When a string is modified, the run time will create a new string and return it, leaving the original to be garbage collected. Most of the time this is a fast and simple way to do it, but when a string is being modified repeatedly it begins to be a burden on performance: all of those allocations eventually get expensive. Here’s a simple example of a program that appends to a string 50,000 times, followed by one that uses a StringBuilder object to modify the string in place. The StringBuilder code is much faster, and if you run them it becomes immediately obvious.

namespace ConsoleApplication1.Feedback{
  using System;
  
  public class Feedback{
    public Feedback(){
      text = "You have ordered: \n";
    }

    public string text;

    public static int Main(string[] args) {
      Feedback test = new Feedback();
      String str = test.text;
      for(int i=0;i<50000;i++){
        str = str + "My Text";
      }
      System.Console.Out.WriteLine("done");
      return 0;
    }
  }
}
namespace ConsoleApplication1.Feedback{
  using System;

  public class Feedback{
    public Feedback(){
      text = "You have ordered: \n";
    }

    public string text;

    public static int Main(string[] args) {
      Feedback test = new Feedback();
      System.Text.StringBuilder SB = 
        new System.Text.StringBuilder(test.text);
      for(int i=0;i<50000;i++){
        SB.Append("My Text");
      }
      System.Console.Out.WriteLine("done");
      return 0;
    }
  }
}

Try looking at Perfmon to see how much time is saved without allocating thousands of strings. Look at the “% time in GC” counter under the .NET CLR Memory list. You can also track the number of allocations you save, as well as collection statistics.

Tradeoffs   There is some overhead associated with creating a StringBuilder object, both in time and memory. On a machine with fast memory, a StringBuilderbecomes worthwhile if you’re doing about five operations. As a rule of thumb, I would say 10 or more string operations is a justification for the overhead on any machine, even a slower one.