Data Annotations in Entity Framework

The [Table] attribute specifies the name of the table associated with an entity.

[Table("Products")]  
public class Product  
{  
    public int Id { get; set; }  
    public string Name { get; set; }  
}  

The [Column] attribute allows specifying the name of the database column that corresponds to a property.

public class Student
{
    public int StudentId { get; set; }

    [Column("Full_Name")] // Maps the property "Name" to the column "Full_Name"
    public string Name { get; set; }
}

The [DataType] attribute allows specifying the data type of a property.

public class Student
{
    public int StudentId { get; set; }

    [DataType(DataType.EmailAddress)] // Specifies that this property should be an email address
    public string Email { get; set; }
}

The [Key] attribute is used to mark a property as the primary key of the entity.

public class Student
{
    [Key] // Marks this property as the primary key
    public int StudentId { get; set; }

    public string Name { get; set; }
}

The [DatabaseGenerated] attribute controls how values of a property are generated in the database. It is especially useful for auto-increment fields, calculated values, or automatically generated dates.

public class Product
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]  
    public int Id { get; set; } // Automatically generated upon insertion (IDENTITY)

    [DatabaseGenerated(DatabaseGeneratedOption.Computed)]  
    public DateTime LastUpdated { get; set; } // Calculated by the database (e.g., GETDATE())

    [DatabaseGenerated(DatabaseGeneratedOption.None)]  
    public string ProductCode { get; set; } // Not automatically generated (manual value)
}

The [Required] attribute is used to indicate that a property cannot be null in the database. It is similar to the NOT NULL constraint in SQL.

public class Student
{
    public int StudentId { get; set; }

    [Required] // Indicates that the name is required
    public string Name { get; set; }
}

The [MaxLength] attribute allows setting a limit for the length of a string.

public class Product  
{  
    [MaxLength(100)]  
    public string Name { get; set; } // NVARCHAR(100)  
}  

The [StringLength] attribute defines a maximum length and optionally a minimum one.

[StringLength(50, MinimumLength = 3)]  
public string ShortDescription { get; set; }  

The [Range] attribute is used to specify a range of valid values for a numeric property.

public class Course
{
    public int CourseId { get; set; }

    [Range(1, 10)] // The range of allowed values for the number of credits
    public int Credits { get; set; }
}

The [ForeignKey] attribute is used to establish a relationship between two entities. In this case, it can be used to specify that a property is a foreign key referring to another entity.

public class Enrollment
{
    public int EnrollmentId { get; set; }

    [ForeignKey("Student")] // Establishes the relationship with the Student entity
    public int StudentId { get; set; }
    
    public Student Student { get; set; }  // Navigation property
}

This attribute allows EF Core to know that StudentId is a foreign key referencing the Student table.

The [InverseProperty] attribute is used to specify the inverse navigation property in a relationship between two entities.

This is useful when there are multiple relationships between the same entities, and EF Core needs help determining how they relate.

public class Student
{
    public int StudentId { get; set; }
    public string Name { get; set; }

    [InverseProperty("Student")] // Establishes the inverse navigation property
    public ICollection<Enrollment> Enrollments { get; set; }
}

This attribute tells EF Core that the Enrollments property in Student is the inverse of the Student property in Enrollment, avoiding ambiguities in relationship mapping.