Avoiding a null reference exception on a variable in a foreach loop that could be null

Question

I have a quadtree which has a remove method, and every time I remove an item from the game I call this method. The problem is that in the foreach loop, the child variable is null. How can I bypass this method so if the variable is null it won't give me an error?

private void CollapseChildren(RectangleTreeNode node)
    {
        foreach (RectangleTreeNode child in node.Children)
        {
            if (child == null)
            {
                return;
            }

            while (child.Items.Count > 0)
            {
                MoveUp(child.Items[0]);
            }
        }

        node.Children = null;
    }

Here's the remove method.

private void Remove(RectangleTreeNode node, RectangleTreeItem item)
    {
        node.Items.Remove(item);
        item.Parent = null;

        while (node != null)
        {
            node.ItemCount--;
            if (node.ItemCount < 6)
            {
                CollapseChildren(node);
            }
            node = node.Parent;
        }
    }

If the value is null then I shouldn't have to loop through the children in nodes.Children because if I do loop through then I get the error at the child variable. Meaning the node doesn't have a child.

If the value is null then I shouldn't have to loop through the children in nodes.Children because if I do loop through then I get the error at the child variable. Meaning the node doesn't have a child — Christian Frantz
– Christian Frantz, Commented Oct 11, 2014 at 2:37
Our company policy is never to set collections null like you do with node.Children = null, always create an empty collection like node.Children = new RectangleTreeNode[0]. This avoids errors and simplifies logic considerably — Dour High Arch
– Dour High Arch, Commented Oct 11, 2014 at 3:14
@DourHighArch: That is a good policy. Also, you can use the same empty array for every empty collection of that type, and avoid the memory allocation. It's an empty array, it's not going to change and one is as good as another. — Eric Lippert
– Eric Lippert, Commented Oct 11, 2014 at 3:27

Grant Winney · Accepted Answer · 2014-10-11 02:42:07Z

1

You're already checking whether or not each child is null.

If there's a risk of node.Children being null, then check for that too, before accessing it:

private void CollapseChildren(RectangleTreeNode node)
{
    if (node.Children == null)
        return;

    foreach (RectangleTreeNode child in node.Children)
    {
       ...
       ...
    }
}

answered Oct 11, 2014 at 2:42

Grant Winney

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3 Comments

Christian Frantz Over a year ago

Still get the error. So it must not be in the method.

Christian Frantz Over a year ago

I tried the answer below and it worked! I just had them in the wrong order

Christian Frantz Over a year ago

I missed the if(child == null) inside the foreach loop. I put it outside for some reason

HTX9 · Accepted Answer · 2014-10-11 02:43:02Z

1

You'll have to check if node.Children is null prior to invoking the foreach loop, so your method becomes:

private void CollapseChildren(RectangleTreeNode node)
{
    if (node.Children == null)
    {
        // exit the method, or do whatever is appropriate when node.Children is null
        return;
    }

    foreach (RectangleTreeNode child in node.Children)
    {
        if (child == null)
        {
            return;
        }

        while (child.Items.Count > 0)
        {
            MoveUp(child.Items[0]);
        }
    }

    node.Children = null;
}

answered Oct 11, 2014 at 2:43

HTX9

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