Why is String.Concat not optimized to StringBuilder.Append?

Question

I found concatenations of constant string expressions are optimized by the compiler into one string.

Now with string concatenation of strings only known at run-time,

Answer 1

For a single concatenation of multiple strings (e.g. a + b + c + d + e + f + g + h + i + j) you really want to be using String.Concat IMO. It has the overhead of building an array for each call, but it has the benefit that the method can work out the exact length of the resulting string before it needs to allocate any memory. StringBuilder.Append(a).Append(b)... only gives a single value at a time, so the builder doesn't know how much memory to allocate.

As for doing it in loops - at that point you've added a new local variable, and you've got to add code to write back to the string variable at exactly the right time (calling StringBuilder.ToString()). What happens when you're running in the debugger? Wouldn't it be pretty confusing not to see the value building up, only becoming visible at the end of the loop? Oh, and of course you've got to perform appropriate validation that the value isn't used at any point before the end of the loop...