实体的主键字段设置为0而不是null

Question

I have a Person entity type like so:

public partial class Person
{    
    public int PersonID { get; set; }
    public byte Gender { get; set; }
    public string IDNumber { get; set; }
}

When I create a new Person for the purpose of saving him to the db, the primary key, PersonID, is set to 0, instead of null.

the code:

var theNewGuy = new MyEntities.Person();

right after this line, theNewGuy.PersonID = 0. The question is, how would I tell it to insert a new Person and assign it the next available primary key? Obviously, I don't want it to be 0, yet there is already a Person in the db with PersonID = 0;

The save code:

Velo.People.Add(theNewGuy);
Velo.SaveChanges();//throws error

Exception thrown: Violation of PRIMARY KEY constraint 'PK_PERSON'. Cannot insert duplicate key in object 'dbo.Person'. The duplicate key value is (0). The statement has been terminated.

Answer 1

Primary keys cannot be Null. A PK must be unique, and Null isn't. Also, int is not a nullable type, so it's default value is 0.

EntityFramework should be able to figure out that PersonID is Person 's key, but just in case try sticking the [Key] attribute on there. Additionally, you could force it to be an auto increment column by adding the [DatabaseGenerated(DatabaseGeneratedOption.Identity)] attribute to your PersonID property.

Answer 2

Since PersonID is int it never can be null . Thus Entity Framework assigns 0 as default value. But no worries - if you will add such newly created Entity to database, everything will be fine - it will be saved, given new PersonID based on what's already in database, and entity in memory will be refreshed to reflect the new id given by database.

Answer 3

If you define public int? PersonID { get; set; } (using ? in the type of object) yo can use null as value.

However, how it has been commented, a primary key it would not be null, but perhaps in you particular case in an itermediate state is can be useful.

If you are creating your entity model from a database, as a primary key it would not be null, in the design of the database you can't set this field as nullable, so you must add the "?" modified manually.