c# – 带有标量结果的LINQ是否会触发延迟加载

我阅读了Entity Framework团队的 Loading Related Entities帖子,并对最后一段感到有些困惑：

Sometimes it is useful to kNow how many entities are related to another entity in the database without actually incurring the cost of loading all those entities. The Query method with the LINQ Count method can be used to do this. For example:

using (var context = new BloggingContext())
{
    var blog = context.Blogs.Find(1);

    // Count how many posts the blog has 
    var postCount = context.Entry(blog)
                          .Collection(b => b.Posts)
                          .Query()
                          .Count();
}

为什么这里需要查询计数方法？
难道我们不能简单地使用LINQ的COUNT方法吗？

var blog = context.Blogs.Find(1);
var postCount = blog.Posts.Count();

是否会触发延迟加载并且所有集合都将被加载到内存中,而不是我会得到我想要的标量值？

解决方法

第一种方法是不加载所有行,因为从IQueryable调用Count方法,但第二种方法是加载所有行,因为它是从ICollection调用的.

我做了一些测试来验证它.我用Table1和Table2测试了它,其中Table1具有PK“Id”并且Table2具有FK“Id1”(1：N).我从这里使用了EF profiler http://efprof.com/.

第一种方法：

var t1 = context.Table1.Find(1);

var count1 = context.Entry(t1)
                        .Collection(t => t.Table2)
                        .Query()
                        .Count();

否选择*来自表2：

SELECT TOP (2) [Extent1].[Id] AS [Id]
FROM   [dbo].[Table1] AS [Extent1]
WHERE  [Extent1].[Id] = 1 /* @p0 */

SELECT [GroupBy1].[A1] AS [C1]
FROM   (SELECT COUNT(1) AS [A1]
        FROM   [dbo].[Table2] AS [Extent1]
        WHERE  [Extent1].[Id1] = 1 /* @Entitykeyvalue1 */) AS [GroupBy1]

第二种方法：

var t1 = context.Table1.Find(1);
var count2 = t1.Table2.Count();

表2加载到内存中：

SELECT TOP (2) [Extent1].[Id] AS [Id]
FROM   [dbo].[Table1] AS [Extent1]
WHERE  [Extent1].[Id] = 1 /* @p0 */

SELECT [Extent1].[Id]  AS [Id],[Extent1].[Id1] AS [Id1]
FROM   [dbo].[Table2] AS [Extent1]
WHERE  [Extent1].[Id1] = 1 /* @Entitykeyvalue1 */

为什么会这样？

Collection(t => t.Table2)的结果是一个实现ICollection的类,但它没有加载所有行,并且有一个名为IsLoaded的属性. Query方法的结果是IQueryable,这允许调用Count而不预加载行.

t1.Table2的结果是ICollection,它正在加载所有行以获取计数.顺便说一句,即使您只使用t1.Table2而不要求计数,行也会被加载到内存中.