Question

First of all i’m new in here and new with csharp. just exhausted while doing some exercises for learning csharp. cant find a solution why i get the error “NullReferenceException was unhandled” and

1) How to overcome this? according to my researches its related with initialisation but couldnt make it. i debug and watched the values all buttons gets gets null values is it because of this? how can i solve it? what to do? which code to add?

(***all informing knowledge about class initialisations and arrays and null stuffs are welcome. i wanna learn all the points and wanna be expert 😛 and people can optimize the code as well. all extra informations are welcome.)

2) And why compiler doesnt show error but error comes while running the code?

ok now i have the code blow

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace temp1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            Button[] btn = new Button[5];

            for (int i = 0; i < 5; i++)
            {
                btn[i] = new Button();
                btn[i].Width = 50;
                btn[i].Height = 50;
                flowLayoutPanel1.Controls.Add(btn[i]);
                btn[i].Click += new EventHandler(btn_Click);
            }
        }

        void btn_Click(object sender, EventArgs e)
        {
            Button[] btn = sender as Button[];
            btn[3].Text = "button name changed";  // here problems occurs
            //btn[3].BackColor = Color.Red;   // here problems occurs
            // btn[3].PerformClick();    // here problems occurs
        }
    }
}

Answer 1

it should be:

void btn_Click(object sender, EventArgs e)
{
    Button btn = sender as Button;
    btn.Text = "button name changed";  // here problems occurs
}

the sender is the button itself, not the array

UPDATE ACCORDING TO THE COMMENT:

you should do something like this:

public partial class Form1 : Form
{
    private Button[] m_ButtonsArray;

    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        m_ButtonsArray = new Button[5];

        for (int i = 0; i < 5; i++)
        {
            m_ButtonsArray [i] = new Button();
            m_ButtonsArray [i].Width = 50;
            m_ButtonsArray [i].Height = 50;
            flowLayoutPanel1.Controls.Add(m_ButtonsArray [i]);
            m_ButtonsArray [i].Click += new EventHandler(btn_Click);
        }
    }

    void btn_Click(object sender, EventArgs e)
    {
        m_ButtonsArray[3].Text = "button name changed";  // here problems occurs
    }
}

UPDATE – Changing the 2nd button when the 5th button was clicked:

public partial class Form1 : Form
{
    private Button[] m_ButtonsArray;

    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        m_ButtonsArray = new Button[5];

        for (int i = 0; i < 5; i++)
        {
            m_ButtonsArray [i] = new Button();
            m_ButtonsArray [i].Width = 50;
            m_ButtonsArray [i].Height = 50
            m_ButtonsArray [i].Tag = i;;
            flowLayoutPanel1.Controls.Add(m_ButtonsArray [i]);
            m_ButtonsArray [i].Click += new EventHandler(btn_Click);
        }
    }

    void btn_Click(object sender, EventArgs e)
    {
        Button btn = (Button)sender;
        if (((int)btn.Tag) == 5)
        {
           m_ButtonsArray[2].Text = "your text here";
        }
    }
}

explanation:
when i create the button, i add the index as a tag (the button knows it’s index with the Tag property)
when a button is clicked, i check the sender object – casting it to Button, for the index that kept in the Tag. if the Tag value is 5 then i reference the second button (i can reference it though m_ButtonsArray) and change the values of it