IsNothing versus Is Nothing


Does anyone here use VB.NET and have a strong preference for or against using IsNothing as opposed to Is Nothing (for example, If IsNothing(anObject) or If anObject Is Nothing...)? If so, why?

EDIT: If you think they're both equally acceptable, do you think it's best to pick one and stick with it, or is it OK to mix them?

This question is tagged with nothing

~ Asked on 2008-08-08 09:45:29

8 Answers


If you take a look at the MSIL as it's being executed you'll see that it doesn't compile down to the exact same code. When you use IsNothing() it actually makes a call to that method as opposed to just evaluating the expression.

The reason I would tend to lean towards using "Is Nothing" is when I'm negating it becomes "IsNot Nothing' rather than "Not IsNothing(object)" which I personally feel looks more readable.

~ Answered on 2008-08-08 12:28:37


I find that Patrick Steele answered this question best on his blog: Avoiding IsNothing()

I did not copy any of his answer here, to ensure Patrick Steele get's credit for his post. But I do think if you're trying to decide whether to use Is Nothing or IsNothing you should read his post. I think you'll agree that Is Nothing is the best choice.

Edit - VoteCoffe's comment here

Partial article contents: After reviewing more code I found out another reason you should avoid this: It accepts value types! Obviously, since IsNothing() is a function that accepts an 'object', you can pass anything you want to it. If it's a value type, .NET will box it up into an object and pass it to IsNothing -- which will always return false on a boxed value! The VB.NET compiler will check the "Is Nothing" style syntax and won't compile if you attempt to do an "Is Nothing" on a value type. But the IsNothing() function compiles without complaints. -PSteele – VoteCoffee

~ Answered on 2008-08-11 14:12:57


You should absolutely avoid using IsNothing()

Here are 4 reasons from the article IsNothing() VS Is Nothing

  1. Most importantly, IsNothing(object) has everything passed to it as an object, even value types! Since value types cannot be Nothing, it’s a completely wasted check.
    Take the following example:

    Dim i As Integer
    If IsNothing(i) Then
       ' Do something 
    End If

    This will compile and run fine, whereas this:

    Dim i As Integer
    If i Is Nothing Then
        '   Do something 
    End If

    Will not compile, instead the compiler will raise the error:

    'Is' operator does not accept operands of type 'Integer'.
    Operands must be reference or nullable types.

  2. IsNothing(object) is actually part of part of the Microsoft.VisualBasic.dll.
    This is undesirable as you have an unneeded dependency on the VisualBasic library.

  3. Its slow - 33.76% slower in fact (over 1000000000 iterations)!

  4. Perhaps personal preference, but IsNothing() reads like a Yoda Condition. When you look at a variable you're checking it's state, with it as the subject of your investigation.

    i.e. does it do x? --- NOT Is xing a property of it?

    So I think If a IsNot Nothing reads better than If Not IsNothing(a)

~ Answered on 2014-06-30 16:07:10


I agree with "Is Nothing". As stated above, it's easy to negate with "IsNot Nothing".

I find this easier to read...

If printDialog IsNot Nothing Then
End If

than this...

If Not obj Is Nothing Then
End If

~ Answered on 2008-08-08 13:11:51


VB is full of things like that trying to make it both "like English" and comfortable for people who are used to languages that use () and {} a lot. And on the other side, as you already probably know, most of the time you can use () with function calls if you want to, but don't have to.

I prefer IsNothing()... but I use C and C#, so that's just what is comfortable. And I think it's more readable. But go with whatever feels more comfortable to you.

~ Answered on 2008-08-08 09:49:51


I'm leaning towards the "Is Nothing" alternative, primarily because it seems more OO.

Surely Visual Basic ain't got the Ain't keyword.

~ Answered on 2008-08-08 10:57:27


I initially used IsNothing but I've been moving towards using Is Nothing in newer projects, mainly for readability. The only time I stick with IsNothing is if I'm maintaining code where that's used throughout and I want to stay consistent.

~ Answered on 2008-08-08 13:24:51


Is Nothing requires an object that has been assigned to the value Nothing. IsNothing() can take any variable that has not been initialized, including of numeric type. This is useful for example when testing if an optional parameter has been passed.

~ Answered on 2016-03-06 04:23:28

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