Lazy Loading in Entity Framework

Lazy loading is delaying the loading of related data, until you specifically request for it. It is the opposite of eager loading. For example, the Student entity contains the StudentAddress entity. In the lazy loading, the context first loads the Student entity data from the database, then it will load the StudentAddress entity when we access the StudentAddress property as shown below.

using (var ctx = new SchoolDBEntities())
{
    //Loading students only
    IList<Student> studList = ctx.Students.ToList<Student>();

    Student std = studList[0];

    //Loads Student address for particular Student only (seperate SQL query)
    StudentAddress add = std.StudentAddress;
}

The code shown above will result in two SQL queries. First, it will fetch all students:

SELECT 
[Extent1].[StudentID] AS [StudentID], 
[Extent1].[StudentName] AS [StudentName], 
[Extent1].[StandardId] AS [StandardId]
FROM [dbo].[Student] AS [Extent1]

Then, it will send the following query when we get the reference of StudentAddress:

exec sp_executesql N'SELECT 
[Extent1].[StudentID] AS [StudentID], 
[Extent1].[Address1] AS [Address1], 
[Extent1].[Address2] AS [Address2], 
[Extent1].[City] AS [City], 
[Extent1].[State] AS [State]
FROM [dbo].[StudentAddress] AS [Extent1]
WHERE [Extent1].[StudentID] = @EntityKeyValue1',N'@EntityKeyValue1 int',@EntityKeyValue1=1

Disable Lazy Loading

We can disable lazy loading for a particular entity or a context. To turn off lazy loading for a particular property, do not make it virtual. To turn off lazy loading for all entities in the context, set its configuration property to false.

using System;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Data.Entity.Core.Objects;
using System.Linq;
    
public partial class SchoolDBEntities : DbContext
{
    public SchoolDBEntities(): base("name=SchoolDBEntities")
    {
        this.Configuration.LazyLoadingEnabled = false;
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
    }
}

Rules for lazy loading:

1. context.Configuration.ProxyCreationEnabled should be true.
2. context.Configuration.LazyLoadingEnabled should be true.
3. Navigation property should be defined as public, virtual. Context will NOT do lazy loading if the property is not defined as virtual.