EF Core Key Attribute

The Key attribute is used to denote the property that uniquely identifies an entity (the EntityKey), and which is mapped to the Primary Key field in a database:

public class Order
{
    [Key]
    public int OrderNumber { get; set; }
    public DateTime DateCreated { get; set; }
    public Customer Customer { get; set; }
    ...
)

In the example above, the name of the key field in the Order class does not meet the Entity Framework Core's naming convention for entity keys, so the Key attribute is required to configure the OrderNumber property as the key field.

Composite Keys

Entity Framework Core supports composite keys - primary key values generated from two or more fields in the database. However, unlike in previous versions of Entity Framework, you cannot use the Key attribute to define composite keys. This can only be done via the Fluent API.

Fluent API

The Fluent API equivalent to the Key attribute is the HasKey method.