Data Annotations in Entity Framework Code First

Introduction

Entity Framework provides two ways to add configuration to model classes.

• DataAnnotation Attribute - Data Annotation Attributes can only add the some property of the table.
  
  Namespace ( System.ComponentModel.DataAnnotations)
  
  
• DataAnnotation Schema Attribute - Data Annotation Schemas are used to change or modify the structure of table.
  
  Namespace (System.ComponentModel.DataAnnotations.Schema)

Data Annotation Attributes

• Key: Key is mainly used to define the primary or composite key on table.
  
  
• ConcurrencyCheck: ConcurrencyCheck annotation allows you to flag one or more properties to be used for concurrency checking in the database when a user edits or deletes an entity.
  
  
• StringLength: StringLength is user to define the length of string allowed in table
  
  
• MinLength: MinLength define the minimum required length of string.
  
  
• MaxLength: MaxLength define the minimum required length of string.
  
  
• Required: The Required annotation will ensure that property has data in it.

Data Annotation Schema Attributes:

• Table: Provides the alternate table name.
  
• Column: Used to provide the column name or column schema.
  
• Foreign Key: Create a relationship 
  
• Index: To defined the clustered/non clustered index.
  
• Complex Type: Make the class ascComplex type.

Example:

Here is the example code of how these attributes can be used.

Employee Class:

using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
namespace EntityFrameworkCodeFirst
{
    [Table("tblEmp")]
    public class Employee
    {
        //Defined Primary Key
        [Key]
        public int ID
        {
            get;
            set;
        }
        [Required]
        public string Title
        {
            get;
            set;
        }
        [StringLength(25)]
        public string EmployeeName
        {
            get;
            set;
        }
        [MinLength(5)]
        public decimal Salary
        {
            get;
            set;
        }
        //Foreign key for Department
        [ForeignKey("DepartmentID")]
        public int DepartmentID
        {
            get;
            set;
        }
    }
}

Department Class

public class Department
{
    public int DepartmentID
    {
        get;
        set;
    }
    public string DepartmentName
    {
        get;
        set;
    }
    public List < Employee > Employees
    {
        get;
        set;
    }
}