What is the difference between 'foo = Nothing' and 'foo is Nothing' in VB.NET?

Question

In VB.NET, what is the difference between

if foo is Nothing Then
      doStuff()
End If

and

if foo=Nothing Then
    doStuff()
End If

---

Update I received the following answer:

foo is Nothing simply checks if foo is not assigned to any reference. foo = Nothing checks if the reference held by foo is equal to nothing.

After running the three statements,

Dim foo as Object
Dim bar as Integer
foo = bar

foo is Nothing evaluates to false and foo = Nothing evaluates to true.

However, if bar is declared as an Object and not initialized, then foo is Nothing and foo = Nothing both evaluate to true! I think this is because Integer is a value type and Object is a reference type.

Answer 1

It depends on the type.

• For value types, Is doesn’t work, only =, and Nothing refers to the default instance of that type (i.e. the instance that you get by calling New T() for a given type T).

• For reference types, Is performs a reference comparison (identical to object.ReferenceEquals(a, Nothing)). a = Nothing usually does not work, unless Operator = has explicitly been defined for that class.

  If, furthermore, Operator = has been implemented correctly, then foo = Nothing and foo Is Nothing should yield the same result (but the same isn’t true for any other value instead of Nothing) but foo Is Nothing will be more efficient since it’s a compiler intrinsic while Operator = will call a method.

• For nullable value types (i.e. instances of Nullable(Of T)), special rules apply: like all other operators, = is lifted (notice the error in that blog post …) by the compiler to the underlying type. The result of comparing two Nullables is thus not Boolean but Boolean? (note the ?). However, because of so-called “null propagation” for lifted operators, this will always return Nothing, no matter the value of foo. Quoting the Visual Basic 10 language specification (§1.86.3):

  If ether (sic!) operand is Nothing, the result of the expression is a value of Nothing typed as the nullable version of the result type.

  So if the users want to compare a Nullable variable to Nothing, they must use the foo Is Nothing syntax for which, once again, the compiler generates special code to make it work (§1.79.3 of the Visual Basic 10 language specification). _{Hat tip to Jonathan Allen for (correctly) persisting that I was wrong; hat tip to Jared Parsons for passing me a link to the Visual Basic 10 specification.}

(The above assumes that Option Strict On is used, as you always should. In case that isn’t the case, the results will differ slightly since calling foo = Nothing may perform a late-bound call.)

Answer 2

foo is Nothing simply checks if `foo` is not assigned to any reference.

foo=Nothing checks if the reference held by `foo` is equal to `nothing`

In VB, both statements will evaluate to the same value if foo has not been initialised