MVC 5实体框架6 - 删除嵌套/分层数据

Question

i've run into some trouble, when playing around with hierarchical data and trying to delete a node that has children.

I have set up a sample tree similar to this article and decorated the ul-li items with a little bit of bootstrap (add, delete and edit icons).

I'm struggeling now at the recursive deletion, as i am using an IEnumerable (Children), which is the inverse result of parentNodeID. So when deleting a node, the children were updated and the system throws an enumeration error (collection has changed). I had such an error a few years ago, but i don't remember, how i fixed it. Any advice?

So here is what i got:

~/Models/Tree.cs

public class Tree
{
    [Key]
    public int NodeID { get; set; }

    [Display(Name ="Element name")]
    public string NodeName { get; set; }

    [Display(Name ="Element identifier")]
    public string NodeIdentifier { get; set; }

    public int? ParentNodeID { get; set; }
    public virtual Tree Parent { get; set; }
    public virtual ICollection<Tree> Children { get; set; }
}

~/Controllers/TreesController.cs

// Partial - only Delete Actions

// GET: Trees/Delete/5
public async Task<ActionResult> Delete(int? id)
{
    if (id == null)
    {
        return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
    }
    Tree tree = await db.Trees.FindAsync(id);
    if (tree == null)
    {
        return HttpNotFound();
    }

    if (tree.Children.Count > 0)
    {
        ViewBag.HasChildren = true;
    }
    else
    {
        ViewBag.HasChildren = false;
    }

    return View(tree);
}

// POST: Trees/Delete/5
[HttpPost, ActionName("Delete")]
[ValidateAntiForgeryToken]
public async Task<ActionResult> DeleteConfirmed(int id)
{
    Tree tree = await db.Trees.FindAsync(id);
    DeleteRecursive(tree);
    db.Trees.Remove(tree);
    await db.SaveChangesAsync();
    return RedirectToAction("Index");
}

public void DeleteRecursive(Tree tree)
{
    foreach(var child in tree.Children)
    {
        DeleteRecursive(child);
    }
    db.Trees.Remove(tree);
}

~/Views/Trees/Index.cshtml

<div class="container">
@helper BuildTree(IEnumerable<MVCMusicStore.Models.Tree> tree, int? parentID = null)
{
    var nodes = tree.Where(t => t.ParentNodeID == parentID).OrderBy(n => n.NodeIndexOrder);
    if (nodes.Any())
    {
        if (nodes.First().ParentNodeID == null)
        {
                <ul id="tree">
                    @foreach (var node in nodes)
                    {
                        <li>
                            @node.NodeIndexOrder - @node.NodeName &nbsp; <a href="@Url.Action("Edit","Trees",new { id = node.NodeID })" data-toggle="tooltip" data-placement="right" title="Bearbeiten"><i class="glyphicon glyphicon-edit"></i></a>&nbsp; <a href="@Url.Action("Delete","Trees",new { id = node.NodeID })" data-toggle="tooltip" data-placement="right" title="Löschen"><i class="glyphicon glyphicon-trash"></i></a>
                            @BuildTree(tree, node.NodeID)
                        </li>
                    }
                </ul>
        }
        else
        {
                <ul>
                    @foreach (var node in nodes)
                    {
                        <li>
                            @node.NodeIndexOrder - @node.NodeName &nbsp; <a href="@Url.Action("Edit","Trees",new { id = node.NodeID })" data-toggle="tooltip" data-placement="right" title="Bearbeiten"><i class="glyphicon glyphicon-edit"></i></a>&nbsp; <a href="@Url.Action("Delete","Trees",new { id = node.NodeID })" data-toggle="tooltip" data-placement="right" title="Löschen"><i class="glyphicon glyphicon-trash"></i></a>
                            @BuildTree(tree, node.NodeID)
                        </li>
                    }
                </ul>
        }         
    }
    else
    {

        <p>
            <a href="@Url.Action("Create","Trees", new { parentNodeID = parentID })" data-toggle="tooltip" data-placement="right" title="Neu"><i class="glyphicon glyphicon-plus"></i></a>
        </p>
    }
 }

@BuildTree(Model,null)
</div>

Edit - Solution:

public void DeleteRecursive(Tree tree)
{
    foreach(var child in tree.Children.ToArray<Tree>())
    {
        DeleteRecursive(child);
    }
    db.Trees.Remove(tree);
}

Using an array to delete from does not alter the original collection.

Credits to Johnathon Sullinger: Delete item in nested collections of Nth level

Answer 1

public void DeleteRecursive(Tree tree)
{
    foreach(var child in tree.Children.ToArray<Tree>())
    {
        DeleteRecursive(child);
    }
    db.Trees.Remove(tree);
}

Using an array to delete from does not alter the original collection.