If an array is empty, it looks like you can't check it's length using ".length". What's the best way to check if an array is empty?
+32
A:
You can absolutely check an empty array's length. However, if you try to do that on a null reference you'll get an exception. I suspect that's what you're running into. You can cope with both though:
if (array == null || array.Length == 0)
If that isn't the cause, please give a short but complete program demonstrating the problem. If that was the cause, it's worth taking a moment to make sure you understand null references vs "empty" collections/strings/whatever.
Jon Skeet
2010-07-09 14:08:08
In what situation would an array have .Length == 0? An array dimensioned to size 0?
Soo
2010-07-09 14:09:48
...an empty array.
David Neale
2010-07-09 14:12:20
@Soo: Yes, exactly.
Adam Robinson
2010-07-09 14:12:33
The important thing to note about the code above is that in csharp, the or operator (`||`) is short circuiting. It sees that the array is null (statement is true) and does NOT attempt to evaluate the second statement. If it did, you'd still get a NullReferenceException and would have to nest your if statements. Not all languages are like this, VB.Net being one example. The default `Or` operator is NOT short-circuiting; they have `OrElse` for that. http://support.microsoft.com/kb/817250
Tim Coker
2010-07-09 14:16:40
@Tim, wow, that's interesting thanks for posting that tip. I learn something new everyday on SO.
Soo
2010-07-09 14:19:53
@Tim: Btw, the C# equivalent of VB's `Or` (non-short-circuiting) would by a single `|`.
Lucas
2010-07-09 14:20:57
:) Glad to help.
Tim Coker
2010-07-09 14:30:53
JeremyP
2010-07-09 14:43:12
@JeremyP: Yup, AndAlso.
Jon Skeet
2010-07-09 14:49:44
@Lucas, that's even more interesting,Thanks for sharing these tips everyone!
Soo
2010-07-09 18:15:36
@Downvoter: Care to comment?
Jon Skeet
2010-08-20 15:35:00
+7
A:
Yeah, for safety I would probably do:
if(array == null || array.Length == 0)
Kevin
2010-07-09 14:08:19
+4
A:
You can use .Length == 0 if the length is empty and the array exists, but are you sure it's not null?
Brian R. Bondy
2010-07-09 14:08:29
+2
A:
This is the best way. Please note Array is an object in NET so you need to check for null before.
Arseny
2010-07-09 14:09:15
A:
do you mean empty or null, two different things,
if the array is instantiated but empty, then length is correct, if it has not been instantiated then test vs null
Pharabus
2010-07-09 14:09:21
+1
A:
Your suggested test is fine, so long as the array is intialised...
Martin.
Martin Milan
2010-07-09 14:10:37
+2
A:
As other have already suggested it is likely you are getting a NullReferenceException which can be avoided by first checking to see if the reference is null. However, you need to ask yourself whether that check is actually warranted. Would you be doing it because the reference really might be null and it being null has a special meaning in your code? Or would you be doing it to cover up a bug? The nature of the question leads me to believe it would be the latter. In which case you really need to examine the code in depth and figure out why that reference did not get initialized properly in the first place.
Brian Gideon
2010-07-09 14:17:26
+2
A:
Jon Skeet answered correctly. Just remeber that the order of the test in the "IF" is important. Check for the null before the lenght. I also prefer to put the null on the left side of the equal...just a habit I got from Java that made the code more efficient and fast... I don't think it's important in a lot of application today, but it's good practice.
if(null == array || array.Length == 0)
Sylvain Perron
2010-07-09 14:21:53
you could atleast say why you knocked this down... what's wrong with this?
Wills
2010-07-09 19:01:09
+1
A:
You can use
if (array == null || array.Length == 0)
OR
if (!(array != null && array.Length >= 0))
NOTE!!!!! To insure that c# will implement the short circuit correctly; you have to compare that the object with NULL before you go to the children compare of the object.
Waleed A.K.
2010-07-09 17:43:30
+1
A:
check if the array is null first so you would avoid a null pointer exception
logic in any language: if array is null or is empty :do ....
Saher
2010-07-09 18:08:00