When you’re concatenating only a few strings, it’s faster to add them than to use StringBuilder. Suppose you want to decompile the following code:

string a = "a";string b = "b";string c = "c";string d = "d";string s;s = a + b + c + d;

You will see that it is actually just calling this static method:

s = String.Concat(a, b, c, d);

This is about two to three times as fast as creating a StringBuilder object to concate them. It’s also worth noting that the optimizer does not optimize adding strings on different lines. For example, this code:

s = a;s += b;s += c;s += d;

is still faster than using a StringBuilder (until you add about six strings), but much slower than adding them all on one line.