Weird Issue with DateTime calculation

Question

I've probably been working too hard, but can someone explain to me the following, taken from the Immediate window?

(int)DateTime.Now.Date.DayOfWeek = 4

and

(int)DayOfWeek.Sunday = 0

and

(int)DateTime.Now.Date.DayOfWeek - (int)DayOfWeek.Sunday = 4

but
(int)DayOfWeek.Sunday - (int)DateTime.Now.Date.DayOfWeek = Could not evaluate expression`

Thanks for reading.

EDIT:

Its the Immediate window that's giving me this weird result, not regular code.

Screenshot: http://ploader.net/files/0c2556df475b3075634d7fd2b0575794.PNG

EDIT2:

The community seem to think its a bug in VS2010. I wonder if @EricLippert or @JonSkeet could spare a minute to confirm this or, if its not, offer an explanation about this behaviour?

Pulmonic

This works fine for me. Can you post all of your code? Perhaps there is something else going on here? – Drewdrewett

Its not code, just the output of ad hoc expressions in the Immediate window. – Pulmonic

@Drewdrewett My guess is that he is talking about the immediate window when debugging - I've tried it and it doesn't work, but I have no idea why. – Overburdensome

Oh i see now. The immidate window wouldn't know what to do if you just paste that expression into it. If you'd like it to print the Debugger.Print(<expression>); or set a var x = <expression>; but just saying x-y in that window doesn't do anything. So the exception is thrown – Drewdrewett

hmm. But if the operands are reversed it works fine doesn't it? I am baffled. Where's Jon Skeet when you need him ...? – Pulmonic

Oh your right, I missed that. Seems very odd. – Drewdrewett

Are you really asking the reason 0 - 4 can not be expressed as a valid DateTime operation? Try taking the absolute value of that expression. I understand this only a problem within the Immediate windows ( I sadly cannot access your image ) honestly it makes total sense to me. You cannot express a day as a negative number. – Taimi

@Raymond He is casting to an int to be expressed as an int...all other variations work. – Violoncello

@Raymond - as @AaronMcIver points out, its nothing to do with Datetime - I'm casting enums and subtracting one from the other. - try it yourself during a debug session. Its only a specific order of the operands (zero first) in the calculation that doesn't work. – Pulmonic

Answer 1

It looks specific to the constant 0 and a non-literal value. The following works just fine:

int zero = 0;
zero - (int)DateTime.Now.Date.DayOfWeek
-4

While the following fails:

int four = 4;
0 - four
Could not evaluate expression

Update: I couldn't find a similar bug report, so I created one: https://connect.microsoft.com/VisualStudio/feedback/details/679501/integer-literal-0-integer-variable-could-not-evaluate-expression-immediate-window

Update #2: Microsoft is able to reproduce the issue and has resolved it as "Won't Fix", meaning there's hope for the next version of Visual Studio, but not for VS2010.

Gilbart

I'd agree with that. Surely its a bug? Do I get some sort of prize for bringing this to the attention of the eggheads on the Visual Studio project team? ;-) – Pulmonic

Probably not, but if Eric Lippert reads this, you might get it fixed in the next release. – Gilbart

Answer 2

I have no idea, it looks like a bug to me.

// This doesn't work
0 - (int)DateTime.Now.Date.DayOfWeek

// But loads of similar variations do:
1 - (int)DateTime.Now.Date.DayOfWeek
-1 - (int)DateTime.Now.Date.DayOfWeek
a - (int)DateTime.Now.Date.DayOfWeek
0 - (int)DayOfWeek.Thursday

In any case everything behaves as expected in the compiled code.

Overburdensome

Thank God for that. I thought I was starting to go crazy. – Pulmonic