Performance: string concatenation vs String.Format vs interpolated string

After writing my previous post about interpolated strings in C#, I was wondering what is the most performant way to concatenate strings. So, I use BenchmarkDotnet to quickly get the result.

Here's the code of the benchmark:

// Full code: https://gist.github.com/meziantou/b120126088457f5f87ad200f4f7bf6b0
class Program
{
    static void Main(string[] args)
    {
        BenchmarkRunner.Run<StringBenchmark>();
    }
}

[OrderProvider(SummaryOrderPolicy.FastestToSlowest)]
[CoreJob]
[MemoryDiagnoser]
class StringBenchmark
{
    [Benchmark]
    public string StringConcatenation1() => "test" + 0 + "test0";

    [Benchmark]
    public string StringFormat1() => string.Format("test{0}{1}", 0, "test0");

    [Benchmark]
    public string StringInterpolation1() => $"test{0}{"test0"}"; // Should be the same as StringFormat

    [Benchmark]
    public string FormattableString1() => Format($"test{0}{"test0"}"); // Force the creation of a FormattableString

    // ...
    // same tests with 5, 10, 20, 50 100 arguments

    private static string Format(FormattableString fs) => fs.ToString();
}

The 4 methods are almost equivalents. But, you can notice that string concatenation is a little faster for a very small number of arguments.

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