Lazy Loading:

One of the important functions of Entity Framework is lazy loading. Lazy loading means delaying the loading of related data, until you specifically request for it. For example, Student class contains StudentAddress as a complex property. So, the context first loads all the students from the database, then it will load the address of a particular student when we access 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]

The, 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

However, you can also turn off lazy loading for a particular property or an entire 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)
    {
        throw new UnintentionalCodeFirstException();
    }
}

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.

Learn how to load entities explicitly in the next section.