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

Assume myObj is null. Is it safe to write this开发者_高级运维?

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.

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)

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.

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?

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.

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

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

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.

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...

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.