CODEWITHSUNDEEP
.netvb.net
Stefan Steiger
Stefan Steiger

Reputation: 82186

Why does this code even compile?

Question:
Why does the below code (not written by me) even compile ?
I mean apart from the fact that option strict is off and option infer is on... 

If Not actdate.DayOfWeek = DayOfWeek.Saturday And Not actdate.DayOfWeek.Sunday Then
...
End If
**Edit:**
Just for those not fluent in VB, it's oviously the same as: 
if (!(actdate.DayOfWeek == DayOfWeek.Saturday) & !actdate.DayOfWeek.Sunday) {
...
}
which basically answers the question already, since the thing to remember is that the VB-`AND` is actually a bitwise and.

Upvotes: 8

Views: 1356

Answers (1)

Hans Passant
Hans Passant

Reputation: 941457

The accepted answer is not correct, operator precedence in VB.NET ensures that the logical version of And operator is used, same one as AndAlso. Both the left-hand and right-hand operands are of type Boolean thanks to the Not operators being used. Precedence in VB.NET is relational > Not > And. In C# it is ! > relational > &. Or to put it another way, you don't need parentheses in VB.NET like you do in C#.

The Not operator in Visual Basic accepts a Boolean or numeric expression. Just like in C#, an enum value is implicitly convertable to an integral value type that matches the Enum's base type. Integer in this case. A numeric value of 0 converts to False. Since DayOfWeek.Sunday's underlying value is 0, the Not expression always produces True.

So this is acceptable syntax. You do however get a warning for this code, very similar to the error you get in C#:

warning BC42025: Access of shared member, constant member, enum member or nested type through an instance; qualifying expression will not be evaluated.

Produced by the Sunday enum member used in the actdate.DayOfWeek property expression. That is certainly a code smell. Short from not ignoring warnings, you can turn that warning into an error. Project + Properties, Compile tab, Warning configuration section. Change the "Instance variable accesses shared member" setting from Warning to Error.

Upvotes: 8

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