What would be faster in this context String.Format or String.Replace?

Question

string str = 'my {0} long string {1} need formatting';

Should I be doing the following,

str = string.Format(str, "really", "doesn't");

or creating a method like so and calling str = str.ReplaceWithValues("really", "doesn't");

 public string ReplaceWithValues(this string str, params object[] values) {
    string ret = str;
    for (int i = 0; i < values.Length; i++) {
        ret = str.Replace(string.Concat("{", i, "}"), values.ToString());
    }
    return ret;
}

It seems like StringBuilder.AppendFormat() isn't efficient when it comes to doing simple replacements like this since it goes character by character through the string.

Answer 1

Why do you want to reinvent String.Format?

I'd just use the framework method - it does exactly what you want, is robust, and is going to make sense to those that follow...

---

Just to satisfy your curiousity:

It seems like StringBuilder.AppendFormat() isn't efficient when it comes to doing simple replacements like this since it goes character by character through the string.

String.Format, FYI, uses StringBuilder.AppendFormat internally. That being said, StringBuilder.AppendFormat is quite efficient. You mention that it goes "character by character" through the string - however, in your case, you're using multiple calls to Replace and Concat instead. A single pass through the string with a StringBuilder is likely to be much quicker. If you really need to know- you could profile this to check. On my machine, I get the following timings if I run both of the above 1,000,000 times:

String.Format -  1029464 ticks
Custom method -  2988568 ticks

Answer 2

If all you want is to concatenate some strings, why not just do that?

string result = "my " + x + " long string " + y + " need formatting";

or

string result = string.Concat("my ", x, " long string ", y, " need formatting");

Answer 3

The custom procedure will increase its cost with each additional placeholder and produce throwaway strings for the garbage collector with each intermediate call to Replace.

Besides the likelihood that string.Format is much faster than multiple calls to Replace, string.Format includes overloads to culture-sensitive operations as well.

The flexibility and intuitiveness of string.Format is at least as compelling as the speed.

Answer 4

In C# the + operator actually turns in to a string.Concat(), and I always thought String.Format("Hello: {0} {1}", "Bob", "Jones") was the fastest. It turns out, after firing up a sample ran outside the debugger, in release mode, that "Bob" + "Jones" is much faster. There is a cutoff point though. I believe around 8 concatenations or so, string.Format becomes faster.