执行原始的 SQL 查询

// Commenting out LINQ to show how to do the same thing in SQL.
            //IQueryable<EnrollmentDateGroup> = from student in db.Students
            //           group student by student.EnrollmentDate into dateGroup
            //           select new EnrollmentDateGroup()
            //           {
            //               EnrollmentDate = dateGroup.Key,
            //               StudentCount = dateGroup.Count()
            //           };

 //var data = db.Students.GroupBy(s => s.EnrollmentDate).Select(s => new EnrollmentDateGroup() { EnrollmentDate = s.Key, StudentCount = s.Count() });
            // SQL version of the above LINQ code.
            string query = "SELECT EnrollmentDate, COUNT(*) AS StudentCount "
                + "FROM Person "
                + "WHERE Discriminator = ‘Student‘ "
                + "GROUP BY EnrollmentDate";
            IEnumerable<EnrollmentDateGroup> data = db.Database.SqlQuery<EnrollmentDateGroup>(query);

禁止创建代理

有时需要禁止实体框架创建代理实例。例如，人们通常认为序列化非代理实例要比序列化代理实例容易得多。可通过清除 ProxyCreationEnabled 标记来关闭代理创建功能。上下文的构造函数便是可执行此操作的一个位置。例如：

public class BloggingContext : DbContext 
{ 
    public BloggingContext() 
    { 
        this.Configuration.ProxyCreationEnabled = false; 
    }  
 
    public DbSet<Blog> Blogs { get; set; } 
    public DbSet<Post> Posts { get; set; } 
}


请注意，在无需代理执行任何操作的情况下，EF 不会为类型创建代理。这意味着，也可以通过使用封装和/或没有虚拟属性的类型，避免生成代理。

Entity Framework Automatic Detect Changes 
 

When using most POCO entities the determination of how an entity has changed (and therefore which updates need to be sent to the database) is handled by the Detect Changes algorithm. Detect Changes works by detecting the differences between the current property values of the entity and the original property values that are stored in a snapshot when the entity was queried or attached. The techniques shown in this topic apply equally to models created with Code First and the EF Designer.

By default, the Entity Framework performs Detect Changes automatically when the following methods are called:
DbSet.Find
DbSet.Local
DbSet.Remove
DbSet.Add
DbSet.Attach
DbContext.SaveChanges
DbContext.GetValidationErrors
DbContext.Entry
DbChangeTracker.Entries

 

Disabling automatic detection of changes

If you are tracking a lot of entities in your context and you call one of these methods many times in a loop, then you may get significant performance improvements by turning off detection of changes for the duration of the loop. For example:

using (var context = new BloggingContext()) 
{ 
    try 
    { 
        context.Configuration.AutoDetectChangesEnabled = false; 
 
        // Make many calls in a loop 
        foreach (var blog in aLotOfBlogs) 
        { 
            context.Blogs.Add(blog); 
        } 
    } 
    finally 
    { 
        context.Configuration.AutoDetectChangesEnabled = true; 
    } 
}


Don’t forget to re-enable detection of changes after the loop — We‘ve used a try/finally to ensure it is always re-enabled even if code in the loop throws an exception.

An alternative to disabling and re-enabling is to leave automatic detection of changes turned off at all times and either call context.ChangeTracker.DetectChanges explicitly or use change tracking proxies diligently. Both of these options are advanced and can easily introduce subtle bugs into your application so use them with care.