C# - Why are DateTime.MinValue and MaxValue not compile-time constants?

Question

I wanted to have an optional date parameter for a method (defaulted to MinValue), in order to check if the user had actually supplied a value or not (supplying MinValue was invalid), but I'm not allowed as apparently it's not a compile-time constant.

According to the MSDN page, "The value of this constant is equivalent to 00:00:00.0000000, January 1, 0001."

So why is that not compile-time constant? And why is it different from passing in Int32.MinValue, which is allowed?

@Downvoter Why the downvote? I felt the question was clear, had a precise answer and was relevant to the site. — Alex
– Alex, Commented Jul 28, 2011 at 12:16

Vasea · Accepted Answer · 2011-07-28 09:38:00Z

24

You cannot define a DateTime constant (or structs). From MSDN allowed types for const are:

One of the types: byte, char, short, int, long, float, double, decimal, bool, string, an enum type, or a reference type.

answered Jul 28, 2011 at 9:38

Vasea

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2 Comments

Vasea Over a year ago

@jazmatician DateTime is a value type. See msdn.microsoft.com/en-us/library/system.datetime(v=vs.110).aspx

Michael Blackburn Over a year ago

I knew that was wrong when I typed it, but couldn't put my finger on it. :)

CodesInChaos · Accepted Answer · 2011-07-28 09:46:19Z

21

Workaround: Use a nullable as parameter. IMO this is cleaner anyways since the special value is clearly different and not just a normal value.

void A(DateTime? p=null) { }

Another alternative is:

void A(DateTime p=default(DateTime)) { }

Which shows that a default parameter can use default(T) as valid default parameter value for user defined types.

Or just overload the method for the different number of parameters.

answered Jul 28, 2011 at 9:46

CodesInChaos

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3 Comments

Alex Over a year ago

this answers my implied question of 'how can I do what I'm trying to do?', thanks :)

eternalmatt Over a year ago

You could try using readonly if you're looking to ensure that the value doesn't change. And I believe this can be used in conjunction with nullable type ?.

Alex Over a year ago

@eternalmatt I don't follow - how does this help having an optional DateTime parameter? If it's optional it'll need a default value, and since I can't provide a compile-time constant it seems CodeInChaos' solution is best.

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