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c#syntaxexpressionshort-circuiting

This is useful but I'm not sure why it works

protected override Boolean IsValid(String propertyValue)
{
    return !String.IsNullOrEmpty(propertyValue) && propertyValue.Trim().Length > 0;
}

This C# validation method does exactly what I want, but I wasn't aware that you could use expression short-circuiting like this.

When propertyValue is null, doesn't execution still need to evaluate the second part of the expression to yield a boolean result, and if so why doesn't the second part then throw a null ref exception for the Trim().Length > 0 call?

I assume that the second part is evaluating to false or to null, but I am curious to know what is going on here.

Apologies if I'm missing something really obvious, please enlighten me.

God - I was missing something obvious what an idiot! - total blank on the fact that when the first part is false the second part is irrelevant and I even KNEW it was short-ciruciting - sorry for wasting people's time, what's the emoticon for embarassment?

Added another tag to reflect my embarrassment at typing before thinking.

Solution

  • This is what short-circuiting does. When you have

    e1 && e2

    and e1 evaluates to false, e2 is not evaluated at all.

    (And for

    e1 || e2

    if e1 is true, e2 is not evaluated at all.)

    This is important to understand in the case where e2 has "effects" (does I/O, throws an exception, ...) as short-circuiting yields different program semantics than full evaluation would.