C# 6 String Interpolation Does Not Concatenate

Well, I learned something new today that's slightly disappointing. I had thought that C# 6 string interpolation concatenated strings or perhaps used the StringBuilder or some such under the hood. It turns out it merely creates a good, old-fashioned String.Format statement out of it.

Given this source code:

	void Main()
	{
	}
	string NormalConcat()
	{
	var x = "B";
	return "A" + x + "C";
	}
	string Interpolation()
	{
	var x = "B";
	return $"A{x}C";
	}

view raw CompareConcatWithInterpolation.cs hosted with ❤ by GitHub

The resulting IL (compiled) code is the following (obtained using LINQPad):

	NormalConcat:
	IL_0000: nop
	IL_0001: ldstr "B"
	IL_0006: stloc.0 // x
	IL_0007: ldstr "A"
	IL_000C: ldloc.0 // x
	IL_000D: ldstr "C"
	IL_0012: call System.String.Concat
	IL_0017: stloc.1
	IL_0018: br.s IL_001A
	IL_001A: ldloc.1
	IL_001B: ret
	Interpolation:
	IL_0000: nop
	IL_0001: ldstr "B"
	IL_0006: stloc.0 // x
	IL_0007: ldstr "A{0}C"
	IL_000C: ldloc.0 // x
	IL_000D: call System.String.Format
	IL_0012: stloc.1
	IL_0013: br.s IL_0015
	IL_0015: ldloc.1
	IL_0016: ret

view raw CompareConcatWithInterpolation.il hosted with ❤ by GitHub

Note the following two statements:

ldstr       "A{0}C"
call        System.String.Format

These indicate that String.Format is being called with the familiar-looking format string "A{0}C".


To compile the C# code and create IL code, I used Joe Albahari's excellent LINQPad program.