder sb = new StringBuilder();
s.Start();
for(var i = 0; i < n; i++){
    sb.Clear();
    sb.Append(random.Next().ToString());
    sb.Append(" ");
    sb.Append(random.Next().ToString());
    result = sb.ToString();
}
s.Stop();
sbElapsedMilliseconds = s.ElapsedMilliseconds;
sbElapsedTicks = s.ElapsedTicks;
s.Reset();

Console.WriteLine(n.ToString() + " x result = string.Format(\"{0} {1}\", p.FirstName, p.LastName); took: " + (fElapsedMilliseconds) + "ms - " + (fElapsedTicks) + " ticks");
Console.WriteLine(n.ToString() + " x result = (p.FirstName + \" \" + p.LastName); took: " + (cElapsedMilliseconds) + "ms - " + (cElapsedTicks) + " ticks");
Console.WriteLine(n.ToString() + " x sb.Clear();sb.Append(random.Next().ToString()); sb.Append(\" \"); sb.Append(random.Next().ToString()); result = sb.ToString(); took: " + (sbElapsedMilliseconds) + "ms - " + (sbElapsedTicks) + " ticks");
Console.WriteLine("****************");
Console.WriteLine("Press Enter to Quit");
Console.ReadLine();

Sample Output:

1000000 x result = string.Format("{0} {1}", p.FirstName, p.LastName); took: 513ms - 1499816 ticks
1000000 x result = (p.FirstName + " " + p.LastName); took: 393ms - 1150148 ticks
1000000 x sb.Clear();sb.Append(random.Next().ToString()); sb.Append(" "); sb.Append(random.Next().ToString()); result = sb.ToString(); took: 405ms - 1185816 ticks

Answer 5

Pity the poor translators

If you know your application will stay in English, then fine, save the clock ticks. However, many cultures would usually see Lastname Firstname in, for instance, addresses.

So use string.Format(), especially if you're going to ever have your application go anywhere that English is not the first language.

Answer 6

Here are my results over 100,000 iterations:

Console.WriteLine("{0} {1}", p.FirstName, p.LastName); took (avg): 0ms - 689 ticks
Console.WriteLine(p.FirstName + " " + p.LastName); took (avg): 0ms - 683 ticks

And here is the bench code:

Stopwatch s = new Stopwatch();

var p = new { FirstName = "Bill", LastName = "Gates" };

//First print to remove the initial cost
Console.WriteLine(p.FirstName + " " + p.LastName);
Console.WriteLine("{0} {1}", p.FirstName, p.LastName);

int n = 100000;
long fElapsedMilliseconds = 0, fElapsedTicks = 0, cElapsedMilliseconds = 0, cElapsedTicks = 0;

for (var i = 0; i < n; i++)
{
    s.Start();
    Console.WriteLine(p.FirstName + " " + p.LastName);
    s.Stop();
    cElapsedMilliseconds += s.ElapsedMilliseconds;
    cElapsedTicks += s.ElapsedTicks;
    s.Reset();
    s.Start();
    Console.WriteLine("{0} {1}", p.FirstName, p.LastName);
    s.Stop();
    fElapsedMilliseconds += s.ElapsedMilliseconds;
    fElapsedTicks += s.ElapsedTicks;
    s.Reset();
}

Console.Clear();

Console.WriteLine("Console.WriteLine(\"{0} {1}\", p.FirstName, p.LastName); took (avg): " + (fElapsedMilliseconds / n) + "ms - " + (fElapsedTicks / n) + " ticks");
Console.WriteLine("Console.WriteLine(p.FirstName + \" \" + p.LastName); took (avg): " + (cElapsedMilliseconds / n) + "ms - " + (cElapsedTicks / n) + " ticks");

So, I don't know whose reply to mark as an answer :)

Answer 7

Concatenating strings is fine in a simple scenario like that - it is more complicated with anything more complicated than that, even LastName, FirstName. With the format you can see, at a glance, what the final structure of the string will be when reading the code, with concatenation it becomes almost impossible to immediately discern the final result (except with a very simple example like this one).

What that means in the long run is that when you come back to make a change to your string format, you will either have the ability to pop in and make a few adjustments to the format string, or wrinkle your brow and start moving around all kinds of property accessors mixed with text, which is more likely to introduce problems.

If you're using .NET 3.5 you can use an extension method like this one and get an easy flowing, off the cuff syntax like this:

string str = "{0} {1} is my friend. {3}, {2} is my boss.".FormatWith(prop1,prop2,prop3,prop4);

Finally, as your application grows in complexity you may decide that to sanely maintain strings in your application you want to move them into a resource file to localize or simply into a static helper. This will be MUCH easier to achieve if you have consistently used formats, and your code can be quite simply refactored to use something like

string name = String.Format(ApplicationStrings.General.InformalUserNameFormat,this.FirstName,this.LastName);

Answer 8

For very simple manipulation I'd use concatenation, but once you get beyond 2 or 3 elements Format becomes more appropriate IMO.

