String.Concat vs String.Format Performance

Time for another micro-optimization!

When building strings it is almost always easiest to write and maintain a typical format statement. However, what is the cost of that over just concatenating strings? When building strings for cache keys (which I know are going to get called a lot) I try to use String.Concat instead of String.Format. Let's look at why!

Below is a table showing a comparison the performance difference between String.Concat and String.Format. The Y axis is the number of arguments being concatenated. The X axis is the number of milliseconds it takes to complete 100,000 runs.

Number of Args	String.Concat Milliseconds	String.Format Milliseconds	Concat Percent Faster
2	4ms	10ms	150%
3	3ms	13ms	333%
4	4ms	16ms	300%
5	12ms	21ms	75%
6	14ms	24ms	71%
7	16ms	28ms	75%
8	18ms	31ms	72%

Here is an example of the code that was used to test this:

// This averaged 4 milliseconds

var stopwatchConcat2 = Stopwatch.StartNew();

for (var i = 0; i < 100000; i++)

    String.Concat("1", "2");

stopwatchConcat2.Stop();

// This averaged 10 milliseconds

var stopwatchFormat2 = Stopwatch.StartNew();

for (var i = 0; i < 100000; i++)

    String.Format("1{0}", "2");

stopwatchFormat2.Stop();

Enjoy,
Tom