Why is String.IsNullOrEmpty faster than String.Length?

Question

ILSpy shows that String.IsNullOrEmpty is implemented in terms of String.Length. But then why is String.IsNullOrEmpty(s) faster than

Answer 1

I believe your test is not correct:

This test shows that string.IsNullOrEmpty is always slower than s.Length==0 because it performs an additional null check:

var strings = "A,B,,C,DE,F,,G,H,,,,I,J,,K,L,MN,OP,Q,R,STU,V,W,X,Y,Z,".Split(',');
var testers = new Func[] { 
    s => s == String.Empty, 
    s => s.Length == 0, 
    s => String.IsNullOrEmpty(s), 
    s => s == "" ,
};
int n = testers.Length;
var stopwatches = Enumerable.Range(0, testers.Length).Select(_ => new Stopwatch()).ToArray();
int count = 0;
for(int i = 0; i < n; ++i) { // iterate testers one by one
    Stopwatch sw = stopwatches[i];
    var tester = testers[i];
    sw.Start();
    for(int j = 0; j < 10000000; ++j) // increase this count for better precision
        count += strings.Count(tester);
    sw.Stop();
}
for(int i = 0; i < testers.Length; i++)
    Console.WriteLine(stopwatches[i].ElapsedMilliseconds);

Results:

6573
5328
5488
6419

You can use s.Length==0 when you are ensure that target data does not contains null strings. In other cases I suggest you use the String.IsNullOrEmpty.