Why does one often see "null != variable" instead of "variable != null" in C#?

Question

Hi,

In c#, is there any difference in the excecution speed for the order in which you state the condition?

if (null != variable) ...
if (variable != null) ...

Since recently, I saw the first one quite often, and it caught my attention since I was used to the second one.

If there is no difference, what is the advantage of the first one?

Answer 1

I guess this is a C programmer that has switched languages.

In C, you can write the following:

int i = 0;
if (i = 1)
{
    ...
}

Notice the use of a single equal sign there, which means the code will assign 1 to the variable i, then return 1 (an assignment is an expression), and use 1 in the if-statement, which will be handled as true. In other words, the above is a bug.

In C# however, this is not possible. There is indeed no difference between the two.

Answer 2

In earlier times, people would forget the '!' (or the extra '=' for equality, which is more difficult to spot) and do an assignment instead of a comparison. putting the null in front eliminates the possibility for the bug, since null is not an l-value.

Most modern compilers give a warning when you do an assignment in a conditional nowadays, and C# actually gives an error. Most people just stick with the var == null scheme since it's easier to read for some people.

Answer 3

It's a hold-over from C. In C, if you either use a bad compiler or don't have warnings turned up high enough, this will compile with no warning whatsoever (and is indeed legal code):

// Probably wrong
if (x = 5)

when you actually probably meant

if (x == 5)

You can work around this in C by doing:

if (5 == x)

A typo here will result in invalid code.

Now, in C# this is all piffle. Unless you're comparing two Boolean values (which is rare, IME) you can write the more readable code, as an "if" statement requires a Boolean expression to start with, and the type of "x=5" is Int32, not Boolean.

I suggest that if you see this in your colleagues' code, you educate them in the ways of modern languages, and suggest they write the more natural form in future.

Answer 4

To me it's always been which style you prefer

@Shy - Then again if you confuse the operators then you should want to get a compilation error or you will be running code with a bug - a bug that come back and bite you later down the road since it produced unexpected behaviour

Answer 5

One more thing... If you are comparing a variable to a constant (integer or string for ex.), putting the constant on the left is good practice because you'll never run into NullPointerExceptions :

int i;
if(i==1){        // Exception raised: i is not initialized. (C/C++)
  doThis();
}

whereas

int i;
if(1==i){        // OK, but the condition is not met.
  doThis();
}

Now, since by default C# instanciates all variables, you shouldn't have that problem in that language.