How Can LINQ Efficiently Identify Elements Unique to One List Among Multiple Lists?

Use LINQ to determine unique elements in a list

We need to identify elements that only exist in one list and not in other lists. LINQ provides a neat solution to this challenge.

Consider the following code snippet:

class Program
{
    static void Main(string[] args)
    {
        // 初始化两个Person对象列表。
        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; }
}

Suppose we want to determine which people in peopleList2 do not exist in peopleList1. To do this, we can make use of LINQ's Where() method.

One way is to use the Any() method:

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

This query checks each person in peopleList2 to see if there is anyone in peopleList1 with the same ID. If there is no match, the person is included in the results.

Or, a more concise way of expressing the same logic is:

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

In this case, All() checks whether the ID of all people in peopleList1 is different from the ID of the current person in peopleList2. If this condition is true, the person is included in the results.

Note: It is important to note that both methods perform O(n*m) operations, which may cause performance issues on large datasets. Additional methods may be needed to optimize performance. The provided LINQ syntax provides a simple solution to the problem, but it is always recommended to evaluate it based on the performance requirements of the project.

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