Question

I’m reviewing some code in VB.net, and in a validation object they have written the following

If Not IsDate(Entity.SelectedDate) Then
            ErrorList.Add(New CValidationError("MainReport", "Please select a weekend date"))

SelectedDate is of type Date. It seems to me it would be impossible to ever hit this condition. Is this true?

Answer 1

Here’s what I see in Reflector:

Public Shared Function IsDate(ByVal Expression As Object) As Boolean
    Dim time As DateTime
    If (Expression Is Nothing) Then
        Return False
    End If
    If TypeOf Expression Is DateTime Then
        Return True
    End If
    Dim str As String = TryCast(Expression,String)
    Return ((Not str Is Nothing) AndAlso Conversions.TryParseDate(str, (time)))
End Function

Now, the question is: if passed a Date (VB.NET’s keyword for DateTime values), could this method ever return false?

No.

If (Expression Is Nothing)

This will never be true for a boxed value type.

If TypeOf Expression Is DateTime

This will always be true if the method is explicitly passed a Date.

Even if TypeOf A Is B returned false when B is a subclass of A (which it doesn’t), you could still assume this would always return true since DateTime, as a value type, cannot be inherited.

So you’re good.

My best guess is that this code originally called IsDate on a String or Object that was not strongly typed; at some point, someone must have updated the SelectedDate property to be typed as Date without bothering to update this validation code.