Is relying on && short-circuiting safe in .NET?

问题

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.

回答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 operation x && y corresponds to the operation x & y, except that y is evaluated only if x is true
...
The operation x && y is evaluated as (bool)x ? (bool)y : false. In other words, x is first evaluated and converted to type bool. Then, if x is true, y is evaluated and converted to type bool, and this becomes the result of the operation. Otherwise, the result of the operation is false.

(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 as T.false((T)x) ? (T)x : T.&((T)x, y), where T.false((T)x) is an invocation of the operator false declared in T, and T.&((T)x, y) is an invocation of the selected operator &. In addition, the value (T)x shall only be evaluated once.

In other words, x is first evaluated and converted to type T and operator false is invoked on the result to determine if x is definitely false.
Then, if x is definitely false, the result of the operation is the value previously computed for x converted to type T.
Otherwise, y is evaluated, and the selected operator & is invoked on the value previously computed for x converted to type T and the value computed for y to produce the result of the operation.

(C# Language Specification Version 4.0 - 7.12.2 User-defined conditional logical operators)

回答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.

回答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?

回答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.

回答5:

sure, it's safe on C#, if the first operand is false then the second is never evaluated.

回答6:

It is perfectly safe. C# is one of those languages.

回答7:

Yes, C# and most languages compute the if sentences from left to right.

VB6 by the way will compute the whole thing, and throw an exception if it's null...

回答8:

an example is

if(strString != null && strString.Length > 0)

This line would cause a null exception if both sides executed.

Interesting side note. The above example is quite a bit faster than the IsNullorEmpty method.

回答9:

In C#, && and || are short-circuited, meaning that the first condition is evaluated and the rest is ignored if the answer is determined.

In VB.NET, AndAlso and OrElse are also short-circuited.

In javaScript, && and || are short-circuited too.

I mention VB.NET to show that the ugly red-headed step-child of .net also has cool stuff too, sometimes.

I mention javaScript, because if you are doing web development then you probably might also use javaScript.

来源：https://stackoverflow.com/questions/4820610/is-relying-on-short-circuiting-safe-in-net