IndexOf vs Dictionary

a.cs

public void Old(string s) {
	var matchCount = 0;
	for (var i=0;i<4000;i++) {
		var key = String.Format(",{0},", i);
		if (s.IndexOf(key) >= 0) {
			matchCount++;
		}
	}
	Console.WriteLine(String.Format("s matches: {0}", matchCount));
}

public void New(Dictionary<string, bool> d) {
	var matchCount = 0;
	for (var i=0;i<4000;i++) {
		var key = String.Format(",{0},", i);
		if (d.Keys.Contains(key)) {
			matchCount++;
		}
	}
	Console.WriteLine(String.Format("d matches: {0}", matchCount));
}

void Main()
{
	Stopwatch watch;
	var sb = new StringBuilder();
	var d = new Dictionary<string, bool>();
	for (var i=0;i<2000;i++) {
		var key = String.Format(",{0},", i);
		sb.Append(key);
		d[key] = true;
	}
	string s = sb.ToString();	
	Console.WriteLine("s length: " + s.Length);
	
	watch = new Stopwatch();
	watch.Start();
	Old(s);
	watch.Stop();
	Console.WriteLine(String.Format("{0}ms", watch.ElapsedMilliseconds));
	
	watch = new Stopwatch();
	watch.Start();
	New(d);
	watch.Stop();
	Console.WriteLine(String.Format("{0}ms", watch.ElapsedMilliseconds));
}

/*
s length: 10890
s matches: 2000
74ms
d matches: 2000
1ms
*/