Question

The following compiles in VB.NET (with Option Strict On) and outputs False:

Dim b As Boolean? = Nothing
Dim myString = If(b, "True", "False")

Why does that work?

• The documentation clearly states that the three-argument version of If requires a Boolean as the first parameter:

  argument1 Required. Boolean. Determines which of the other arguments to evaluate and return.

• and there is no implicit conversion from Boolean? to Boolean:

  Dim b1 As Boolean? = Nothing
  Dim b2 As Boolean = b1   ' Fails with the following error:
                           '   Option Strict On disallows implicit conversions
                           '   from 'Boolean?' to 'Boolean'.
  

So, why does this work? Is it a bug (or “hidden feature”) in the compiler or is it a bug in the documentation and Boolean? is actually a valid type for the first argument of If(a, b, c)?

PS: In C#, b ? x : y does not compile if b is of type bool?.

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EDIT: I’ve reported this issue to Microsoft Connect. Someone from MS has replied and confirmed that the documentation will be updated to include the Boolean? case.

Answer 1

There’s two “why’s”. Why does it and why did they do it that way. I can answer the first, the second is Microsoft’s.

If you inspect the code generated from VB.Net using Reflector you’ll see this:

Dim b As Nullable(Of Boolean) = Nothing
Dim myString As String = IIf(b.GetValueOrDefault, "True", "False")

Or C#:

bool? b = null;
string myString = b.GetValueOrDefault() ? "True" : "False";

So the compiler itself is inserting the GetValueOrDefault for the Nullable(of T)