UnionBy 方法的使用

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如下範例，
1、指定一元素當 key，而當 key 值相同時，則視為同一筆資料。
2、key 還可以一次指定多個。
3、當在比較時，被視為相同資料時，則以基底為準。

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List<Person> list1 = new List<Person>{
    new Person { Id = 1, Name = "John", Body = "fat" },
    new Person { Id = 2, Name = "Alice", Body = "fat" }
};

List<Person> list2 = new List<Person>{
    new Person { Id = 2, Name = "Alice", Body = "thin" },
    new Person { Id = 3, Name = "Kate", Body = "thin" }
};

var result = list1.UnionBy(list2, p => p.Id);
foreach (var person in result)
{
    Console.WriteLine($"Id: {person.Id}, Name: {person.Name}, Body: {person.Body}");
}
Console.WriteLine();
//output:
//Id: 1, Name: John, Body: fat
//Id: 2, Name: Alice, Body: fat
//Id: 3, Name: Kate, Body: thin

var result2 = list2.UnionBy(list1, p => p.Id);
foreach (var person in result2)
{
    Console.WriteLine($"Id: {person.Id}, Name: {person.Name}, Body: {person.Body}");
}
Console.WriteLine();
//output:
//Id: 2, Name: Alice, Body: thin
//Id: 3, Name: Kate, Body: thin
//Id: 1, Name: John, Body: fat

var result3 = list1.UnionBy(list2, p => new { p.Id, p.Name });
foreach (var person in result3)
{
    Console.WriteLine($"Id: {person.Id}, Name: {person.Name}, Body: {person.Body}");
}
Console.WriteLine();
//output:
//Id: 1, Name: John, Body: fat
//Id: 2, Name: Alice, Body: fat
//Id: 3, Name: Kate, Body: thin

var result4 = list1.UnionBy(list2, p => new { p.Id, p.Name, p.Body });
foreach (var person in result4)
{
    Console.WriteLine($"Id: {person.Id}, Name: {person.Name}, Body: {person.Body}");
}
Console.WriteLine();
//output:
//Id: 1, Name: John, Body: fat
//Id: 2, Name: Alice, Body: fat
//Id: 2, Name: Alice, Body: thin
//Id: 3, Name: Kate, Body: thin

class Person
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Body { get; set; }
}