HasValue() or ?? operand when dealing with nullable types in LINQ-to-Entity,

Question

I have had the following code to assign a value to nullable int variable:

ParentCommentId = lac.ParentCommentId ?? lac.ParentCommentId.Value,

However, with this code I was receiving the Nullable object must have a value error.

Then, I revised the code as follows:

ParentCommentId = lac.ParentCommentId.HasValue ? lac.ParentCommentId.Value : null,

And, now everything works fine. I wonder why ?? operand does not work in this case. Am I using it wrong? In what cases, ?? would be more suitable?

Answer 1

Nullable object must have a value is a runtime exception that occurs when you try to access .Value of a nullable with .HasValue false.

Your code:

ParentCommentId = lac.ParentCommentId ?? lac.ParentCommentId.Value

gets translated to:

if (lac.ParentCommentId.HasValue)
{
    ParentCommentId = lac.ParentCommentId.Value;
}
else
{
    ParentCommentId = lac.ParentCommentId.Value;
}

As you can see, both branches are doing the same and you would be accessing .Value even if HasValue is false (and will result in that exception).

The operator ?? is used to take the first not-null value. You can write

var value = value1 ?? value2 ?? value3;

The first of value1, value2, value3 that is not null will be assigned to value, if all are null then value will be set to null.