Perhaps I am asking this question out of ignorance, but why are value type
references handled differently than object references? I understand that
value type instantiations are held on the stack versus the heap, but I must
be missing something about the way in which references to these objects are
held (does each reference actually store a copy of the value since they are
passed by value? if so, how does example #2 below work?). I suppose, if I
were not lazy, that I could examine the bytecode generated, but I figured
that somebody on this list could provide a good explanation.
For instance, I am mostly annoyed by the handling of references to NULL,
which apparently value type references are incapable of. Let me illustrate
an example.
Say I am instantiating an entity object that represents some logical
coupling of information stored in a relational database (common task,
right?). Now, assume that I have a date column in one of the tables in the
database that I am interested in representing within my object, which
happens to hold a NULL value for the entity that I am loading (again, fairly
common).
It seems that my options for representing a NULL DateTime are as follows:
1)
DataReader reader = // ...
DateTime myDateColumn = // ...
// ...
if (reader["my_date_column"] is DBNull) {
myDateColumn = DateTime.MinValue; // or some other "special value"
indicating NULL
} else {
myDateColumn = (DateTime)reader["my_date_column"];
}
One of the problems with this approach, is that wherever myDateColumn is
referenced, the code needs to know that DateTime.MinValue signifies a NULL
or empty value. Isn't this what NULL is for?!?
2)
DataReader reader = // ...
object myDateColumn = // ...
// ...
if (reader["my_date_column"] is DBNull) {
myDateColumn = null;
} else {
myDateColumn = reader["my_date_column"];
}
Obviously a better - and more true - representation of the value of this
field; however, then I lose type safety, am forced to recast myDateColumn to
DateTime whenever I access it, etc., etc. This gets messy after a while.
Why is it that I cannot simply do:
3)
DataReader reader = // ...
DateTime myDateColumn = // ...
// ...
if (reader["my_date_column"] is DBNull) {
myDateColumn = null;
} else {
myDateColumn = (DateTime)reader["my_date_column"];
}
In addition, I find it odd that, should I go with #1 above, the following
does not work:
object xxx = (myDateColumn.Equals(DateTime.MinValue) ? null : myDateColumn);
...while the following does:
object xxx = (myDateColumn.Equals(DateTime.MinValue) ? null :
(object)myDateColumn);
Thanks in advance.
Best Regards,
joe duffy
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