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Use StringBuilder when you’re concatenating strings in a very long loop or in a loop within an unknown size – especially if you don’t know for sure (at compile time) how many iterations you’ll make through the loop. For example, reading a file a character at a time, building up a string as you go.
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Use String Concatenation operator when you can specify everything which needs to be concatenated in one statement. (If you have an array of things to concatenate, consider calling String.Concat explicitly – or String.Join if you need a delimiter.), avoid using the (+=) or the normal (+) for strings concatenation.
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If you need the intermediate results of the concatenation for something other than feeding the next iteration of concatenation, StringBuilder isn’t going to help you. For instance, if you build up a full name from a first name and a last name, and then add a third piece of information (the nickname, maybe) to the end, you’ll only benefit from using StringBuilder if you don’t need the (first name + last name) string for other purpose (as we do in the example which creates a Person object).