加快性能-LINQ在一个LIST中获得项目，而不在另一个列表中

Question

我有两个列表，我想返回不在另一个列表中的项目。 这是我的代码：

var Results = ListOne.Where(x => ListTwo.All(a => a.EmployeeNum != x.EmployeeNum && a.Sched != x.Sched));

这大约需要9-10秒才能完成。 ListOne大约有1200条记录，ListTwo大约有33000条记录。

Answer 1

使用HashSet<T> ，因为它具有O(1)搜索时间，可以提高性能，例如

var hashSet = new HashSet<T>(ListTwo.Select(x => Tuple.Create(x.EmployeeNum, x.Sched)));
var results = ListOne.Where(x => !hashSet.Contains(Tuple.Create(x.EmployeeNum, x.Sched)));

Answer 2

您还可以创建自己的IEqualityComparer（假设您有一个名为Employee的类）：

var results = ListTwo.Except(ListOne, new EmployeeComparer());

IEqualityComparer实现：

public class EmployeeComparer : IEqualityComparer<Employee>
{
    public int GetHashCode(Employee co)
    {
        if (co == null)
        {
            return 0;
        }

        return co.EmployeeNum.GetHashCode();
    }

    public bool Equals(Employee x1, Employee x2)
    {
        if (object.ReferenceEquals(x1, x2))
        {
            return true;
        }

        if (object.ReferenceEquals(x1, null) || object.ReferenceEquals(x2, null))
        {
            return false;
        }

        return x1.EmployeeNum == x2.EmployeeNum && x1.Sched == x2.Sched;
    }
}

Answer 3

尝试这个

var Results = ListOne.AsParallel().Where(x => ListTwo.All(a => a.EmployeeNum != x.EmployeeNum && a.Sched != x.Sched)).ToList();