为什么列出<a> doesn&#39;t convert to List<b> on conversion using Select operator

Question

Consider the following piece of code, Here I can typecast IEnumerable<A> to List<A>, as that's what it contains internally, but when I use Select to transform the same collection to IEnumerable<B>, it is not List<B>, which I can only achieve by calling ToList(), which is a penalty in my actual code, due to millions records in the first collection. Is there a better way to achieve it, I wonder why List<A> doesn't transform into List<B>, if List is the underlying memory allocation

void Main()
{
    A a = new A { Id = 1 };

    IEnumerable<A> aList = new List<A> {a};

    ((aList as List<A>) != null); // True

    var bList = aList.Select(x => new B { Id = x.Id});

    ((bList as List<B>) != null);  // False
}

public class A
{
    public int Id { get; set;}
}

public class B
{
    public int Id { get; set; }
}

Answer 1

您的bList是IEnumerable<B>，其中aList的值尚未转换。例如，当您foreach超过该列表或在其上调用.ToList()时，就会发生转换。这是＆＃34;懒惰＆＃34;评论中提到的方面。

所以你的第二个测试是正确的：它是不是一个List<B>。