nullable object must have a value

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.Value;
   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 regular DateTime object.

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

Note that oldDTE is not null. I've removed the Value from myNewDT.MyDateTime but the same exception is thrown due to a generated setter.

Yuliy

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.

When using LINQ extension methods (e.g. Select, Where), the lambda function might be converted to SQL that might not behave identically to your C# code. For instance, C#'s short-circuit evaluated || and && are converted to SQL's eager AND and OR. This can cause problems when you're checking for null in your lambda.

Example:

MyEnum? type = null;
Entities.Table.Where(a => type == null || 
    a.type == (int)type).ToArray();  // Exception: Nullable object must have a value

Try dropping the .value

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;

Assign the members directly without the .Value part:

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

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

I got this message when trying to access values of a null valued object.

sName = myObj.Name;

this will produce error. First you should check if object not null

if(myObj != null)
  sName = myObj.Name;

This works.

来源：https://stackoverflow.com/questions/1896185/nullable-object-must-have-a-value

标签

nullable

invalidoperationexception