Another reason to prefer String.Format is that .NET strings are immutable and doing it this way creates fewer temporary/intermediate copies.

Answer 9

While I totally understand the style preference and picked concatenation for my first answer partly based on my own preference, part of my decision was based on the thought that concatenation would be faster. So, out of curiosity, I tested it and the results were staggering, especially for such a small string.

Using the following code:

    System.Diagnostics.Stopwatch s = new System.Diagnostics.Stopwatch();

    var p = new { FirstName = "Bill", LastName = "Gates" };

    s.Start();
    Console.WriteLine("{0} {1}", p.FirstName, p.LastName);
    s.Stop();
    Console.WriteLine("Console.WriteLine(\"{0} {1}\", p.FirstName, p.LastName); took: " + s.ElapsedMilliseconds + "ms - " + s.ElapsedTicks + " ticks");

    s.Reset();
    s.Start();
    Console.WriteLine(p.FirstName + " " + p.LastName);
    s.Stop();

    Console.WriteLine("Console.WriteLine(p.FirstName + \" \" + p.LastName); took: " + s.ElapsedMilliseconds + "ms - " + s.ElapsedTicks + " ticks");

I got the following results:

Bill Gates
Console.WriteLine("{0} {1}", p.FirstName, p.LastName); took: 2ms - 7280 ticks
Bill Gates
Console.WriteLine(p.FirstName + " " + p.LastName); took: 0ms - 67 ticks

Using the formatting method is over 100 times slower!! Concatenation didn't even register as 1ms, which is why I output the timer ticks as well.

Answer 10

For basic string concatenation, I generally use the second style - easier to read and simpler. However, if I am doing a more complicated string combination I usually opt for String.Format.

String.Format saves on lots of quotes and pluses...

Console.WriteLine("User {0} accessed {1} on {2}.", user.Name, fileName, timestamp);
vs
Console.WriteLine("User " + user.Name + " accessed " + fileName + " on " + timestamp + ".");

Only a few charicters saved, but I think, in this example, format makes it much cleaner.

Answer 11

A better test would be to watch your memory using Perfmon and the CLR memory counters. My understanding is that the whole reason you want to use String.Format instead of just concatenating strings is, since strings are immutable, you are unnecessarily burdening the garbage collector with temporary strings that need to be reclaimed in the next pass.

StringBuilder and String.Format, although potentially slower, are more memory efficient.

What is so bad about string concatenation?

Answer 12

Generally I prefer the former, as especially when the strings get long it can be much easier to read.

The other benefit is I believe one of performance, as the latter actually performs 2 string creation statements before passing the final string to the Console.Write method. String.Format uses a StringBuilder under the covers I believe, so multiple concatenations are avoided.

It should be noted however that if the parameters you are passing in to String.Format (and other such methods like Console.Write) are value types then they will be boxed before passed in, which can provide its own performance hits. Blog post on this here.

Answer 13

A week from now Aug 19, 2015, this question will be exactly seven (7) years old. There is now a better way of doing this. Better in terms of maintainability as I haven't done any performance test compared to just concatenating strings (but does it matter these days? a few milliseconds in difference?). The new way of doing it with C# 6.0:

var p = new { FirstName = "Bill", LastName = "Gates" };
var fullname = $"{p.FirstName} {p.LastName}";

This new feature is better, IMO, and actually better in our case as we have codes where we build querystrings whose values depends on some factors. Imagine one querystring where we have 6 arguments. So instead of doing a, for example:

var qs = string.Format("q1={0}&q2={1}&q3={2}&q4={3}&q5={4}&q6={5}", 
    someVar, anotherVarWithLongName, var3, var4, var5, var6)

in can be written like this and it's easier to read:

var qs=$"q1={someVar}&q2={anotherVarWithLongName}&q3={var3}&q4={var4}&q5={var5}&q6={var6}";

Answer 14

Starting from C# 6.0 interpolated strings can be used to do this, which simplifies the format even more.

var name = "Bill";
var surname = "Gates";
MessageBox.Show($"Welcome to the show, {name} {surname}!");

An interpolated string expression looks like a template string that contains expressions. An interpolated string expression creates a string by replacing the contained expressions with the ToString represenations of the expressions’ results.

Interpolated strings have similar performance to String.Format, but improved readability and shorter syntax, due to the fact that values and expressions are inserted in-line.

Please also refer to this dotnetperls article on string interpolation.

If you are looking for a default way to format your strings, this makes sense in terms of readability and performance (except if microseconds is going to make a difference in your specific use case).

