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When the user clicks on BtnAdd_Click() in FormB then the image displays in the PictureBox in FormA will move to the next image incrementImage().

I want to check whether the BtnAdd_Click() was clicked in FormB and passed to FormA to perform the incrementImage() function. The code that I tried below and gives an error Object reference not set to an instance of an object.

What I have tried:

FormA. cs
private FormB formB;

public FormA()
            formB = new FormB();
            formB.ButtonWasClicked += new FormB.ClickButton(formB_ButtonWasClicked);

void formB_ButtonWasClicked()

public void incrementImage()


            if (nCurrentItem > nTotalNumber)
                nCurrentItem = nTotalNumber;

            else if (nCurrentItem < nTotalNumber)
                Image img;
                using (var bmpTemp = new Bitmap(ImageFileNames[nCurrentItem]))
                    img = new Bitmap(bmpTemp);
                PicBox.Image = img;

            else if (nCurrentItem == nTotalNumber)
                label5.Visible = true;

            imgCounter.Text = firstImage.ToString() + " / " + nTotalNumber.ToString();

FormB. cs
public delegate void ClickButton();
public event ClickButton ButtonWasClicked;

private void BtnAdd_Click(object sender, EventArgs e)

    if (listBoxFailCategories.SelectedIndex == -1)
        MessageBox.Show("Please select any defect category");
        var tempDefectCategory = "";

        foreach (string item in listBoxFailCategories.SelectedItems)
            string defectFolder = Path.Combine(Value, item);
            File.Copy(Image, Path.Combine(defectFolder, Path.GetFileName(Image)), true);

            added = true;

            tempDefectCategory = tempDefectCategory + item.ToString() + ",";

            FullPathName = Path.Combine(defectFolder, Path.GetFileName(Image));

        DefectCategory = tempDefectCategory;

        ButtonWasClicked(); //error here


Updated 26-Oct-21 21:21pm

The code that I tried below and gives an error Object reference not set to an instance of an object.

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterday's shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, it will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, the debugger will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

But if you are trying to pass information between forms, you should probably look at how you are doing it. Exactly how you should do it depends on the "relationship" between the two forms.
Have a look at these, one of them will fit your circumstances.
The form that creates an instance of another:
MyForm mf = new MyForm();
Is the "parent", the other form is the "child".
(This doesn't imply any formal MDI relationship)

Transferring information between two forms, Part 1: Parent to Child[^]
Transferring information between two forms, Part 2: Child to Parent[^]
Transferring information between two forms, Part 3: Child to Child[^]
Maciej Los 28-Oct-21 15:51pm
formB.ButtonWasClicked += new Form6.ClickButton(formB_ButtonWasClicked);

What is Form6?
Vektor 2021 27-Oct-21 3:17am
Richard MacCutchan 27-Oct-21 3:44am
If you use copy and paste you would avoid such problems, which tend to confuse those of us trying to help.

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