SelectMany() 究竟是做什么用的？

Question

没有输入和输出很难解释这个。我制作了一个简单的汽车示例，以避免我的项目中不必要的细节。

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 } }
};

我想输出扩展名类：

{
    { 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() 的确切用途。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 将每辆汽车的车轮投射到一个新对象（在我的代码中是具有您指定属性的匿名类型）。外层 SelectMany() 将这些序列压平成一个列表。

Answer 2

也许这个解决方案会有帮助？这会将您的 IEnumerable 拆分为固定大小的 IEnumerable

public static class EnumerableExtension
{
    private static IEnumerable<T> GetChunk<T>(IEnumerator<T> source, int size)
    {
        yield return source.Current;

        for (int i = 1; i < size; i++)
        {
            if (source.MoveNext())
            {
                yield return source.Current;
            }
            else
            {
                yield break;
            }
        }
    }

    public static IEnumerable<IEnumerable<T>> SplitToChunks<T>(this IEnumerable<T> source, int size)
    {
        if (size < 1)
        {
            throw new ArgumentOutOfRangeException();
        }

        using (var enumerator = source.GetEnumerator())
        {
            while (enumerator.MoveNext())
            {
                yield return GetChunk(enumerator, size);
            }
        }
    }

}

抱歉，没有正确阅读问题）

Answer 3

这是我发现冗长的 Linq 语法更具可读性的罕见情况之一：

 var query = from car in cars
             from carwheel in car.carwheels
             select new { car.carname, carwheel };

SelectMany() 究竟是做什么用的？

Exactly what SelectMany() is for?

c#

enumerable