linq select 和selectMany的区别

news2024/9/24 3:27:45

Select 和 SelectMany 都是 LINQ 查询方法，但它们之间有一些区别。

Select 方法用于从集合中选择特定的属性或对集合中的元素进行转换，并返回一个新的集合。例如：

var numbers = new List<int> { 1, 2, 3, 4, 5 };
var squaredNumbers = numbers.Select(x => x * x);

// squaredNumbers 的值为 { 1, 4, 9, 16, 25 }

SelectMany 方法用于对集合中的元素进行嵌套操作，并返回一个扁平化的结果集。例如：

var teams = new List<Team>
{
    new Team { Name = "Team A", Members = new List<string> { "Alice", "Bob" } },
    new Team { Name = "Team B", Members = new List<string> { "Charlie", "David" } }
};

var allMembers = teams.SelectMany(t => t.Members);

// allMembers 的值为 { "Alice", "Bob", "Charlie", "David" }

因此，Select 用于简单的投射和转换操作，而 SelectMany 用于处理嵌套结构并将其扁平化。
实例：

public class Select
{
	public Select(params string[] str)
	{
		str?.ToList()?.ForEach(x => Attrs.Add(x));
	}
	public List<string> Attrs { get; set; } = new List<string>();
}
void Main()
{
	var demoList = new List<Select>();
	demoList.Add(new Select("s1", "s2"));
	demoList.Add(new Select("s11", "s22"));
	demoList.Add(new Select("s111", "s222"));
	var firstList = demoList.Select(x => x.Attrs).ToList();
	var secondList = demoList.SelectMany(x => x.Attrs).ToList();
}