Since C# strings are immutable, an existing string cannot 
be modified. So, if one tries to change a string either 
with the concatenation operator (+) or with the Insert, 
PadLeft, PadRight, Replace, or SubString methods, an 
entirely new string is created—leaving the original string 
intact. 

Therefore, operations which would alter strings—instead—
cause additional memory to be allocated. Memory is a scarce 
resource. And, memory allocations are expensive in terms of 
memory and performance. Consequently, sometimes String 
class usage should be avoided. 

The StringBuilder class is designed for situations when one 
needs to work with a single string and make an arbitrary 
number of iterative changes to it. Many StringBuilder class 
methods are the same as those of the String class. However, 
the string content of a StringBuilder class can be changed 
without the necessity of allocating additional memory. 
Thus, operations on the StringBuilder class will be much 
faster than operations on the String class in certain 
situations. Paradoxically, just the the opposite can be 
true in other situations. 

The String class is optimized and quite efficient for most 
cases. On the other hand, if strings must be modified, then 
the String class can be a real resource waster. It must be 
appreciated that the String class is really very 
intelligent in its memory handling in most everyday 
programming situations. 

Instead of the String class, use the StringBuilder class 
when a single string must be modified repeatedly.