延迟加载是在第一次访问引用实体/实体的属性时从数据库自动加载实体或实体集合的过程.延迟加载意味着延迟加载相关数据，直到您特别请求它为止.
使用POCO实体类型时，懒惰通过创建派生代理类型的实例，然后覆盖虚拟属性以添加加载钩子来实现加载.
延迟加载几乎是默认加载.
如果您保留默认配置，并且没有在查询中明确告诉您想要延迟加载以外的其他内容，那么您将获得延迟加载.
例如，当使用Student实体类时，将在第一次访问Enrollments导航属性时加载相关的Enrollments.
导航属性应定义为public，virtual.如果属性未定义为虚拟，则上下文 NOT 执行延迟加载.
以下是Student类其中包含Enrollments的导航属性.
public partial class Student {
public Student() {
this.Enrollments = new HashSet<Enrollment>();
}
public int ID { get; set; }
public string LastName { get; set; }
public string FirstMidName { get; set; }
public System.DateTime EnrollmentDate { get; set; }
public virtual ICollection<Enrollment> Enrollments { get; set; }
}让我们看一个简单的例子，其中首先从数据库加载学生列表然后它将加载注册一个特殊的学生，只要你需要它.
class Program {
static void Main(string[] args) {
using (var context = new UniContextEntities()) {
//Loading students only
IList<Student> students = context.Students.ToList<Student>();
foreach (var student in students) {
string name = student.FirstMidName + " " + student.LastName;
Console.WriteLine("ID: {0}, Name: {1}", student.ID, name);
foreach (var enrollment in student.Enrollments) {
Console.WriteLine("Enrollment ID: {0}, Course ID: {1}",
enrollment.EnrollmentID, enrollment.CourseID);
}
}
Console.ReadKey();
}
}
}编译并执行上述代码时，您将收到以下输出.
ID: 1, Name: Ali Alexander Enrollment ID: 1, Course ID: 1050 Enrollment ID: 2, Course ID: 4022 Enrollment ID: 3, Course ID: 4041 ID: 2, Name: Meredith Alonso Enrollment ID: 4, Course ID: 1045 Enrollment ID: 5, Course ID: 3141 Enrollment ID: 6, Course ID: 2021 ID: 3, Name: Arturo Anand Enrollment ID: 7, Course ID: 1050 ID: 4, Name: Gytis Barzdukas Enrollment ID: 8, Course ID: 1050 Enrollment ID: 9, Course ID: 4022 ID: 5, Name: Yan Li Enrollment ID: 10, Course ID: 4041 ID: 6, Name: Peggy Justice Enrollment ID: 11, Course ID: 1045 ID: 7, Name: Laura Norman Enrollment ID: 12, Course ID: 3141
延迟加载和序列化不能很好地混合，如果你不小心，你可能最终只是因为启用了延迟加载而查询整个数据库.在序列化实体之前关闭延迟加载是一个好习惯.
延迟加载Enrollments集合可以通过使Enrollments属性非虚拟来关闭，如下例所示.
public partial class Student {
public Student() {
this.Enrollments = new HashSet<Enrollment>();
}
public int ID { get; set; }
public string LastName { get; set; }
public string FirstMidName { get; set; }
public System.DateTime EnrollmentDate { get; set; }
public ICollection<Enrollment> Enrollments { get; set; }
}可以为所有实体关闭延迟加载通过将Configuration属性上的标志设置为false来实现上下文，如以下示例所示.
public partial class UniContextEntities : DbContext {
public UniContextEntities(): base("name = UniContextEntities") {
this.Configuration.LazyLoadingEnabled = false;
}
protected override void OnModelCreating(DbModelBuilder modelBuilder) {
throw new UnintentionalCodeFirstException();
}
}关闭延迟加载后，现在再次运行上面的示例时，您会看到注册不是已加载并且仅检索学生数据.
ID: 1, Name: Ali Alexander ID: 2, Name: Meredith Alons ID: 3, Name: Arturo Anand ID: 4, Name: Gytis Barzduka ID: 5, Name: Yan Li ID: 6, Name: Peggy Justice ID: 7, Name: Laura Norman ID: 8, Name: Nino Olivetto
我们建议您在一个步骤中执行上述示例为了更好地理解，逐步的方式.
