c# - Performance issue: comparing to String.Format

Question

A while back a post by Jon Skeet planted the idea in my head of building a CompiledFormatter class, for using in a loop instead of String.Format().

The idea is the portion of a call to String.Format() spent parsing the format string is overhead; we should be able to improve performance by moving that code outside of the loop. The trick, of course, is the new code should exactly match the String.Format() behavior.

This week I finally did it. I went through using the .Net framework source provided by Microsoft to do a direct adaption of their parser (it turns out String.Format() actually farms the work to StringBuilder.AppendFormat()). The code I came up with works, in that my results are accurate within my (admittedly limited) test data.

Unfortunately, I still have one problem: performance. In my initial tests the performance of my code closely matches that of the normal String.Format(). There's no improvement at all; it's even consistently a few milliseconds slower. At least it's still in the same order (ie: the amount slower doesn't increase; it stays within a few milliseconds even as the test set grows), but I was hoping for something better.

It's possible that the internal calls to StringBuilder.Append() are what actually drive the performance, but I'd like to see if the smart people here can help improve things.

Here is the relevant portion:

private class FormatItem
{
    public int index; //index of item in the argument list. -1 means it's a literal from the original format string
    public char[] value; //literal data from original format string
    public string format; //simple format to use with supplied argument (ie: {0:X} for Hex

    // for fixed-width format (examples below) 
    public int width;    // {0,7} means it should be at least 7 characters   
    public bool justify; // {0,-7} would use opposite alignment
}

//this data is all populated by the constructor
private List<FormatItem> parts = new List<FormatItem>(); 
private int baseSize = 0;
private string format;
private IFormatProvider formatProvider = null;
private ICustomFormatter customFormatter = null;

// the code in here very closely matches the code in the String.Format/StringBuilder.AppendFormat methods.  
// Could it be faster?
public String Format(params Object[] args)
{
    if (format == null || args == null)
        throw new ArgumentNullException((format == null) ? "format" : "args");

    var sb = new StringBuilder(baseSize);
    foreach (FormatItem fi in parts)
    {
        if (fi.index < 0)
            sb.Append(fi.value);
        else
        {
            //if (fi.index >= args.Length) throw new FormatException(Environment.GetResourceString("Format_IndexOutOfRange"));
            if (fi.index >= args.Length) throw new FormatException("Format_IndexOutOfRange");

            object arg = args[fi.index];
            string s = null;
            if (customFormatter != null)
            {
                s = customFormatter.Format(fi.format, arg, formatProvider);
            }

            if (s == null)
            {
                if (arg is IFormattable)
                {
                    s = ((IFormattable)arg).ToString(fi.format, formatProvider);
                }
                else if (arg != null)
                {
                    s = arg.ToString();
                }
            }

            if (s == null) s = String.Empty;
            int pad = fi.width - s.Length;
            if (!fi.justify && pad > 0) sb.Append(' ', pad);
            sb.Append(s);
            if (fi.justify && pad > 0) sb.Append(' ', pad);
        }
    }
    return sb.ToString();
}

//alternate implementation (for comparative testing)
// my own test call String.Format() separately: I don't use this.  But it's useful to see
// how my format method fits.
public string OriginalFormat(params Object[] args)
{
    return String.Format(formatProvider, format, args);
}

Additional notes:

I'm wary of providing the source code for my constructor, because I'm not sure of the licensing implications from my reliance on the original .Net implementation. However, anyone who wants to test this can just make the relevant private data public and assign values that mimic a particular format string.

Also, I'm very open to changing the FormatInfo class and even the parts List if anyone has a suggestion that could improve the build time. Since my primary concern is sequential iteration time from front to end maybe a LinkedList would fare better?

[Update]:

Hmm... something else I can try is adjusting my tests. My benchmarks were fairly simple: composing names to a "{lastname}, {firstname}" format and composing formatted phone numbers from the area code, prefix, number, and extension components. Neither of those have much in the way of literal segments within the string. As I think about how the original state machine parser worked, I think those literal segments are exactly where my code has the best chance to do well, because I no longer have to examine each character in the string.

Another thought:

This class is still useful, even if I can't make it go faster. As long as performance is no worse than the base String.Format(), I've still created a strongly-typed interface which allows a program to assemble it's own "format string" at run time. All I need to do is provide public access to the parts list.

Answer 1

Here's the final result:

I changed the format string in a benchmark trial to something that should favor my code a little more:

The quick brown {0} jumped over the lazy {1}.

As I expected, this fares much better compared to the original; 2 million iterations in 5.3 seconds for this code vs 6.1 seconds for String.Format. This is an undeniable improvement. You might even be tempted to start using this as a no-brainer replacement for many String.Format situations. After all, you'll do no worse and you might even get a small performance boost: as much 14%, and that's nothing to sneeze at.

Except that it is. Keep in mind, we're still talking less than half a second difference for 2 million attempts, under a situation specifically designed to favor this code. Not even busy ASP.Net pages are likely to create that much load, unless you're lucky enough to work on a top 100 web site.

Most of all, this omits one important alternative: you can create a new StringBuilder each time and manually handle your own formatting using raw Append() calls. With that technique my benchmark finished in only 3.9 seconds. That's a much greater improvement.

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In summary, if performance doesn't matter as much, you should stick with the clarity and simplicity of the built-in option. But when in a situation where profiling shows this really is driving your performance, there is a better alternative available via StringBuilder.Append().