Does string interpolation evaluate duplicated usage?

Question

If I have a format string that utilizes the same place holder multiple times, like:

emailBody = $"Good morning {person.GetFullName()}, blah blah blah, {person.GetFullName()} would you like to play a game?"; 

does person.GetFullName() get evaluated twice, or is the compiler smart enough to know these are the same value, and should be evaluated once?

Answer 1

Yes, it will be evaluated twice. It can't know that it is the same value. For example:

Random rng = new Random(); Console.WriteLine($"{rng.Next(5)}, {rng.Next(5)}, {rng.Next(5)}"); 

It's exactly the same as if you just used the expression as a method argument - if you called:

Console.WriteLine("{0} {1}", person.GetFullName(), person.GetFullName()); 

... you'd expect GetFullName to be called twice then, wouldn't you?

If you only want to evaluate it once, do so beforehand:

var name = person.GetFullName(); emailBody = $"Good morning {name}, blah blah blah, {name} would you like to play a game?"; 

Or just use a normal string.Format call:

emailBody = string.Format(     "Good morning {0}, blah blah blah, {0} would you like to play a game?",     person.GetFullName());