LINQ：按子列表中的值過濾

Question

我已經獲得了這樣的列表：

int[] listOfUserIds = new int[]{1,2,5};

var groups = db.some_table.Where(x => x.isOpen == true)
                        .Select(t => new Models.XModel() {
                            Id = t.Id,
                            Name = t.name,
                            Users = t.Users.Where(x => x.Age > 25).Select(user => new Models.UsersModel()
                            {
                                Name = user.Name,
                                UserId = user.UserId
                            })
                        });

現在，從這個groups列表中，我想獲取至少有一個UserId在listOfUserIds內的user的listOfUserIds 。

如何在沒有 for 循環的情況下輕松實現這一目標？

Answer 1

在您的Where子句中再添加 1 個條件：

x.Users.Any(user => listOfUserIds.Contains(user.UserId))

您的代碼：

var groups = db.some_table.Where(x => x.isOpen == true && x.Users.Any(user => listOfUserIds.Contains(user.UserId)))
                        .Select(t => new Models.SomeModel() {
                            Id = t.Id,
                            Name = t.name,
                            Users = t.Users.Where(x => x.Age > 25).Select(user => new Models.UsersModel()
                            {
                                Name = user.Name,
                                UserId = user.UserId
                            })
                        });

為了提高性能，您可以構建一個HashSet<int> ：

int[] listOfUserIds = new int[]{1,2,5};
HashSet<int> hash = new HashSet<int>(listOfUserIds);

var groups = db.some_table.Where(x => x.isOpen == true && x.Users.Any(user => hash.Contains(user.UserId)))
                            .Select(t => new Models.SomeModel() {
                                Id = t.Id,
                                Name = t.name,
                                Users = t.Users.Where(x => x.Age > 25).Select(user => new Models.UsersModel()
                                {
                                    Name = user.Name,
                                    UserId = user.UserId
                                })
                            });

使用HashSet<int> ，這個操作hash.Contains(x.Id)是O(1)而不是O(n)和listOfUserIds