如何指定派生结构的默认状态应该在C＃中？

Question

I've seen different questions on SO about not being able to use parameterless constructors or not setting field initializers, but I think my question kind of goes a step beyond that.

What I would like to know is, how would I go about setting up the "default" value of a struct I need to define? Say I'm making a TaxID struct, and the default value needs to be 999-99-9999 for whatever reason. This is already done with other structs in .NET, namely the DateTime. Any time you declare a DateTime the value is immediately set to Jan 1 0001 12:00 AM. Why does it get set to this value and not 0/0/0000 00:00:0000 AM? There must be something going on behind the scenes that makes the value actually make "sense" at it's "default", even given the restrictions put on us by c# with regards to structs.

Answer 1

In general, this should be avoided. If you need to require a good, default case which would require construction, you should consider changing to a class.

One of the core design guidelines for structs is: Do ensure that a state where all instance data is set to zero, false, or null (as appropriate) is valid.

For details, see the design guidelines .

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I just looked this section up in the design guidelines 2nd edition, and they have an example in detail there using properties and non-conventional overrides to work around this, as well. The basic concept was to save the value privately in a way that 0 is the "good default", and do some form of transform in every property and method override (including ToString). In their case, they used a positive integer as an example, and always save curVal-1 in the private member, so the default value (0) is treated like a value of 1. They added a constructor with an int val, and save value-1 internally, etc.

This seems like a lot of hidden, unexpected overhead, though - so I'd personally still recommend (and use) a class in this case.

--- Edit in response to comments ---

DateTime, as your example, is the way it is because 0 == 1/1/0001 at Midnight. DateTime uses a single, ulong to represent ticks from 1/1/0001. That is why ( from here ):

"The DateTime value type represents dates and times with values ranging from 12:00:00 midnight, January 1, 0001 Anno Domini (Common Era) through 11:59:59 PM, December 31, 9999 AD (CE) "

This is the full range of ulong in ticks. A "0" in the struct of DateTime is treated as 1/1/0001 when you convert to a string - the values aren't 1 + 1 + .... - it's a single 0.

Answer 2

Any time you declare a DateTime the value is immediately set to Jan 1 0001 12:00 AM. Why does it get set to this value and not 0/0/0000 00:00:0000 AM? There must be something going on behind the scenes that makes the value actually make "sense" at it's "default", even given the restrictions put on us by c# with regards to structs.

More likely it implicitly initializes a ticks field to default(int) = 0, and 0 ticks means DateTime.MinValue when you go to get the value.

Answer 3

Nothing sinister is going on. The internal representation of a DateTime is the number of 100 nanosecond "ticks" since 12:00:00 midnight, January 1, 0001 Anno Domini. When the DateTime is created, the tick count is zero. If you look at DateTime with Ildasm, you will see it's a bit more complicated than that, but you get the idea. :)

Answer 4

You cannot declare a default constructor in a C# struct. You can simulate this behavior by creating a private field and writing a property to return the default value if it's 1/1/1

Answer 5

Just for completeness:

Value types in C# (including arrays, structs, enums) are always initialized to all zeros. Reference types are also initially zero, better know as null .

In case of enums, zero is the default value, and is always a possible value even if it's not a defined in the enum. Personally, I usually define 0 as None, Empty, Unknown or something similar:

enum MyEnum { A = 1, B = 2, C = 3 };
// ...
MyEnum value = new MyEnum(); // value has a nameless 0

In case of arrays, if it's an array of reference types, they are all initially null (which you can think of as a pointer to address 0). If it's an array of value types, they are all initially zero or the equivalent (see other answers about DateTime.Ticks and why a "zeroed" DateTime is equal to 1/1/0001 12:00am).

Coming back to structs: the same applies. They will always have a default parameterless constructor and you cannot avoid this. All value type members will initially be zero (or equivalent) and all reference type members will be null . Think about int, short, decimal, char, DateTime (ahem), TimeSpan, Size, Point, Rectangle, Color, Guid... they are all initialized to all zeros.