String output: format or concat in C#?

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.

Try this code.

It's a slightly modified version of your code.

1. I removed Console.WriteLine as it's probably a 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 1,000 milliseconds and 100 milliseconds. In both situations, you will get 0 ms after dividing it by 1,000,000.

Code:

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

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 are 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.