IsNothing versus Is Nothing

Question

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.

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

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 its 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)

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
    'blah
End If

than this...

If Not obj Is Nothing Then
    'blah
End If

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.

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

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