Three of my coworkers just told me that there's no reason to use a StringBuilder in place of concatenation using the + operator. In other words, this is fine to do with a bunch of strings: myString1 + myString2 + myString3 + myString4 + mySt...
The rationale that they used was that since .NET 2, the C# compiler will build the same IL if you use the + operator as if you used a StringBuilder.
This is news to me. Are they correct?
No, they are not correct. String concatenation creates a new string whereas StringBuilder uses a variable size buffer to build the string, only creating a string object when ToString() is called.
There are many discussions on string concatenation techniques all over the Internet if you would like to read further on the subject. Most focus on the efficiency of the different methods when used in loops. In that scenario, StringBuilder is faster over string concatenation using string operators for concatenations of 10 or more strings, which should indicate that it must be using a different method than the concatenation.
That said, if you're concatenating constant string values, the string operators will be better because the compiler will factor them away, and if your performing non-looped concatenation, using the operators will be better as they should result in a single call to string.Concat.