Remove Controls from List with different Parents

Question

I am trying to delete Controls dynamically, without knowing the parent. But I keep getting a "System.NullReferenceException" in mscorlib.dll when I debug.

My Code:

//Delete Controls
        List<PictureBox> toDelete = severalControlsFromDifferentPanels;
        for (int i = toDelete.Count - 1; i >= 0; --i)
        {
            Control parent = toDelete[i].Parent;
            parent.Controls.Remove(toDelete[i]);
        }

What am I missing here? Am I overseeing something obvious? thanks in advance!

Aren't you still referring to a deleted object somewhere in your code? — MCollard, Feb 10 '14 at 14:41
where did you get that `severalControlsFromDifferetPanels`, from a method? are you sure all controls has a parent? — Selman Genç, Feb 10 '14 at 14:42

score 3 · Accepted Answer · answered Feb 10 '14 at 14:36

3

Check if pictureBox has parent before accessing it:

foreach(PictureBox pictureBox in toDelete)
    if (pictureBox.Parent != null)
        pictureBox.Parent.Controls.Remove(pictureBox);

To make this code even more readable you can create extension method:

public static void RemoveFromParent(this Control control)
{
    if (control == null)
       throw new ArgumentNullException();

    if (control.Parent == null)
        return;

    control.Parent.Controls.Remove(control);
}

Thus removing controls will look like:

foreach(PictureBox pictureBox in toDelete)
    pictureBox.RemoveFromParent();

answered Feb 10 '14 at 14:36

Sergey Berezovskiy

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1

@Selman22 oops, didn't see error message. I believe that is caused by control without parent. Changed code, thanks – Sergey Berezovskiy Feb 10 '14 at 14:42
1

You were right! Some controls actually had no parent. I used this weird for-loop because of this post: http://stackoverflow.com/a/1582317/2958399 (i see now, its not even necessary ;)) Thanks for the quick answer! – Sip Feb 10 '14 at 14:47

score 0 · Answer 2 · answered Feb 10 '14 at 14:45

Something is null; trying to access a property or method on a null object will throw that exception.

Place a breakpoint on the first line and step through your code.

 List<PictureBox> toDelete = severalControlsFromDifferetPanels;

 // if severalControlsFromDifferetPanels is null, then toDelete.Count will throw
 for (int i = toDelete.Count - 1; i >= 0; --i)
 {
     // if toDelete[i] is null, then accessing .Parent will throw
     Control parent = toDelete[i].Parent;

     // if parent is null, then .Controls will throw
     parent.Controls.Remove(toDelete[i]);
 }

We can't tell any more than that from the code you've provided.

if `parent` is not null, if `parent.Controls` can not be null :) — Selman Genç, Feb 10 '14 at 14:47

Remove Controls from List with different Parents

2 Answers2