Configurations in Entity Framework Core

Last Updated: August 14, 2020

In the previous tutorial I explained the Conventions in Entity Framework Core. If you want to override the conventions then apply the configurations and customize the EF Core model to database mappings.

This tutorial is a part of Entity Framework Core series.

There are 2 Configuration Methods in EF Core:

1. Data Annotation Attributes
2. Fluent API

Data Annotation Attributes

These are the .NET Attributes that can be applied to the domain classes and their properties. The Data Annotation attributes are included in separate namespace called System.ComponentModel.DataAnnotations.

Let me demonstrate how to apply these Data Annotation Attributes to the domain classes in order to configure database tables and override the default Conventions of Entity Framework Core.

In the below code I have applied Data Annotation Attributes to the domain classes and their properties.

[Table("TBL_City")]
public class City
{
    [Key]
    public int KeyId { get; set; }
 
    [Column("CityName", TypeName = "varchar(25)")]
    public string Name { get; set; }
 
    [NotMapped]
    public int Population { get; set; }
 
    [ForeignKey("FKid")]
    public Country Country { get; set; } 
}

[Table("TBL_Country")]
public class Country
{
    [Key]
    public int KeyId { get; set; }
 
    [MaxLength(20)]
    public string Name { get; set; }
}

public class CompanyContext : DbContext
{
    public DbSet<City> City { get; set; }
 
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        if (!optionsBuilder.IsConfigured)
        {
            optionsBuilder.UseSqlServer(@"Server=vaio;Database=Company;Trusted_Connection=True;");
        }
    }
}

Once you apply Migration you will get 2 database tables created with different names (due to [Table("Name")] attribute on the classes). These tables are:

1. TBL_City
2. TBL_Country

Data Annotation Attributes are widely used in validating input controls placed on the UI. I have covered there uses in my tutorial called Model Validation in ASP.NET Core from Beginning to Expert.

TBL_City

The TBL_City has:

KeyId column as primary key.
CityName column with data type varchar(25).
FKid as foreign key column.

Since the Population property has [NotMapped] attribute so no column is made for it on the TBL_City database table.

The TBL_Country has:

KeyId column as primary key.
Name column with data type nvarchar(20).

See the below image which shows the snapshot of these 2 database tables.

Understanding how Data Annotation Attributes work

Attribute	Description
Table	Applied on entity class to give a name to database table.
Column	Applied on a property to give column name, order and data type.
Key	Sets the property as primary key for the table.
ForeignKey	Applied to a property to mark it as foreign key.
NotMapped	Can be applied to entity class or property for not generating a corresponding table or column in the database.
MaxLength	Sets the max length for the table column.
Required	Can be applied on properties to make the corresponding column on the table as not null.