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statement validation

Why isn't this statement valid ?
DateTime? st = this.SunSt.Text != "  :" ? DateTime.Parse(this.SunSt.Text) : null;

I get an error that
Type of conditional expression cannot be determined because there is no implicit conversion between 'System.DateTime' and '<null>'

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2 Solutions
DateTime is a ValueType and a DateTime variable can never have a value of null.

Try something like this:
    DateTime d = (this.SunSt.Text != "  :" ? DateTime.Parse(this.SunSt.Text) : DateTime.MinValue);
or this:
    object obj = (this.SunSt.Text != "  :" ? DateTime.Parse(this.SunSt.Text) : null);
    if (obj != null) { /* do something */ }
psokoloAuthor Commented:
I thought that DateTime? data type is nullable (I am using NET 2.0) isn't it ?

I am trying to pass value to data access method.
psokoloAuthor Commented:
for example this should not give error :
DateTime? date = null;
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> I thought that DateTime? data type is nullable (I am using NET 2.0) isn't it ?
No, it's not (see http://msdn2.microsoft.com/en-us/library/system.datetime.aspx). If you need to be able to assign a null value to your variable, you will have to use a non-struct/ValueType type (e.g. an object variable).
> for example this should not give error :
> DateTime? date = null;

It doesn't give an error, but after the statement, the variable still isn't null. It contains the "Default" value for the type, which happens to be DateTime.MinValue.
psokoloAuthor Commented:
I work with Typed Datesets. If I pass DateTime.MinValue to Insert method in TableAdapter it should insert null value to DB ?

Good to know that the values are not actually null.
Yes, Typed DataSets do a lot of work for you, and this could include converting DateTime.MinValue to DBNull.Value in some cases.
The compiler has answered you why it cannot compile.

when you use inline conditional statement (using ? character) then both sides (true and false) has to be of the same (or castable) type.

so you could correct your statement like this:

DateTime? st = this.SunSt.Text != "  :" ? DateTime.Parse(this.SunSt.Text) : (DateTime?)null;

and the compiler knows, that it should treat boh return values as DateTime?.

When you compare nullable type against null value then even the nullable type isn't really null it can evaluate true, so:

DateTime? dt = null;
if(dt == null)
   do something.

will execute "do something" block. But it is much better to check HasValue for null checking.

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