Hierarchy Problem -> Replace Recursion with Linq Join?

Question

I have a self referential table, which has ID, ParentID (nullable).

So, the table contains many nodes, each node could be the root in the hierarchy (parent is null),

Answer 1

If you want to select all direct children of a node, a simple query like the following should do the job:

from item in table
where item.ID == parentID;
select item

If you want to select all descendants of a node, this is not possible with LINQ, because it requires recursion or a stack which LINQ (and SQL) doesn't provide.

See also:

• StackOverflow: LINQ to SQL for self-referencing tables?
• CodeProject: T-SQL - How to get all descendants of a given element in a hierarchical table
• StackOverflow: Expressing recursion in LINQ

Answer 2

I know this is an old post but you should check out this extension:

http://www.scip.be/index.php?Page=ArticlesNET23

I've been using it and it is working great.

Answer 3

Basically I'm going with something like this as discussed in the SO link you proivded.

public IQueryable GetCategories(Category parent)
{
    var cats = (parent.Categories);
    foreach (Category c in cats )
    {
        cats  = cats .Concat(GetCategories(c));
    }
    return a;
}

CTEs are probably the best solution but I'd like to keep things all in the same tier for now.

Answer 4

Here is a quick one I just wrote:

class MyTable
{
    public int Id { get; set; }
    public int? ParentId { get; set; }
    public MyTable(int id, int? parentId) { this.Id = id; this.ParentId = parentId; }
}

List<MyTable> allTables = new List<MyTable> {
    new MyTable(0, null), 
    new MyTable(1, 0),
    new MyTable(2, 1)
};

Func<int, IEnumerable<MyTable>> f = null;
f = (id) =>
{
    IEnumerable<MyTable> table = allTables.Where(t => t.Id == id);

    if (allTables
        .Where(t => t.ParentId.HasValue && t.ParentId.Value == table
            .First().Id).Count() != 0)
        return table
            .Union(f(
            allTables.Where(t => t.ParentId.HasValue && t.ParentId.Value == table
                .First().Id).First().Id));
    else return table;

};

But I believe it is possible to do using SQL with a Union ALL.