String vs StringBuilder

Recently I was asked the question if I knew the difference between a String and StringBuilder. The expected answer, which was not my answer, was String is immutable while StringBuilder is mutable. My ill-informed response was, “String is not immutable. I constantly change the values of Strings in my code.” The response I got was, “While String values can be changed, you’re actually creating a new String object in memory every time you do so. You should’t concatenate a string inside a loop. For that scenario, StringBuilder would give you better performance.” Sill thinking I was right, I decided to do a performance test myself with the following code:

void Test1()
    string str = "s";

    for (int index = 0; index <= 100; index++)
        str += index.ToString();


void Test2()
    StringBuilder str = new StringBuilder("s", 200);

    for (int index = 0; index <= 100; index++)


Test1 concatenates a string in for loop while Test2 concatenates a StringBuilder in a for loop.
I used Visual Studio’s performance analysis (Debug >> Start Performance Analysis) to monitor CPU usage and managed memory allocation.

Results for Test1

As you can see, CPU usage was relatively high between 0-1 seconds and there was a high amount of memory allocation for System.String.

Results for Test2

In Test2, CPU usage was lower between 0-1 seconds and there was significantly less memory usage throughout the test. If you’re wondering why I’m only looking at the 0-1 second range in the CPU usage report, it’s because each test took less then a second to run.

What I learned was that I was completely wrong with my response. StringBuilder does offer better performance when concatenating within a loop. I can now honestly say I’ve been schooled in the art of .NET performance and memory usage. Being a .NET developer for the past 6 years, I felt like a phony. Scott Hanselman knows what I’m talking about. I guess when it comes to being a developer there is always room for growth.

Always pass on what you have learned.

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