C# String Interpolation Performance

Time for a follow up to my String.Concat vs String.Format Performance post from back in 2014!

I recently found out that string interpolation is not nearly as efficient as I would have thought. I also suspected that it was just doing a string concatenation, but it is actually doing a string format. This leads to a pretty significant performance degradation; the following test runs one million iterations of each.

Number of Args	Interpolation Milliseconds	String.Format Milliseconds	String.Concat Milliseconds	String Add Milliseconds	StringBuilder Milliseconds
2	262	260	19	18	34
3	367	367	25	24	35
4	500	513	31	32	41
5	646	635	67	66	44
6	740	723	79	76	49
7	802	819	86	85	52
8	938	936	97	98	58

So, what's the lesson? Don't use string interpolation in high performance areas (such as your logger)!

Enjoy,
Tom