StringBuilder and string performance

The hidden cost of +=

When you write report += line; C# cannot change the old string. It creates a brand-new string, copies all the old characters into it, adds the new ones, and throws the old string away for the garbage collector. In a loop of 10,000 lines, the copying grows and grows: line 10,000 copies everything from the 9,999 lines before it.

StringBuilder basics

StringBuilder lives in the System.Text namespace, which is not in the default implicit usings, so add using System.Text; at the top of the file.

using System.Text;

var sb = new StringBuilder();
sb.Append("Invoice #");
sb.Append(2041);
sb.AppendLine();                 // adds a new line
sb.AppendLine("Item     Qty  Price");
sb.AppendLine($"{"Pen",-8} {2,3} {20.00m,6:F2}");
string result = sb.ToString();
Console.WriteLine(result);

Other useful members

Comparing the two ways

using System.Diagnostics;
using System.Text;

var watch = Stopwatch.StartNew();
string s = "";
for (int i = 0; i < 20_000; i++) s += i + ",";
Console.WriteLine($"+= took {watch.ElapsedMilliseconds} ms");

watch.Restart();
var sb = new StringBuilder();
for (int i = 0; i < 20_000; i++) sb.Append(i).Append(',');
string t = sb.ToString();
Console.WriteLine($"StringBuilder took {watch.ElapsedMilliseconds} ms");
Console.WriteLine(s == t);   // True

When NOT to use StringBuilder

• Joining two to five pieces once: $"{first} {last}" is clearer and just as fast.
• Joining all items of an array or list with a separator: string.Join(", ", items) is simpler.
• Very short loops (a handful of rounds): the difference is too small to matter; choose what reads better.

Common mistakes and fixes

• Forgetting using System.Text;: error CS0246, the type StringBuilder could not be found.
• Printing the builder instead of calling ToString(): Console.WriteLine(sb) actually works (it calls ToString for you), but passing sb where a string is expected does not compile. Call sb.ToString().
• Calling ToString() inside the loop: that makes a new string each time, which undoes the benefit. Call it once at the end.
• Measuring speed in a Debug build with one tiny run: measure bigger loops, ideally in a Release build (dotnet run -c Release).