How do I translate a LINQ expression with more tha

问题:

This question already has an answer here:

• Nested “from” LINQ query expressed with extension methods 3 answers

Iam using the Entity Framework. How would i write the following Linq-Code in Lambda C#?

var users = (from u in context.Users.ToList()
                         from e in u.Events
                         where e.EventName == eventName //Name of the Event, the user is related to
                         select u.FirstName + u.LastName).ToList();

Can't get through Lambda in this situation.

回答1:

The equivalent in "Method Chains" (that's what it's called) looks like this:

context.Users.SelectMany(u => u.Events
                               .Where(e => e.EventName == eventName)
                               .Select(e => u.FirstName + u.LastName))
             .ToList();

Please note that I omitted the call to ToList() on context.Users because it seems unnecessary.

回答2:

context.Users.SelectMany(u => u.Events.Where(e => e.EventName == eventName).
                                       Select(e => u.FirstName + u.LastName)).
              ToList();

回答3:

var users = context.Users
            .Where(u => u.Events.Any(q => q.EventName == eventName))
            .Select(m => m.FirstName + m.LastName).ToList();

Update, adding test related classes;

public class User
{
    public List<Event> Events { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class Event
{
    public string EventName { get; set; }
}

and code;

string eventName = "a";

List<User> t = new List<User>();

Event event1 = new Event() { EventName = "a" };
Event event2 = new Event() { EventName = "b" };
Event event3 = new Event() { EventName = "c" };

User user1 = new User() { FirstName = "x", LastName = "y" };
User user2 = new User() { FirstName = "x1", LastName = "y1" };
User user3 = new User() { FirstName = "x2", LastName = "y2" };

user1.Events = new List<Event>();
user2.Events = new List<Event>();
user3.Events = new List<Event>();

user1.Events.Add(event1);
user1.Events.Add(event2);

user2.Events.Add(event2);
user2.Events.Add(event3);

user3.Events.Add(event1);
user3.Events.Add(event3);

t.Add(user1);
t.Add(user2);
t.Add(user3);

var bb = t.Where(u => u.Events.Any(q => q.EventName == eventName)).Select(m => m.FirstName + m.LastName).ToList();