"Object reference not set to an instance of an object" when trying to change a picture box image

Question

I'm trying to change the image of a pictureBox based on the number of occurance of a list:

list<string> items = new List<string>();
items.Add("Item1");
items.Add("Item2");
items.Add("Item3");
items.Add("Item4");
items.Add("Item5");

foreach (var item in items.OfType<string>().Select((x, i) => new { x, i }))
{
 int ItemNumber = item.i + 1;
 string ItemNumberStr = ItemNumber.ToString();
 PictureBox pbox = (PictureBox)this.Controls["Picturebox" + ItemNumberStr];
 pbox.Image = Properties.Resources.white_square_button;
 Label labl = (Label)this.Controls["label" + ItemNumberStr];
 labl.Text = item.x;
}

This is done within a foreach event in which item.i is the number of occurance represented as an int, then converted to a string to determine the number of pictureBox I'm trying to modify. But when I do this I recieve error "Object reference not set to an instance of an object" here:

pbox.Image = Properties.Resources.white_square_button;

This also happens with the label aswell.

What am I doing wrong?

What is null here? pbox? Properties.Resources? Something else? — Michael Petrotta, Jul 06 '14 at 19:00
It looks like you don't have an element defined with `this.Controls["Picturebox" + ItemNumberStr]`. Make sure that you have such element before trying to access its value. — Darin Dimitrov, Jul 06 '14 at 19:05
I believe the problem might be your naming convention. Are you sure the controls are named Picturebox1 ... Picturebox5, letter-case matching? Also, if the controls are nested, in another container, they will not be visible in the parent. If this is Windows Forms app, consider this.Controls.Find("PictureBox"+ItemNumberStr, true). — Darek, Jul 06 '14 at 19:06
@Darek Did that and it indeed found the control i defined above. — Joscplan, Jul 06 '14 at 19:16
Almost all cases of `NullReferenceException` are the same. Please see "[What is a NullReferenceException in .NET?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-in-net)" for some hints. — John Saunders, Jul 06 '14 at 20:26

score 2 · Answer 1 · answered Jul 06 '14 at 19:05

2

ControlCollection[String] will not throw an exception if there are no control with such name:

        Control control = this.Controls["I am not here"];
        MessageBox.Show((control == null).ToString());

It is doubtful that Properties.Resources property is null, so the most probable is that the control you are trying to access do not exist or named a bit different.

answered Jul 06 '14 at 19:05

Eugene Podskal

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The control is named exactly as I defined it. – Joscplan Jul 06 '14 at 19:14
Can you post the exact exception message you are getting? At which line, which object is null? – Eugene Podskal Jul 06 '14 at 19:17
The problem is that pbox is taken as null I think `pbox null System.Windows.Forms.PictureBox` – Joscplan Jul 06 '14 at 19:22
If `pbox` is null then it is not being named correctly or it hasn't been added to the Form.Controls collection. There is no other possibility short of some real problem in Form's framework code. – Eugene Podskal Jul 06 '14 at 19:25
Thank you, I found the solution. Check my answer below. – Joscplan Jul 06 '14 at 19:56

score 1 · Answer 2 · edited May 23 '17 at 10:26

The PictureBoxes I was trying to modify were in a panel and as mentioned here: this.Controls doesn't contain all controls by doing this.Controls["Picturebox" + ItemNumberStr] it would return me a null reference because the PictureBoxes were assigned to that label. So what I did is replace this with panel1 (The name of the panel in which the PictureBoxes were) and now it works like a charm.

list<string> items = new List<string>();
items.Add("Item1");
items.Add("Item2");
items.Add("Item3");
items.Add("Item4");
items.Add("Item5");

foreach (var item in items.OfType<string>().Select((x, i) => new { x, i }))
{
 int ItemNumber = item.i + 1;
 string ItemNumberStr = ItemNumber.ToString();
 PictureBox pbox = (PictureBox)panel1.Controls["Picturebox" + ItemNumberStr];
 pbox.Image = Properties.Resources.white_square_button;
 Label labl = (Label)panel1.Controls["label" + ItemNumberStr];
 labl.Text = item.x;
}

change "this" to "groupBox1" fixed my problem, thanks – Saghachi Nov 28 '22 at 06:21 — Saghachi, Nov 28 '22 at 06:21

"Object reference not set to an instance of an object" when trying to change a picture box image

2 Answers2