Getting NullReferenceException Error While Trying To Remove Dynamical Content in C#

Question

I am trying to develop a program that calculates the average grade of a semester with Windows Forms Application. Well, first I have a combobox and a button for deciding the number of lesson I take at that semester. When I select the number of lesson and click the button, it creates the amount of textbox(es) I want. There is nothing to worry about at this part. http://www.imageupload.co.uk/images/2015/01/26/Ads%C4%B1z.png http://www.imageupload.co.uk/images/2015/01/26/Ads%C4%B1z1.jpg

The problem that bugs me is the deletion of the dynamical contents. I think deleting and re-creating contents dynamically is a well idea for changing the amount of lesson more than once. This is my code for my solution.

public partial class Form1 : Form
{
    int Syc = 0;
    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        for (int k = 1; k <= 15; k++) {
            comboBox1.Items.Add(k);
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {

        int En = 50;
        int Item=Convert.ToInt32(comboBox1.SelectedItem);
        TextBox[] d1 = new TextBox[Item];
        ComboBox[,] d2 = new ComboBox[Item,2]; //Planning to use it afterwards
        if (Syc == 1) Sil(d1); // Controls if the combobox changed before
        Syc = 1;
        for (int k = 0; k < Item; k++) {
            d1[k] = new TextBox();
            d1[k].Name = "textBox" + (k+1).ToString();
            d1[k].Location = new Point(10,En);
            d1[k].Size = new System.Drawing.Size(100, 15);
            d1[k].Text = "";
            this.Controls.Add(d1[k]);
            En += 25;

        }
    }
    private void Sil(TextBox[] d1) { //This part should have deleted
                         //the dynamical content that created before
        for (int k = d1.Length-1; k >=0; k--) {
            this.Controls.Remove(d1[k]);
            d1[k].Dispose();
        }
    }
}

Well, I get a NullReferenceError at the line where d1[k].Dispose exists in Sil function. I feel really confused because there shouldn't be any reason for this kind of error. I created the instances of the textbox(es) and I even tried this again in Sil function. What do I miss here or what kind of solution would you suggest?

Please post the full stack trace of the error message. Also, in most cases debugging can solve such errors. — Quality Catalyst, Jan 26 '15 at 03:40
Debug the code. You are messing up with array indexing and variable handling .. — Kavindu Dodanduwa, Jan 26 '15 at 03:44

score 0 · Answer 1 · answered Jan 26 '15 at 03:41

Where do you reset Syc to 0?
If you select 4, resulting in 4 TextBoxs and Syc=1, then select 6, how is your code not going to call Dispose on null pointers at d1[4] and d1[5]?

Set some breakpoints and see. (Hint: I'm pretty sure you want your call to Sil to precede your call to d1 = new TextBox[], and that somewhere in there, you're going to want to set Syc = 0.)

Getting NullReferenceException Error While Trying To Remove Dynamical Content in C#

1 Answers1