Answer 15

1. Formatting is the “.NET” way of doing it. Certain refactoring tools (Refactor! for one) will even propose to refactor the concat-style code to use the formatting style.
2. Formatting is easier to optimize for the compiler (although the second will probably be refactored to use the 'Concat' method which is fast).
3. Formatting is usually clearer to read (especially with “fancy” formatting).
4. Formatting means implicit calls to '.ToString' on all variables, which is good for readability.
5. According to “Effective C#”, the .NET 'WriteLine' and 'Format' implementations are messed up, they autobox all value types (which is bad). “Effective C#” advises to perform '.ToString' calls explicitly, which IMHO is bogus (see Jeff's posting)
6. At the moment, formatting type hints are not checked by the compiler, resulting in runtime errors. However, this could be amended in future versions.

Answer 16

I'd use the String.Format, but I would also have the format string in the resource files so it can be localised for other languages. Using a simple string concat doesn't allow you to do that. Obviously if you wont ever need to localise that string, this isn't a reason to think about. It really depends on what the string is for.

If it's going to be showed to the user, I'd use String.Format so I can localize if I need to - and FxCop will spell check it for me, just in case :)

If it contains numbers or any other non-string things (e.g. dates), I'd use String.Format because it gives me more control over the formatting.

If it's for building a query like SQL, I'd use Linq.

If for concatenating strings inside a loop, I'd use StringBuilder to avoid performance problems.

If it's for some output the user wont see, and isn't going to effect performance I'd use String.Format because I'm in the habit of using it anyway and I'm just used to it :)

Answer 17

If you're dealing with something that needs to be easy to read (and this is most code), I'd stick with the operator overload version UNLESS:

• The code needs to be executed millions of times
• You're doing tons of concats (more than 4 is a ton)
• The code is targeted towards the Compact Framework

Under at least two of these circumstances, I would use StringBuilder instead.

Answer 18

I choose based on readability. I prefer the format option when there's some text around the variables. In this example:

Console.WriteLine("User {0} accessed {1} on {2}.", 
                   user.Name, fileName, timestamp);

you understand the meaning even without variable names, whereas the concat is cluttered with quotes and + signs and confuses my eyes:

Console.WriteLine("User " + user.Name + " accessed " + fileName + 
                  " on " + timestamp + ".");

(I borrowed Mike's example because I like it)

If the format string doesn't mean much without variable names, I have to use concat:

   Console.WriteLine("{0} {1}", p.FirstName, p.LastName);

The format option makes me read the variable names and map them to the corresponding numbers. The concat option doesn't require that. I'm still confused by the quotes and + signs, but the alternative is worse. Ruby?

   Console.WriteLine(p.FirstName + " " + p.LastName);

Performance wise, I expect the format option to be slower then the concat, since format requires the string to be parsed. I don't remember having to optimize this kind of instruction, but if I did, I'd look at string methods like Concat() and Join().

The other advantage with format is that the format string can be put in a configuration file. Very handy with error messages and UI text.

Answer 19

If you intend to localise the result, then String.Format is essential because different natural languages might not even have the data in the same order.

Answer 20

I think this depends heavily on how complex the output is. I tend to choose whichever scenario works best at the time.

Pick the right tool based on the job :D Whichever looks cleanest!

Answer 21

I prefer the second as well but I have no rational arguments at this time to support that position.

Answer 22

Nice one!

Just added

        s.Start();
        for (var i = 0; i < n; i++)
            result = string.Concat(p.FirstName, " ", p.LastName);
        s.Stop();
        ceElapsedMilliseconds = s.ElapsedMilliseconds;
        ceElapsedTicks = s.ElapsedTicks;
        s.Reset();

And it is even faster (I guess string.Concat is called in both examples, but the first one requires some sort of translation).

1000000 x result = string.Format("{0} {1}", p.FirstName, p.LastName); took: 249ms - 3571621 ticks
1000000 x result = (p.FirstName + " " + p.LastName); took: 65ms - 944948 ticks
1000000 x result = string.Concat(p.FirstName, " ", p.LastName); took: 54ms - 780524 ticks

Answer 23

Since I don't think the answers here cover everything, I'd like to make a small addition here.

Console.WriteLine(string format, params object[] pars) calls string.Format. The '+' implies string concatenation. I don't think this always has to do with style; I tend to mix the two styles depending on the context I'm in.

Short answer

The decision you're facing has to do with string allocation. I'll try to make it simple.

Say you have

string s = a + "foo" + b;

If you execute this, it will evaluate as follows:

string tmp1 = a;
string tmp2 = "foo" 
string tmp3 = concat(tmp1, tmp2);
string tmp4 = b;
string s = concat(tmp3, tmp4);

tmp here is not really a local variable, but it is a temporary for the JIT (it's pushed on the IL stack). If you push a string on the stack (such as ldstr in IL for literals), you put a reference to a string pointer on the stack.

