nullable object must have a value

Question

There is paradox in the exception description: Nullable object must have a value (?!)

This is the problem:

I have a DateTimeExtended class, that has

{
  DateTime? MyDataTime;
  int? otherdata;

}

and a constructor

DateTimeExtended(DateTimeExtended myNewDT)
{
   this.MyDateTime = myNewDT.MyDateTime;
   this.otherdata = myNewDT.otherdata;
}

running this code\

DateTimeExtended res = new DateTimeExtended(oldDTE);

throws an InvalidOperationException with the message:

Nullable object must have a value.

myNewDT.MyDateTime.Value - is valid and contain a regulare DateTime object.

What is the meaning of this message and what am I doing wrong ?

* EDIT: oldDTE is NOT NULL I've removed the Value from myNewDT.MyDateTime but same exception thrown.**

Thanks.

Answer 1

Try dropping the .value

Answer 2

Assign the members directly without the .Value part:

DateTimeExtended(DateTimeExtended myNewDT)
{
   this.MyDateTime = myNewDT.MyDateTime;
   this.otherdata = myNewDT.otherdata;
}

Answer 3

In this case oldDTE is null, so when you try to access oldDTE.Value the InvalidOperationException is thrown since there is no value. In your example you can simply do:

this.MyDateTime = newDT.MyDateTime;

Answer 4

You should change the line this.MyDateTime = myNewDT.MyDateTime.Value; to just this.MyDateTime = myNewDT.MyDateTime;

The exception you were receiving was thrown in the .Value property of the Nullable DateTime, as it is required to return a DateTime (since that's what the contract for .Value states), but it can't do so because there's no DateTime to return, so it throws an exception.

In general, it is a bad idea to blindly call .Value on a nullable type, unless you have some prior knowledge that that variable MUST contain a value (i.e. through a HasValue check).

EDIT

Here's the code for DateTimeExtended that does not throw an exception:

class DateTimeExtended
{
    public DateTime? MyDateTime;
    public int? otherdata;

    public DateTimeExtended() { }

    public DateTimeExtended(DateTimeExtended other)
    {
        this.MyDateTime = other.MyDateTime;
        this.otherdata = other.otherdata;
    }
}

I tested it like this:

DateTimeExtended dt1 = new DateTimeExtended();
DateTimeExtended dt2 = new DateTimeExtended(dt1);

Adding the .Value on other.MyDateTime causes an exception. Removing it gets rid of the exception. I think you're looking in the wrong place.

Answer 5

Looks like oldDTE.MyDateTime was null, so constructor tried to take it's Value - which threw.