You cannot store a null value into an int.
int a = null; // error !
What you want is a nullable int:
Nullable<int> a = null;
or
int? a = null;
Both are equivalent
In your example, article.Days
must be nullable
You can configure it into your database and your entity will have nullable int for this property
Then, you will be able to use ??
on it in your linq query
Simple example:
int? a = null;
int b = a ?? 5; // if a is null then store 5, otherwise store value of a