How to allow null values on Foreign keys using EF?

Question

How to allow null values on Foreign keys using EF?

public class User
{
    public User() 
    { 
    }

    public int idUser; { get; set; }
    [Required]
    public string UserName { get; set; }
    public virtual Computer Computer{ get; set; }
}

public class Computer
{
    public Computer() 
    {
    }
    [Key, ForeignKey("User")]
    public int idUser{ get; set; }
    public string ComputerName {get;set;}
    public virtual User User { get; set; }
}

Running this I'm getting not null on the foreign key.

Solution 1

How to allow null values on Foreign keys using EF?

Your database table should allow nulls as well if you are planning to take the following approach:

change

public int idUser; { get; set; }
[Required]
public string UserName { get; set; }
public virtual Computer Computer { get; set; }

To

public int idUser; { get; set; }
[Required]
public string UserName { get; set; }
public virtual Computer Computer{ get; set; }

And change

[Key, ForeignKey("User")] 
public int idUser { get; set; }
public string ComputerName { get; set; }
public virtual User User { get; set; }

To

[Key, ForeignKey("User")] 
public int? idUser { get; set; }
public string ComputerName { get; set; }
public virtual User User { get; set; }

The ? character, makes a field nullable. Make sure it is nullable in your database as well.

Solution 2

I assume you are using code first. If that is the case then you override OnModelCreating in the context class and add the following line

modelBuilder.Entity<Computer>().HasOptional(c => c.User);

================ Ok, you changed your post after I posted my answer.

You can use

int? idUser

for your user ID, this will let EF know that you are allowing a nullable relationship.

Solution 3

You can make use of ? keyword. It makes your type a Nullable type.

For example int doesn't allow nulls normally.

But after appending a ? to int you can assign null to it.

int? iCanHaveNull = null;

Comments

[Lucie kulza]

what about string datatype?

[Max]

@Luciekulza A string is a reference type and nullable by default.

[Lucie kulza]

Need I to write the ? character on User? and int? the same time?

[Max]

Only for User and Computer, since those are the foreign keys you want to be nullable

[Keith]

What the @#!@#$? the ? character is to allow nullable types for basic data (int, double, bool, etc). A Class by definition is already a nullable type. This answer makes no sense.

[Max]

@Keith Database should be equal to code, if he decides to make this foreign key nullable at the database, same should be done in code for cosistency.

[Keith]

That's true, but your answer is still incorrect. you CANNOT use ? on an already nullable type. The ? MUST be on the int? idUser property. If you don't believe me, try your solution. It will NOT compile.

[oatsoda]

This answer is just plain wrong. As per @Keith comment, unless User is a struct then the code won't even compile.

[Sras]

only for int, but what about string or guid