Can you think of top 10 questions in .NET developer interview? I bet one of them is about string builders. Like “how would you concatenate multiple strings?”. With the correct answer “I’d use StringBuilder. Because, you know, string is immutable, allocations, blah, blah, blah…”. And if you pretend to be an experienced developer, your code has 0 (zero) concatenations via +.

Even when it comes to join 2 (3, 4, 5, …) literals or constants, they still use StringBuilder. Or course, it’s less effective, because such concatenations usually happen during compilation (constants are embedded into code as literals).

But what about regular strings? Should one always use StringBuilder? I’ve prepared a small benchmark, using BenchMarkDotNet (source code is available on GitHub) that shows that when the number of strings is less than 20, StringBuilder has no advantage over +. Even more, when number of items is less that 10, + has better performance. And this is exactly the most used scenario of string concatenation. If you have more than 10 items, you’d most likely use the loop with StringBuilder, or String.Join() or (if you’re crazy badass) IEnumerable.Aggregate().

Here are results from tests running on my machine:


BenchmarkDotNet=v0.10.11, OS=Windows 10 Redstone 3 [1709, Fall Creators Update] (10.0.16299.192)
Processor=Intel Core i7-4720HQ CPU 2.60GHz (Haswell), ProcessorCount=8
Frequency=2533208 Hz, Resolution=394.7564 ns, Timer=TSC
.NET Core SDK=2.1.2
  [Host]     : .NET Core 2.0.3 (Framework 4.6.25815.02), 64bit RyuJIT
  DefaultJob : .NET Core 2.0.3 (Framework 4.6.25815.02), 64bit RyuJIT
Method NumberOfItems Mean Error StdDev Median
StringBuilderAction 2 885.5 ns 7.824 ns 7.319 ns 885.3 ns
StringConcatenationAction 2 770.6 ns 6.802 ns 6.362 ns 771.4 ns
StringBuilderAction 5 1,785.5 ns 10.304 ns 9.639 ns 1,784.6 ns
StringConcatenationAction 5 1,674.5 ns 20.619 ns 18.278 ns 1,672.6 ns
StringBuilderAction 10 3,215.0 ns 63.250 ns 62.120 ns 3,186.8 ns
StringConcatenationAction 10 3,313.1 ns 102.415 ns 85.521 ns 3,283.2 ns
StringBuilderAction 20 5,951.5 ns 48.102 ns 40.167 ns 5,937.8 ns
StringConcatenationAction 20 6,947.7 ns 34.813 ns 29.071 ns 6,940.4 ns
StringBuilderAction 50 14,997.1 ns 347.515 ns 985.841 ns 14,672.7 ns
StringConcatenationAction 50 22,794.2 ns 474.567 ns 1,291.094 ns 22,303.9 ns
StringBuilderAction 100 27,873.3 ns 481.219 ns 375.704 ns 27,864.9 ns
StringConcatenationAction 100 59,897.8 ns 1,948.882 ns 3,091.133 ns 59,045.8 ns

There is another, poor man’s benchmark available on dotnetfiddle.net. It might be less accurate, but + always wins.

Having that we can conclude that using StringBuilder to just concatenate a couple of strings is nothing but a Cargo Cult. More code and less performance…

Happy coding!