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!

Answer 1

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();

Answer 2

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.