可以分配的自定义结构什么都没有和/或没有DBNull？

Question

Please can any one advise me if it is possible to decalre a custom structure that can be assigned Nothing and / or DbNull.Values, and also can be instanciated as Nothing?

What I am looking to do is ceate a custom DateTime object that can recieve a DBNull.Value from a database query and also start life as Nothing. IS this possible?

Thanks in advance.

Best regards, Duane

Answer 1

Seems like Nullable<DateTime> ( DateTime? for short, or Date? in VB.NET) gets you almost all the way there. You just need to specially deal with the conversion to/from DBNull on your own.

// You can set a DateTime? to null.
DateTime? d = null;

// You can also set it to a DateTime.
d = DateTime.Now;

// You can check whether it's null in one of two ways:
if (d == null || !d.HasValue) // (These mean the same thing.)
{ }

// Boxing a DateTime? will either result in null or a DateTime value.
SetDatabaseValue(d);

// As for conversions from DBNull, you'll have to deal with that yourself:
object value = GetDatabaseValue();
d = value is DBNull ? null : (DateTime?)value;

Answer 2

If you're working with DataSets, use the Field and SetField DataRow extension methods . These allow you to use Nullable types without worrying about DBNull anymore.

Eg, suppose the "MyDateField" field (of type DateTime) is nullable. Then you can do something like this:

foreach (var row in myDataTable)
{
    // will return null if the field is DbNull
    var currentValue = row.Field<DateTime?>("MyDateField");

    // will set the value to DbNull.Value
    row.SetField<DateTime?>("MyDateField", null);
}