button array NullReferenceException was unhandled

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 :P 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
        }
    }
}

eyossi · Accepted Answer · 2012-06-25 14:58:00Z

3

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

edited Jun 25, 2012 at 14:58

answered Jun 25, 2012 at 11:19

eyossi

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5 Comments

modest and cute girl Over a year ago

thanks but then how can i reach to the 3th button and make its color change

modest and cute girl Over a year ago

yuppi yes it worked thanks. so in my code if it cancel Button[] btn = sender as Button[]; part in button click and move Button[] btn = new Button[5]; to one above form1load part it wokrs also. thanks but then when i use Button btn = sender as Button; ??? i mean in which cases i use that statement??? i wrote that statement without understanding fully. and now i see that i should not write it. but in which cases i use it. and another question is. is it possible to change the name of the button text when i click to the specific button lets sat the 2th button?

eyossi Over a year ago

I will answer your second question first: you can add this line: m_ButtonsArray[i].Tag = i; and then on btn_Click(object sender, EventArgs e) you can do: Button btn = sender as Button; if (((int)btn.Tag) == 2) btn.Text="your text here" that should answer the first question too

modest and cute girl Over a year ago

sorry but coulndt get in which cases i use sender stuff :( and i think i coulndt be clear with the other question which is how can i change the 2th button name when i click to a lets say button 5 with specific button i mean that. thanks.

eyossi Over a year ago

@modestandcutegirl check the new update for your second question

y.baris · Accepted Answer · 2012-06-25 14:08:33Z

1

You can solve like that too...it's so similar for eyossi's solution

public partial class Form1 : Form { private

public Form1()
{
    InitializeComponent();
}
Button[] m_Buttons = Array new Button[5];

private void Form1_Load(object sender, EventArgs e)
{
    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
}

}

answered Jun 25, 2012 at 14:08

y.baris

739 bronze badges

Comments

David · Accepted Answer · 2012-06-25 11:21:35Z

0

This code is where the problem begins:

Button[] btn = sender as Button[];

At compile time there is no indication to the compiler that the sender of type object won't cast to an array of Buttons. But at runtime it won't cast as such, because it's not an array. It's just a Button:

Button btn = sender as Button;

The handler method (btn_Click) isn't singularly handling all buttons at the same time, but rather each one individually. When a button is clicked, that button by itself invokes the handler method.

answered Jun 25, 2012 at 11:21

David

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3 Comments

modest and cute girl Over a year ago

when i made this compiler gives error at the btn[3].Text = "button name changed"; part. i want to reach to the 3th button and change the color.

David Over a year ago

@modestandcutegirl: Since there's no reference to the Button in the form (it's dynamically added), you're going to need to find it inside of flowLayoutPanel1.Controls, where it exists amid the other buttons (and potentially other controls). That's the only place it'll be known from within the handler method.

modest and cute girl Over a year ago

thanks. i wanted to thank to all but cant reply to my onw msg since im new and not much reputation. so im thanking to u seperatly. thanks for ur replies.

Julien Roncaglia · Accepted Answer · 2012-06-25 11:26:14Z

0

(Others already answered correctly why this exception is thrown)

The error can't be shown by the compiler because you are forcing a cast (using as) from an object to something else. The compiler doesn't know what the object is and so can't know that an error will happens.

If you call manually the method in your code and pass it an array of buttons as the first argument the code won't crash.

A good thing in this case would have been to use a standard cast instead of as the exception thrown would have been a lot more explicit and would have happened on the correct line instead of letting a null value propagate and fail latter.

A small remark on your code is that your array is useless, you don't use it anywhere and it will be removed from memory as soon as the load function finish executing (Well not exactly but you could read about the Garbage collector if you want more details).

answered Jun 25, 2012 at 11:26

Julien Roncaglia

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2 Comments

modest and cute girl Over a year ago

the point is that i want to reach to the lets say 3th button int the array and make its color change when i clicked on the button? this is just and exercise i made up. cuz i can have 100s buttons and i made it with for loop and put it in to an array and when i click to a button i may want to make the 2th indexed (the 3th) button to change the name or change the color. and another question is what if i want to color the 3th button when i click to a specific lets say 5th button? and thanks to all of u guys who replied. hope my question is more or less clear now.

modest and cute girl Over a year ago

thank u virtualblackfox for information. can u read my reply to "eyossi" also please. and if u have some says ur welcome. thanks u all for ur kind replies.

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