如何使用方法语法重写此行？

Question

我正在使用SMO,我有这行代码:

var results = (from User user in database.Users
    where user.LoginType == LoginType.WindowsUser
    select new { user.Name, user.Login }).ToList();

但无论出于何种原因,我无法像这样编写查询:

var results = database.Users
              .Where(user => user.LoginType == LoginType.WindowsUser)
              .Select(user => new { Name = user.Name, Login = user.Login })
              .ToList();

我得到的错误是 'UserCollection' does not contain a definition for 'Where' and no extension method 'Where' accepting a first argument of type 'UserCollection' could be found (are you missing a using directive or an assembly reference?)

但据我所知,这两种说法都是一致的.

为什么会这样？

Answer 1

不,他们有点不同.因为你在这里有一个显式类型的范围变量:

from User user in database.Users

您的查询相当于:

var results = database.Users
                      .Cast<User>();
                      .Where(user => user.LoginType == LoginType.WindowsUser)
                      .Select(user => new { user.Name, user.Login })
                      .ToList();

大概是你的UserCollection类型实现IEnumerable,但不是IEnumerable<User>- 这就是你需要演员的原因.

您可以通过隐式键入范围变量来激活查询表达式的相同错误:

// This will fail
var results = (from user in database.Users
               where user.LoginType == LoginType.WindowsUser
               select new { user.Name, user.Login }).ToList();