Counting placeholders

CountPlaceholders.cs

using System;

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine(CountPlaceholders("{0,3} {1:x} {{2}}"));
    }

    static int CountPlaceholders(string format)
    {
        var counter = new CountingFormatter();
        string.Format(counter, format, s_enoughArgs);
        return counter.PlaceholderCount;
    }

    private static readonly object[] s_enoughArgs = new object[99];

    private sealed class CountingFormatter : ICustomFormatter, IFormatProvider
    {
        public int PlaceholderCount;

        public string Format(string format, object arg, IFormatProvider formatProvider)
        {
            PlaceholderCount++;
            return "";
        }

        public object GetFormat(Type formatType)
        {
            if (formatType == typeof(ICustomFormatter))
            {
                return this;
            }

            return null;
        }
    }
}