How does StringBuilder work internally in C#?

Question

How does StringBuilder work?

What does it do internally? Does it use unsafe code? And why is it so fast (compared to the + opera

Answer 1

The StringBuilder uses a string buffer that can be altered, compared to a regular String that can't be. When you call the ToString method of the StringBuilder it will just freeze the string buffer and convert it into a regular string, so it doesn't have to copy all the data one extra time.

As the StringBuilder can alter the string buffer, it doesn't have to create a new string value for each and every change to the string data. When you use the + operator, the compiler turns that into a String.Concat call that creates a new string object. This seemingly innocent piece of code:

str += ",";

compiles into this:

str = String.Concat(str, ",");