?? and || Operator timing

I am starting to learn C# and have just started using the ?? operator, but am slightly confused about the timing. Take the below code example:

    string x = null;
    string y = null;
    string v = null;

    var watch = System.Diagnostics.Stopwatch.StartNew();

    if (string.IsNullOrEmpty(x ?? y ?? v ?? y ?? v))
    {

    }

    watch.Stop();
    var elapsedMs = watch.ElapsedTicks;
    Console.WriteLine(elapsedMs.ToString());

    watch.Restart();

    if (string.IsNullOrEmpty(x) || string.IsNullOrEmpty(y) || string.IsNullOrEmpty(v) || string.IsNullOrEmpty(y) || string.IsNullOrEmpty(v))
    {

    }

    var elapsedMs2 = watch.ElapsedTicks;
    Console.WriteLine(elapsedMs2.ToString());

The result i get is that the first if statement takes around 100 ticks whilst the second usually around 3. I was under the impression that both of these would be similar in timing, or that the second if statement with the || would take longer.

Obviously i am missing something here. I thought that after the first x was evaluated to null that would be it and the rest wouldn't be evaluated. Is anyone able to explain what i am getting wrong here?

Solution

Do you maybe have the definition of ?? backward? a ?? b means:

If a is null, b
Otherwise, a

So if the left side is not null, then we don't evaluate b, since we know a ?? b is a. If a is null, as in your case, we do have to evaluate b.

if (string.IsNullOrEmpty(x ?? y ?? v ?? y ?? v))

runs as:

Evaluate x.
That evaluates as null, so evaluate y.
That evaluates as null, so evaluate v.
That evaluates as null, so evaluate y.
That evaluates as null, so evaluate v.
That evaluates as null. Pass this value (null) into string.IsNullOrEmpty.
That evaluates as true. Enter the block.

if (string.IsNullOrEmpty(x) || string.IsNullOrEmpty(y) || string.IsNullOrEmpty(v) || string.IsNullOrEmpty(y) || string.IsNullOrEmpty(v))

evaluates as:

Evaluate x.
That evaluates as null. Pass this value (null) into string.IsNullOrEmpty.
That evaluates as true. Since || is satisfied if only one of the sides is true, we do not evaluate the right side of the first ||, or any further to the right. Enter the block.