What is the Advantage of Using System.Text.StringBuilder over System.String

Strings are so heavily used in all programming languages that we do not think about them very much. We use them simply and hope to do the right thing. Normally all goes well but sometimes we need more performance so we switch to StringBuilder which is more efficient because it does contain a mutable string buffer. .NET Strings are immutable which is the reason why a new string object is created every time we alter it (insert, append, remove, etc.).

That sounds reasonable, so why do we still use the .NET String class functions and not the faster StringBuilder? Because optimal performance is a tricky thing and the first rule of the performance club is to measure it for yourself. Do not believe somebody telling you (including me!) that this or that is faster in every case. It is very difficult to predict the performance of some code in advance because you have to know so many variables that influence the outcome. Looking at the generated MSIL code does still NOT tell you how fast the code will perform. If you want to see why your function is so slow/fast you have to look at the compiled (JIT ed) x86 assembler code to get the full picture.

Greg Young did some very nice posts about what the JITer does make of your MSIL code at your CPU. In the following article I will show you the numbers for StringBuilder vs String which I did measure with .NET 2.0 a P4 3.0 GHz with 1 GB RAM. Every test was performed 5 million times to get a stable value.