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nullable object must have a value

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What does Nullable object must have a value mean?

According to C# Nullable Documentation If a value of type T has not been assigned to the Nullable object, you can compare it to null and retrieve its HasValue property, but you cannot access its Value property or call its other members.

What is a Nullable object?

In object-oriented computer programming, a null object is an object with no referenced value or with defined neutral ("null") behavior.

What are nullable types in C#?

C# provides a special data types, the nullable types, to which you can assign normal range of values as well as null values. For example, you can store any value from -2,147,483,648 to 2,147,483,647 or null in a Nullable<Int32> variable. Similarly, you can assign true, false, or null in a Nullable<bool> variable.

How do you make a field Nullable in C#?

You can declare nullable types using Nullable<t> where T is a type. Nullable<int> i = null; A nullable type can represent the correct range of values for its underlying value type, plus an additional null value. For example, Nullable<int> can be assigned any value from -2147483648 to 2147483647, or a null value.


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

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

Assign the members directly without the .Value part:

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

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;