.net - 如何使用LINQ to Entity选择递归嵌套实体

Question

我有一个名为Category的实体，该实体包含一个名为ChildCategories的IEnumerable。类别可以具有这些子类别，也可以具有自己的子类别，依此类推。

假设我已经选择了顶级父类别，我想获得所有子类别及其子类别，依此类推，以便获得该类别的所有层次子类别。我希望这是flat花一现，并返回初始类别。我尝试创建类似

    public static IEnumerable<T> AllChildren<T>(this IEnumerable<T> items, 
        Func<T, IEnumerable<T>> children, bool includeSelf)
    {
        foreach (var item in items)
        {
            if (includeSelf)
            {
                yield return item;
            }
            if (children != null)
            {
                foreach (var a in children(item))
                {
                    yield return a;
                    children(a).AllChildren(children, false);
                }
            }
        }
    }

使用SelectMany方法后，它会变得扁平，但是还没有完全理解它。

Answer 1

在他的博客文章Traverse a hierarchical structure with LINQ-to-Hierarchical中，Arjan Einbu描述了一种简化层次结构以便于查询的方法:

Can I make a generic extension method that will flatten any hierarchy? [...]

To do that, we need to analyze which parts of the method needs to be swapped out. That would be the TreeNode’s Nodes property. Can we access that in an other way? Yes, I think a delegate can help us, so lets give it a try:

public static IEnumerable<T> FlattenHierarchy<T>(this T node, 
                             Func<T, IEnumerable<T>> getChildEnumerator)
{
    yield return node;
    if(getChildEnumerator(node) != null)
    {
        foreach(var child in getChildEnumerator(node))
        {
            foreach(var childOrDescendant 
                      in child.FlattenHierarchy(getChildEnumerator))
            {
                yield return childOrDescendant;
            }
        }
    }
}

casperOne describes this in his answer as well以及尝试使用LINQ直接遍历层次结构时固有的问题。