Is cutting a string into two code lines slower? [duplicate]

This program:

var name = "Bobby Tables";
var age = 8;
String msg = $"I'm {name} and" +
    $" I'm {age} years old";

is compiled as if you had written:

var name = "Bobby Tables";
var age = 8;
String msg = String.Concat(String.Format("I'm {0} and", name),
    String.Format(" I'm {0} years old", age));

You see the difficulty in getting rid of the Concat - the compiler has re-written our interpolation literals to use the indexed formatters that String.Format expects, but each string has to number its parameters from 0. Naively concatenating them would cause them both to insert name. To get this to work out correctly, there would have to be state maintained between invocations of the $ parser so that the second string is reformatted as " I'm {1} years old". Alternatively, the compiler could try to apply the same kind of analysis it does for concatenation of string literals. I think this would be a legal optimization even though string interpolation can have side effects, but I wouldn't be surprised if it turned out there was a corner case under which interpolated string concatenation changed program behavior. Neither sounds impossible, especially given the logic is already there to detect a similar condition for string literals, but I can see why this feature didn't make it into the first release.

I would write the code in the way that you feel is cleanest and most readable, and not worry about micro-inefficiencies unless they prove to be a problem. The old saying about code being primarily for humans to understand holds here.

Maybe it would be not as readable as with + but by all means, it is possible. You just have to break line between { and }:

Log.Debug($@"Must resize {image.Width} x {image.Height} image to {
    resizedImage.Width} x {resizedImage.Height} for reasons.");

SO's colouring script does not handle this syntax too well but C# compiler does ;-)

In the specialized case of using this string in HTML (or parsing with whatever parser where multiple whitespaces does not matter), I could recommend you to use @$"" strings (verbatim interpolated string) eg.:

$@"some veeeeeeeeeeery long string {foo} 
whatever {bar}"