The moment you call concat this reference becomes a problem, because there isn't any string reference available that contains both strings. This means that .NET needs to allocate a new block of memory, and then fill it with the two strings. The reason this is a problem, is because allocation is relatively expensive.

Which changes the question to: How can you reduce the number of concat operations?

So, the rough answer is: string.Format for >1 concats, '+' will work just fine for 1 concat. And if you don't care about doing micro-performance optimizations, string.Format will work just fine in the general case.

A note about Culture

And then there's something called culture...

string.Format enables you to use CultureInfo in your formatting. A simple operator '+' uses the current culture.

This is especially an important remark if you're writing file formats and f.ex. double values that you 'add' to a string. On different machines, you might end up with different strings if you don't use string.Format with an explicit CultureInfo.

F.ex. consider what happens if you change a '.' for a ',' while writing your comma-seperated-values file... in Dutch the decimal separator is a comma, so your user might just get a 'funny' surprise.

More detailled answer

If you don't know the exact size of the string beforehand, it's best to use a policy like this to overallocate the buffers you use. The slack space is first filled, after which the data is copied in.

Growing means allocating a new block of memory and copying the old data to the new buffer. The old block of memory can then be released. You get the bottom line at this point: growing is an expensive operation.

The most practical way to do this is to use an overallocation policy. The most common policy is to overallocate buffers in powers of 2. Of course, you have to do it a bit smarter than that (since it makes no sense to grow from 1,2,4,8 if you already know you need 128 chars) but you get the picture. The policy ensures you don't need too many of the expensive operations I described above.

StringBuilder is a class that basically overallocates the underlying buffer in powers of two. string.Format uses StringBuilder under the hood.

This makes your decision a basic trade-off between overallocate-and-append(-multiple) (w/w.o. culture) or just allocate-and-append.

Answer 24

Personally, the second one as everything you are using is in the direct order it will be output in. Whereas with the first you have to match up the {0} and {1} with the proper var, which is easy to mess up.

At least it's not as bad as the C++ sprintf where if you get the variable type wrong the whole thing will blow up.

Also, since the second is all inline and it doesn't have to do any searching and replacing for all the {0} things, the latter should be faster... though I don't know for sure.

Answer 25

I actually like the first one because when there are a lot of variables intermingled with the text it seems easier to read to me. Plus, it is easier to deal with quotes when using the string.Format(), uh, format. Here is decent analysis of string concatenation.

Answer 26

I've always gone the string.Format() route. Being able to store formats in variables like Nathan's example is a great advantage. In some cases I may append a variable but once more than 1 variable is being concatenated I refactor to use formatting.

Answer 27

Oh, and just for completeness, the following is a few ticks faster than normal concatenation:

Console.WriteLine(String.Concat(p.FirstName," ",p.LastName));

Answer 28

The first one (format) looks better to me. It's more readable and you are not creating extra temporary string objects.

Answer 29

I was curious where StringBuilder stood with these tests. Results below...

class Program {
   static void Main(string[] args) {

      var p = new { FirstName = "Bill", LastName = "Gates" };

      var tests = new[] {
         new { Name = "Concat", Action = new Action(delegate() { string x = p.FirstName + " " + p.LastName; }) },
         new { Name = "Format", Action = new Action(delegate() { string x = string.Format("{0} {1}", p.FirstName, p.LastName); }) },
         new { Name = "StringBuilder", Action = new Action(delegate() {
            StringBuilder sb = new StringBuilder();
            sb.Append(p.FirstName);
            sb.Append(" ");
            sb.Append(p.LastName);
            string x = sb.ToString();
         }) }
      };

      var Watch = new Stopwatch();
      foreach (var t in tests) {
         for (int i = 0; i < 5; i++) {
            Watch.Reset();
            long Elapsed = ElapsedTicks(t.Action, Watch, 10000);
            Console.WriteLine(string.Format("{0}: {1} ticks", t.Name, Elapsed.ToString()));
         }
      }
   }

   public static long ElapsedTicks(Action ActionDelg, Stopwatch Watch, int Iterations) {
      Watch.Start();
      for (int i = 0; i < Iterations; i++) {
         ActionDelg();
      }
      Watch.Stop();
      return Watch.ElapsedTicks / Iterations;
   }
}

Results:

Concat: 406 ticks
Concat: 356 ticks
Concat: 411 ticks
Concat: 299 ticks
Concat: 266 ticks
Format: 5269 ticks
Format: 954 ticks
Format: 1004 ticks
Format: 984 ticks
Format: 974 ticks
StringBuilder: 629 ticks
StringBuilder: 484 ticks
StringBuilder: 482 ticks
StringBuilder: 508 ticks
StringBuilder: 504 ticks

Answer 30

According to the MCSD prep material, Microsoft suggests using the + operator when dealing with a very small number of concatenations (probably 2 to 4). I'm still not sure why, but it's something to consider.