String Concatenation Best Practices

Question

Trying to determine if it's a better practice to use string.Format in place of concatenating strings and if so, why is this? Also, are their advantages/disadvantages to one or the other that I should be aware of?

This:

string foo = "I" + " am " + " a " + " string.";

or:

string bar = string.Format("{0} am a {1}.", "I", "string");

Obviously oversimplified examples, just wanting to be clear.

Answer 1

The "best practice" should be the thing that makes your code the most readable and maintanable. The performance difference between concatenating strings versus using string.Format versus using a StringBuilder is so small that it's essentially irrelevant.

Answer 2

Assuming the first method was not optimized at compile time, because strings are immutable it will create many intermediate strings. It'll work from left to right so there will first be "I am ", then "I am a ", and finally "I am a string." which is stored in foo.

String.format will not make intermediate strings. To my understanding it does all the manipulation in a char[] which is then made immutable by being made a String.