Why do I get NullReferenceException when I try to create dynamic buttons in ASP.Net?

Question

I am pretty new at ASP.Net and I'm trying to program a website which creates wanted amount of buttons. I have a textbox and a button, I take the number of button I want from textbox by clicking button, it's simple. And I have this code I wrote. It should have worked but I get a "System.NullReferenceException: Object reference not set to an instance of an object." error at the line where d1[k].ID exists. I searched about this error a little bit and I found out it's because d1[k].ID variable is null, but I don't know what to do. How can I solve this error?

protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void Button1_Click(object sender, EventArgs e)
    {

        int Sayi = Convert.ToInt32(TextBox1.Text);
        Button[] d1=new Button[Sayi];
        int Sy = 20;
        for (int k = 1; k <= Sayi; k++) {
            d1[k].ID = "Btn" + k.ToString();
            d1[k].Width = 100;
            d1[k].Height = 25;
            d1[k].Text = k.ToString();
            d1[k].Attributes.Add("style", "top:"+Sy+"; left:10 ;position:absolute");
            Sy += 20;

        }
    }

On which line do you get the exception. There are a ton of places where that exception could occur. — Complexity, Jan 05 '15 at 10:56
Arrays are initialise to contain the default value in each element which for reference types is null. Add `d1[k] = new Button()` to the begining of your loop. — Ben Robinson, Jan 05 '15 at 11:00

Uriil · Accepted Answer · 2015-01-05T12:27:46.387

2

You creating instance of Button Array, not button by itself, so it should be:

int Sayi = Convert.ToInt32(TextBox1.Text);
    Button[] d1=new Button[Sayi];
    int Sy = 20;
    for (int k = 0; k < Sayi; k++) {
        var b = new Button()
        b.ID = "Btn" + k.ToString();
        b.Width = 100;
        b.Height = 25;
        b.Text = k.ToString();
        b.Attributes.Add("style", "top:"+Sy+"; left:10 ;position:absolute");
        Sy += 20;
        d1[k] = b;
    }

edited Jan 05 '15 at 12:27

answered Jan 05 '15 at 10:59

Uriil

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And have the "k"-loop count from 0 to `Sayi-1` using `for(int k=0; k – Hans Keﬆing Jan 05 '15 at 12:26
@HansKesting ty, did not noticed that – Uriil Jan 05 '15 at 12:28
It worked, thank you. I don't get any NullReferenceException or any error now but still, I don't see the buttons created. Have any idea how I can create buttons? – Canberk Karaoğlan Jan 05 '15 at 15:25
@KinderGray you are not adding them to page(at least in attached code) – Uriil Jan 05 '15 at 15:53
@Uriil I added this.form1.Controls.Add(d1[k]); line and now it works perfectly. Only textBox1 moves when I create buttons. Thank you all for your helps. – Canberk Karaoğlan Jan 05 '15 at 16:51

score 0 · Answer 2 · answered Jan 05 '15 at 11:00

0

d1[k] doesn't have a Button reference, you need to create one at that index before setting properties:

d1[k] = new Button();

answered Jan 05 '15 at 11:00

Grant Thomas

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Why do I get NullReferenceException when I try to create dynamic buttons in ASP.Net?

2 Answers2