使用LINQ获取一个List <>中的项目,这些项目不在另一个List <>中

Question

我会假设有一个简单的LINQ查询来执行此操作,我只是不确定如何.请参阅下面的代码段.

class Program
{
    static void Main(string[] args)
    {
        List<Person> peopleList1 = new List<Person>();
        peopleList1.Add(new Person() { ID = 1 });
        peopleList1.Add(new Person() { ID = 2 });
        peopleList1.Add(new Person() { ID = 3 });

        List<Person> peopleList2 = new List<Person>();
        peopleList2.Add(new Person() { ID = 1 });
        peopleList2.Add(new Person() { ID = 2 });
        peopleList2.Add(new Person() { ID = 3 });
        peopleList2.Add(new Person() { ID = 4 });
        peopleList2.Add(new Person() { ID = 5 });
    }
}

class Person
{
    public int ID { get; set; }
}

我想执行一个LINQ查询给我所有peopleList2不在peopleList1这个例子中的人应该给我两个人(ID = 4&ID = 5)

Answer 1

var result = peopleList2.Where(p => !peopleList1.Any(p2 => p2.ID == p.ID));

Answer 2

如果覆盖People的相等性,那么您也可以使用:

peopleList2.Except(peopleList1)

Except应该比Where(...Any)变体快得多,因为它可以将第二个列表放入哈希表中.Where(...Any)具有运行时,O(peopleList1.Count * peopleList2.Count)而基于HashSet<T>(几乎)的变体具有运行时O(peopleList1.Count + peopleList2.Count).

Except隐式删除重复项.这不应该影响你的情况,但可能是类似案件的问题.

或者,如果您想要快速代码但不想覆盖相等性:

var excludedIDs = new HashSet<int>(peopleList1.Select(p => p.ID));
var result = peopleList2.Where(p => !excludedIDs.Contains(p.ID));

此变体不会删除重复项.

Answer 3

或者,如果你想要它没有否定:

var result = peopleList2.Where(p => peopleList1.All(p2 => p2.ID != p.ID));

基本上它说来自peopleList2的所有内容,其中peopleList1中的所有ID都与peoplesList2中的id不同.

从接受的答案中稍微有点不同的方法:)

Answer 4

由于迄今为止所有解决方案都使用了流畅的语法,因此这里是查询表达式语法的解决方案,对于那些感兴趣的人:

var peopleDifference = 
  from person2 in peopleList2
  where !(
      from person1 in peopleList1 
      select person1.ID
    ).Contains(person2.ID)
  select person2;

我认为它与某些人感兴趣的答案有所不同,甚至认为它很可能对列表不是最理想的.现在对于具有索引ID的表,这肯定是要走的路.

Answer 5

派对迟到了但是Linq与SQL兼容的一个很好的解决方案是:

List<string> list1 = new List<string>() { "1", "2", "3" };
List<string> list2 = new List<string>() { "2", "4" };

List<string> inList1ButNotList2 = (from o in list1
                                   join p in list2 on o equals p into t
                                   from od in t.DefaultIfEmpty()
                                   where od == null
                                   select o).ToList<string>();

List<string> inList2ButNotList1 = (from o in list2
                                   join p in list1 on o equals p into t
                                   from od in t.DefaultIfEmpty()
                                   where od == null
                                   select o).ToList<string>();

List<string> inBoth = (from o in list1
                       join p in list2 on o equals p into t
                       from od in t.DefaultIfEmpty()
                       where od != null
                       select od).ToList<string>();

感谢http://www.dotnet-tricks.com/Tutorial/linq/UXPF181012-SQL-Joins-with-C

Answer 6

克劳斯的答案很棒,但ReSharper会要求你"简化LINQ表达":

var result = peopleList2.Where(p => peopleList1.All(p2 => p2.ID != p.ID));

Answer 7

此Enumerable Extension允许您定义要排除的项列表以及用于查找用于执行比较的键的函数.

public static class EnumerableExtensions
{
    public static IEnumerable<TSource> Exclude<TSource, TKey>(this IEnumerable<TSource> source,
    IEnumerable<TSource> exclude, Func<TSource, TKey> keySelector)
    {
       var excludedSet = new HashSet<TKey>(exclude.Select(keySelector));
       return source.Where(item => !excludedSet.Contains(keySelector(item)));
    }
}

你可以这样使用它

list1.Exclude(list2, i => i.ID);