c# - SelectMany() 究竟是做什么用的？

Question

如果没有输入和输出，很难解释这一点。我制作了一个简单的汽车示例，以避免我的项目出现不必要的细节。

我有这个列表:

List<car> cars = new List<car>()
{
    new car() { carname = "bmw", carwheels = new List<int>() { 1, 2, 3, 4 } },
    new car() { carname = "cadillac", carwheels = new List<int>() { 1, 2, 3, 4 } },
    new car() { carname = "mustang", carwheels = new List<int>() { 1, 2, 3, 4 } }
};

我想用扩展类输出这个:

{
    { carname = "bmw", carwheel = 1 },
    { carname = "bmw", carwheel = 2 },
    { carname = "bmw", carwheel = 3 },
    { carname = "bmw", carwheel = 4 },
    { carname = "cadillac", carwheel = 1 },
    { carname = "cadillac", carwheel = 2 },
    { carname = "cadillac", carwheel = 3 },
    { carname = "cadillac", carwheel = 4 },
    { carname = "mustang", carwheel = 1 },
    { carname = "mustang", carwheel = 2 },
    { carname = "mustang", carwheel = 3 },
    { carname = "mustang", carwheel = 4 }
}

有没有办法通过 IEnumerable 扩展方法进行投影？与 SelectMany() 相反的东西。

Answer 1

它不是 SelectMany() 的相反，而是 SelectMany() 的用途。你可以这样实现:

var result = cars.SelectMany(c => 
            c.carwheels.Select(w => new 
            {
                carname = c.carname,
                carwheel = w
            })).ToList();

内部 Select 将每辆汽车的车轮转换到一个新对象(在我的代码中是具有您指定属性的匿名类型)。外层的 SelectMany() 将这些序列扁平化为一个列表。