Question

Is there a performance difference between String.Replace(char, char) and String.Replace(string, string) when I just need to replace once character with another?

Answer 1

Yes, there is: I ran a quick experiment, and it looks like the string version is about 3 times slower.

    string a = "quickbrownfoxjumpsoverthelazydog";
    DateTime t1 = DateTime.Now;
    for (int i = 0; i != 10000000; i++) {
        var b = a.Replace('o', 'b');
        if (b.Length == 0) {
            break;
        }
    }
    DateTime t2 = DateTime.Now;
    for (int i = 0; i != 10000000; i++) {
        var b = a.Replace("o", "b");
        if (b.Length == 0) {
            break;
        }
    }
    DateTime te = DateTime.Now;
    Console.WriteLine("{0} {1}", t2-t1, te-t2);

1.466s vs 4.583s

This is not surprising, because the overload with strings needs an extra loop to go through all characters of the oldString. This loop runs exactly one time, but the overhead is still there.