Null Difference

Question

What is the difference between

if(null==object)

and

if(object==null)

Please give the advantage for using the above.

score 12 · Answer 1 · answered Nov 19 '08 at 17:29

12

The difference comes if you accidentally type = instead of ==:

if (null = object) - Compiler error
if (object = null) - Bug!

answered Nov 19 '08 at 17:29

Greg

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In C# using assignment operator in place of boolean result is a compile time error. – Pradeep Nov 19 '08 at 17:31
that is good to know.. but the question wasn't originally marked 'C#' :D – warren Nov 19 '08 at 17:33
Indeed... I've removed the tag – Greg Nov 19 '08 at 17:34
:) Agree. It is tagged in .net. Doesn't VB.Net or managed C++ provide this feature? – Pradeep Nov 19 '08 at 17:36
VB uses = for both assignment and equality. – Joel Coehoorn Nov 19 '08 at 18:03
VB uses = for value equality. When checking for nulls or reference equality, VB uses "Is". – Jonathan Allen Nov 20 '08 at 08:41

score 6 · Answer 2 · answered Nov 19 '08 at 17:42

In the good old days, compilers would happily let you make assignments inside conditionals, leading to unintentional errors:

if(a = false)
{
  // I'll never execute
}
if(b = null)
{
  // I'll never execute 
}
b.Method(); // And now I'm null!

So some clever developers started putting their constants first in their conditionals:

if(false = a) // OOPS! Compiler error
{
  // ..
}
if(null = b) // OOPS! Compiler error
{
  // ..
}

So they trained themselves to avoid a whole class of errors. Most modern compilers will no longer let you make that error, but the practice continues.

There is one other advantage to always putting your constants first:

if(myString != null && myString.Equals("OtherString"))
{
  // ...
}

can (in .NET, Java, and most languages with an object-based string type) be reduced to:

if("OtherString".Equals(myString))
{
  // ..
}

score 4 · Answer 3 · answered Nov 19 '08 at 17:34

4

Well, here is something I kind of like... use extensions:

public static class ObjectExtensions
{
    public static bool IsNull(this object target)
    {
        return null == target;
    }
}

Now, you can forget about it completely:

if(item.IsNull())

answered Nov 19 '08 at 17:34

Brian Genisio

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I don't like that. I think is going a much too far with extensions. Also, I don't like calling static methods on member variables. And then it also requires a double take to see someone calling a method on a variable that presumably may be null. ... But it does work I suppose. – BobbyShaftoe Jan 09 '09 at 01:32

score 3 · Answer 4 · answered Nov 19 '08 at 17:29

3

No difference. (null == object) is a practice from C/C++, where "=" is both used as assignment operator, and as comparison operator.

There were too many errors when if (object = null) was used.

answered Nov 19 '08 at 17:29

Sunny Milenov

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I have no connection with this post, but why someone just down voted it? I agree that it is duplicating the opinions but then instead of down voting person should just keep quite. – Pradeep Nov 19 '08 at 17:33
because the answer as originally submitted was neither complete nor helpful - "No difference" isn't enough :) – warren Nov 19 '08 at 17:34
It's not a dup at all (the first 3 answers, including this one) appeared at the same time. And different explanations. Anyway, I just hate when something id downvoted w/o reason. I always put a comment when I downvote. – Sunny Milenov Nov 19 '08 at 17:36
Get over it, Sunny. In the end you'll earn +8 pt, because someone (maybe me) will upvote your answer ;-) – splattne Nov 19 '08 at 17:37
"No difference" was more than enough for smart ones. What was the need to write Julius Caesar there? – Pradeep Nov 19 '08 at 17:38
splattne: it was not about me or about my answer. I post such a comments even on someone else's answers when they are downvoted w/o reason. – Sunny Milenov Nov 19 '08 at 17:40
because he asked for "why" to use one over the other... so it wasn't a complete answer – warren Nov 19 '08 at 17:40
warren: question in 2 parts - answer in 2 parts :). I read the second question later. – Sunny Milenov Nov 19 '08 at 17:42
hehe - but it was only two lines! =D – warren Nov 19 '08 at 17:51
what to tell you, small buffer :) – Sunny Milenov Nov 19 '08 at 18:00

score 2 · Answer 5 · answered Nov 19 '08 at 17:29

2

Some prefer if (null == object) so that if you accidentally type = instead of ==, you get a compile error instead of an assignment.

answered Nov 19 '08 at 17:29

bdukes

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... if you're using C or C++, and haven't got warnings turned up high enough. – Jon Skeet Nov 19 '08 at 18:05

score 2 · Answer 6 · answered Nov 19 '08 at 17:29

2

Logically, there is no difference.

From an error checking point of view, the first is more desirable because if you miss an equals sign (=), the compiler will let you know you can't make an assignment to a constant.

answered Nov 19 '08 at 17:29

warren

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score 0 · Answer 7 · answered Nov 19 '08 at 17:29

In many languages == is the comparison operator = is the assignment operator.

It very easy to type = when you really mean ==.

Therefore the convention of typing constant==variable is preferred.

constant=variable will not compile thus showing you, your error.

variable=constant will compile and will do the wrong thing at runtime.

Null Difference

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