c# - String concatenation vs String Builder. Performance

Question

I have a situation where I need to concatenate several string to form an id of a class. Basically I'm just looping in a list to get the ToString values of the objects and then concatenating them.

foreach (MyObject o in myList)
  result += o.ToString();

The list is NOT expected to have more than 5 elements (although it could but that's a very, very marginal case) and usually will have from 1 to 3 elements, being common for it to just have one or two.

What would be more performance, keeping the concatenation or using an StringBuilder?

StringBuilder bld = new StringBuilder()
foreach (MyObject o in myList)
  bld.Append(o.ToString());

I'm unsure if creating the StringBuilder will take more time than standard concatenation for the most usual case.

This is lazy, items on the list do not change once created so the id is lazily constructed once when called.

As a side note... Should I use a fixed array instead of a List? Would I get any performance or memory improvement if I do? (List is only used as an IEnumerable anyway)

A more general view of the question could be, how many strings are enough to stop concatenating and start building?

Should I even bother to test case the scenario?

if (myList.Count > 4) 
  ConcatWithStringBuilder(myList);

Answer 1

The usual answer is that string concatenation is more efficient for between 4 to 8 strings. It depends on whose blog you read.

Don't write a test to decide on which method to use. If you are unsure of whether it will go over the magic limit, then just use StringBuilder.

Run this code to see the results for yourself:

const int sLen=30, Loops=5000;
DateTime sTime, eTime;
int i;
string sSource = new String('X', sLen);
string sDest = "";
// 
// Time string concatenation.
// 
sTime = DateTime.Now;
for(i=0;i<Loops;i++) sDest += sSource;
eTime = DateTime.Now;
Console.WriteLine("Concatenation took " + (eTime - sTime).TotalSeconds + " seconds.");
// 
// Time StringBuilder.
// 
sTime = DateTime.Now;
System.Text.StringBuilder sb = new System.Text.StringBuilder((int)(sLen * Loops * 1.1));
for(i=0;i<Loops;i++) sb.Append(sSource);
sDest = sb.ToString();
eTime = DateTime.Now;
Console.WriteLine("String Builder took " + (eTime - sTime).TotalSeconds + " seconds.");
// 
// Make the console window stay open
// so that you can see the results when running from the IDE.
// 
Console.WriteLine();
Console.Write("Press Enter to finish ... ");
Console.Read();

Ref. http://support.microsoft.com/kb/306822