Question

In Form1.cs i have

public const int n = 30;
public TabPage[] tp = new TabPage[n];

private void toolStripSeparator1_Click(object sender, EventArgs e)
{
    RenameFunc rf = new RenameFunc();
    rf.ShowDialog();
}

In RenameFunc.cs i have

private void button1_Click_1(object sender, EventArgs e)
{
    Form1 frm1 = new Form1();

    if (textBox1.Text != null)
    /*Line 24 */    frm1.tp[Array.IndexOf(frm1.tp, frm1.tabControl1.SelectedTab)].Text = textBox1.Text;
    Application.Exit();

}

tabControl1 is also seted tu Public

in Line 24 i get error

System.NullReferenceException: Object reference not set to an instance of an object.
at System.Windows.Forms.TabControl.get_SelectedTabInternal()
at System.Windows.Forms.TabControl.get_SelectedTab()
at Notepad1._0.RenameFunc.button1_Click_1(Object sender, EventArgs e) in D:\C#\Notepad1.0\Notepad1.0\RenameFunc.cs:line 24

How to correct ?

Answer 1

I don’t know what tp is, but, I’m sure it’s not initialized and this gives the exception.
The reason is easily found in the previous line

Form1 frm1 = new Form1();

here you create a new instance of Form1. You are not referencing the first Form1 from which your RenameFunc has been called.

Perhaps you could pass a reference to the correct Form1 when you call RenameFunc, for example

RenameFunc rf = new RenameFunc(this); 

and keep that reference in your RenameFunc internal vars

public partial class RenameFunc : Form
{
    private Form1 _caller = null;
    public RenameFunc(Form1 f)
    {
         InitializeComponent();
         _caller = f;       
    }

}

and in button1_Click_1 use that reference instead of new Form1

   if (textBox1.Text != null)           
        _caller.tp[Array.IndexOf(_caller.tp, _caller.tabControl1.SelectedTab)].Text =
                   textBox1.Text;           
   Application.Exit();       

However a little explanation on tp would be beneficial