How to solve object reference not set to an instance of an object.? Asp.net C# web

Question

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SQL-Server-2012

Hi..
I'm facing problem to resolve this error i try again and again to solve this but I'm failed to resolve this issue,I execute the database query in sql it working fine but here this error come.
here error :

C#

Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:


Line 33:                     string value = row["Database_Name"] + "";
Line 34:                     parentNode.ChildNodes.Add(new TreeNode(value));
Line 35:                     lv.Items.Add(value);//HERE ERROR COME RED 
Line 36: 
Line 37:

What I have tried:

Here is my code :

C#

protected void Page_Load(object sender, EventArgs e)
    {
        welcome.InnerText = Session["name"].ToString();
        if (!Page.IsPostBack)
        {
            SqlConnection cnn = new SqlConnection("Data Source=HAMEED_KHAN\\SQLEXPRESS;Initial Catalog=db_compiler;Integrated Security=True");
            SqlCommand cmd = new SqlCommand("SELECT Database_Name FROM  Create_db", cnn);
            DataTable dt = new DataTable();
            Master.FindControl("TreeView1");
            TreeView tv = (TreeView)Master.FindControl("TreeView1");
            ListBox lv = (ListBox)Master.FindControl("ListBox1");
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            cnn.Open();
            da.Fill(dt);
            cnn.Close();

            var parentNode = new TreeNode("Database");
            if (dt.Rows.Count > 0)
            {
                foreach (DataRow row in dt.Rows)
                {
                    string value = row["Database_Name"] + "";
                    parentNode.ChildNodes.Add(new TreeNode(value));
                    lv.Items.Add(value);


                }
            }
            tv.Nodes.Add(parentNode);
        }
    }

Posted 3-Apr-16 0:54am

Hameed Khan

Updated 3-Apr-16 1:29am

v2

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Comments

Kornfeld Eliyahu Peter 3-Apr-16 7:12am

Debug and see if lv is null (probably is)...Now ask yourself:
1. Why?
2. Why in the first place you are using FindControl to load lv and not its ID?

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OriginalGriff · Answer 1 · 2016-04-03T01:29:00

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 yesterdays 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, VS 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, VS 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!