AND/OR (&&/||) logic for multiple condition statements

c# logic conditional statements
miguelarcilla picture miguelarcilla · Dec 19, 2013 · Viewed 40.4k times · Source

If you have an if-statement in C# that checks multiple conditions:

if (a == 5 && b == 9) { ... }

Does b == 9 still get checked if a == 5 condition is false, or does it automatically exit since there's no way this could pass anymore?

Similarly, for an OR if-statement:

if (a == 5 || b == 9) { ... }

Will b == 9 still get checked if a == 5 is true?

Answer

Lasse V. Karlsen picture Lasse V. Karlsen · Dec 19, 2013

Both && and || is "short-circuiting" operators, which means that if the answer is known from the left operand, the right operand is not evaluated.

This means that:

a && b

b will not be evaluated if a is false, since the final answer is already known.

Likewise:

a || b

b will not be evaluated if a is true, since the final answer is already known.

If you want both operands to be evaluated, use the & and | operators instead.

The bonus of this is that you can write expressions that would fail if all operands was evaluated. Here's a typical if-statement:

if (a != null && a.SomeProperty != null && a.SomeProperty.Inner != null)
    ... use a.SomeProperty.Inner

If a was null, and the expression would go on to evaluate a.SomeProperty, it would throw a NullReferenceException, but since && short-circuits, if a is null, the expression will not evaluate the rest and thus not throw the exception.

Obviously, if you replace && with &, it will throw that exception if either a or a.SomeProperty is null.

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