Is relying on && short-circuiting safe in .NET?
Solution 1
Yes. In C# &&
and ||
are short-circuiting and thus evaluates the right side only if the left side doesn't already determine the result. The operators &
and |
on the other hand don't short-circuit and always evaluate both sides.
The spec says:
The
&&
and||
operators are called the conditional logical operators. They are also called the “shortcircuiting” logical operators.
...
The operationx && y
corresponds to the operationx & y
, except thaty
is evaluated only ifx
istrue
...
The operationx && y
is evaluated as(bool)x ? (bool)y : false
. In other words,x
is first evaluated and converted to typebool
. Then, ifx
istrue
,y
is evaluated and converted to typebool
, and this becomes the result of the operation. Otherwise, the result of the operation isfalse
.
(C# Language Specification Version 4.0 - 7.12 Conditional logical operators)
One interesting property of &&
and ||
is that they are short circuiting even if they don't operate on bools, but types where the user overloaded the operators &
or |
together with the true
and false
operator.
The operation
x && y
is evaluated asT.false((T)x) ? (T)x : T.&((T)x, y)
, whereT.false((T)x)
is an invocation of theoperator false
declared inT
, andT.&((T)x
, y) is an invocation of the selectedoperator &
. In addition, the value (T)x shall only be evaluated once.In other words,
x
is first evaluated and converted to typeT
andoperator false
is invoked on the result to determine ifx
is definitelyfalse
.
Then, ifx
is definitelyfalse
, the result of the operation is the value previously computed forx
converted to typeT
.
Otherwise,y
is evaluated, and the selected operator&
is invoked on the value previously computed forx
converted to typeT
and the value computed fory
to produce the result of the operation.
(C# Language Specification Version 4.0 - 7.12.2 User-defined conditional logical operators)
Solution 2
Yes, C# uses logical short-circuiting.
Note that although C# (and some other .NET languages) behave this way, it is a property of the language, not the CLR.
Solution 3
I know I'm late to the party, but in C# 6.0 you can do this too:
if(myObj?.SomeString != null)
Which is the same thing as above.
Also see: What does question mark and dot operator ?. mean in C# 6.0?
Solution 4
Your code is safe - && and || are both short-circuited. You can use non-short-circuited operators & or |, which evaluate both ends, but I really don't see that in much production code.
Solution 5
sure, it's safe on C#, if the first operand is false then the second is never evaluated.
patrick
Updated on July 05, 2022Comments
-
patrick over 1 year
Assume myObj is null. Is it safe to write this?
if(myObj != null && myObj.SomeString != null)
I know some languages won't execute the second expression because the && evaluates to false before the second part is executed.
-
Adam Robinson almost 13 yearsActually, VB.NET behaves this way when using
AndAlso
andOrElse
, not onAnd
andOr
, which is likely what 90% of people always use. -
Brad almost 13 yearsOne of those languages that uses short-circuit evaluation here
-
Ilya Kogan almost 13 years@Adam, the question says "I know some languages..."
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Adam Robinson almost 13 years@Ilya Kogan: I guess that means that I should read the question more closely ;)
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Admin almost 13 yearsThis is... not exactly wrong, but misleading on so many levels. (1) Curly braces languages is just a small subset of the languages that do this. (2) Reversing the input before parsing it is madness, so about no language does this. (3) Regardless of how it's parsed, short-curcuiting is runtime semantics. (4) See comment to harpo's answer regarding VB -
And
doesn't short-curcuit, but&
doesn't either. (Perhaps 2+3 are because you confuse parsing with something that happens at runtime...) -
Joe Enos almost 13 yearsThat last paragraph was a little too interesting...made my head hurt. :)
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CodesInChaos almost 13 yearsIt looks complicated, but it's not that hard. It simply doesn't evaluate the right side if the
false
operator on the left side returnstrue
and returns the left side instead, and else invokes the custom&
operator. So it mimics the short circuiting behaviorbool
s have. The typical application of this is an extendedbool
that can have a third state. For example you could implement something similar toDBBol
orbool?
with that. -
Pedro almost 13 yearsIf you were one of the "beautiful people," maybe someone would up-vote your answer.
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John Grabauskas about 6 yearsYour answer is correct, but not as much detail as CodesInChaos
-
Olivier Jacot-Descombes almost 6 yearsIt is also important to note that this not only an implementation detail (which can change at any time) but part of the C# Specification (which will not change in this respect).
-
Stewart almost 4 yearsEssentially,
a || b
is equivalent toa ? a : b
, anda && b
is equivalent toa ? b : a
, except that a is evaluated only once. This is more noticeable in JavaScript, where operands of arbitrary type can be used. Other languages such as C don't follow this principle - they accept operands of non-boolean types but always return a boolean value. -
Billy Bob over 2 yearsAnswer does not answer the question and is a suggestion on syntactic sugar that should be a comment.
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Drew Delano over 2 years@BillyBob The answer to OPs original question is yes, and that has been well established by all of the other answers on here. This goes more toward answering "does the example code provided even need short circuiting"
-
Nikola Malešević about 2 yearsI wish we had
&&=
and||=
operators.