Conditional statement yields different results then If Then

Question

Trying to get to the bottom of a little mystery.

I have the following If Statement:

Dim myVal As Nullable(Of Guid)
myVal = If(vendor.Address.ID = Guid.Empty, Nothing, vendor.Address.ID)

The end value of myVal here is, inexplicably, Guid.Empty.

If I write that same code as follows:

Dim myVal As Nullable(Of Guid)
If(vendor.Address.ID = Guid.Empty) Then
  myVal = Nothing
Else
  myVal = vendor.Address.ID
End If

Why the difference?

Answer 1

The problem is in how VB.NET infers types using the If operator. Both operands must be the same type. Given Nothing on one side and a Guid on the other, it assumes the result type should be a Guid, so it coerces the Nothing to Guid.Empty. Putting an explicit cast to Nullable(Of Guid) on either side will be enough tell the compiler that that's what you really want.

This should give you the results you expect:

Dim myVal2 As Nullable(Of Guid)
myVal2 = If(vendor.Address.ID = Guid.Empty, CType(Nothing, Nullable(Of Guid)), vendor.Address.ID)

Or this:

Dim myVal2 As Nullable(Of Guid)
myVal2 = If(vendor.Address.ID = Guid.Empty, Nothing, CType(vendor.Address.ID, Nullable(Of Guid)))