journal.nullschool.net

The IIf statement in Visual Basic is a pain when Option Strict is on. The reason is that the return value is Object and thus requires a cast back to the target type:

    Dim a As Integer = IIf(SomeCondition(), 10, 20)
    Error: Option Strict On disallows implicit conversions from 'System.Object' to 'Boolean'.

To fix the compile error, you need to add a cast:

    Dim a As Integer = CInt(IIf(SomeCondition(), 10, 20))

Even worse, use an enum:

    Dim b As Color = CType(IIf(SomeCondition(), Color.Red, Color.Blue), Color)

But by using generic methods and type inferencing, you can create a version of IIf that eliminates the need for the cast:

    Function IIf(Of T)(ByVal Expression As Boolean, ByVal TruePart As T, ByVal FalsePart As T) As T
        If Expression Then Return TruePart Else Return FalsePart
    End Function

Then the above examples could be written as:

    Dim a As Integer = IIf(SomeCondition(), 10, 20)
    Dim b As Color = IIf(SomeCondition(), Color.Red, Color.Blue)

Much nicer. And also much faster because we avoid boxing the parameters and unboxing the result. Now if only IIf could be massaged into a real ternary operator…