Data Annotations in EF Core

What are Data Annotations?

Data Annotations are attributes applied to entity classes and properties to configure entity mappings and validation rules. They provide a simpler alternative to Fluent API for basic configurations.

Common Data Annotations

Key Attributes

using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

public class Product
{
    [Key]
    public int Id { get; set; }
    
    [Required]
    [MaxLength(200)]
    public string Name { get; set; }
    
    [Column(TypeName = "decimal(18,2)")]
    public decimal Price { get; set; }
    
    [StringLength(1000)]
    public string Description { get; set; }
}

Table and Column Mapping

[Table("Products", Schema = "dbo")]
public class Product
{
    [Column("ProductId")]
    public int Id { get; set; }
    
    [Column("ProductName", Order = 1)]
    public string Name { get; set; }
}

Relationships

public class Post
{
    public int Id { get; set; }
    public string Title { get; set; }
    
    [ForeignKey("Blog")]
    public int BlogId { get; set; }
    
    public Blog Blog { get; set; }
}

public class Blog
{
    public int Id { get; set; }
    public string Name { get; set; }
    
    [InverseProperty("Blog")]
    public ICollection<Post> Posts { get; set; }
}

Validation Attributes

public class User
{
    [Required]
    [EmailAddress]
    public string Email { get; set; }
    
    [Range(18, 100)]
    public int Age { get; set; }
    
    [RegularExpression(@"^[a-zA-Z0-9]*$")]
    public string Username { get; set; }
    
    [Url]
    public string Website { get; set; }
    
    [Phone]
    public string PhoneNumber { get; set; }
}

Index Attributes

[Index(nameof(Email), IsUnique = true)]
[Index(nameof(LastName), nameof(FirstName))]
public class User
{
    public int Id { get; set; }
    public string Email { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

Composite Keys

public class OrderItem
{
    [Key]
    [Column(Order = 0)]
    public int OrderId { get; set; }
    
    [Key]
    [Column(Order = 1)]
    public int ProductId { get; set; }
    
    public int Quantity { get; set; }
}

Not Mapped

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
    
    [NotMapped]
    public decimal DiscountedPrice => Price * 0.9m;
}

Timestamp/Concurrency

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    
    [Timestamp]
    public byte[] RowVersion { get; set; }
    
    [ConcurrencyCheck]
    public decimal Price { get; set; }
}

Data Annotations vs Fluent API

When to Use Each

Use Data Annotations For:

• Simple property configurations
• Validation rules
• Basic table/column mappings
• Quick prototyping

Use Fluent API For:

• Complex relationships
• Advanced configurations
• Separation of concerns
• Configuration reusability

Summary

Data Annotations provide attribute-based configuration for EF Core entities. They’re simpler than Fluent API but offer fewer options. Use them for basic configurations and validation, and Fluent API for complex scenarios.