NullReferenceException was unhandled - what is null?

Question

I'm trying to access an event in my main form by clicking a button (btnsearch_Click) and everytime I clicked it, it says 'object reference not set to an instance of an object'.

Here is my code:

USER CONTROL

namespace Purchase_Order
{
    public partial class Search : UserControl
    {
        public event EventHandler btnSearchClicked;

        public Search()
        {
            InitializeComponent();
        }
        private void btnsearch_Click(object sender, EventArgs e)
        {

           btnSearchClicked(sender, e);
        }
   }
}

MAIN FORM

namespace Purchase_Order
{
    public partial class formMain : Form
    {

        public formMain()
        {
            InitializeComponent();            
        }




 private void formMain_Load(object sender, EventArgs e)
        {

Search searchbox = new Search();
searchbox.btnSearchClicked += new EventHandler(SearchClicked);
}


 void SearchClicked(object sender, EventArgs e)
        {
            MySqlConnection con = new MySqlConnection(serverstring);

            try
            {

                string query = "SELECT * FROM tblclassification WHERE INSTR(class_name, @search)";

                MySqlCommand cmd = new MySqlCommand(query, con);
                MySqlDataAdapter da = new MySqlDataAdapter(cmd);

                Search content = new Search();
                cmd.Parameters.AddWithValue("@search", content.btnsearch.Text);

                DataTable dt = new DataTable();
                da.Fill(dt);


                classification control = new classification();
                control.dataGridView1.DataSource = dt;
                control.dataGridView1.DataMember = dt.TableName;

                panelMain.Controls.Clear();
                panelMain.Controls.Add(control);
                MessageBox.Show("OK");

            }
            catch (Exception)
            {
                throw;
            }
            finally
            {
                if (con.State == ConnectionState.Open)
                {
                    con.Close();
                }
            }    
        }

Answer 1

Share the Search instance from formMain_Load with the SearchClicked method

MAIN FORM

 public partial class formMain : Form
    {
        private Search _searchbox;
        ...

private void formMain_Load(object sender, EventArgs e)
{
    _searchbox = new Search();
    _searchbox.btnSearchClicked += new EventHandler(SearchClicked);
}

void SearchClicked(object sender, EventArgs e)
        {


     Search content = _searchbox;

MySqlConnection con = new MySqlConnection(serverstring);

            try
            {

                string query = "SELECT * FROM tblclassification WHERE INSTR(class_name, @search)";

                MySqlCommand cmd = new MySqlCommand(query, con);
                MySqlDataAdapter da = new MySqlDataAdapter(cmd);


                cmd.Parameters.AddWithValue("@search", content.btnsearch.Text);

                DataTable dt = new DataTable();
                da.Fill(dt);


                classification control = new classification();
                control.dataGridView1.DataSource = dt;
                control.dataGridView1.DataMember = dt.TableName;

                panelMain.Controls.Clear();
                panelMain.Controls.Add(control);
                MessageBox.Show("OK");

            }
            catch (Exception)
            {
                throw;
            }
            finally
            {
                if (con.State == ConnectionState.Open)
                {
                    con.Close();
                }
            }    
            }
        }

USER CONTROL

public partial class Search : UserControl
    {
        public event EventHandler btnSearchClicked;

        public Search()
        {
            InitializeComponent();
        }
        private void btnsearch_Click(object sender, EventArgs e)
        {

           btnSearchClicked(sender, e);
        }
   }

Answer 2

To try and expand a little on Habib's answer (I was going to post this as a comment but it's a little lengthy), you are first creating an instance of Search and registering the event in formMain_Load here:

private void formMain_Load(object sender, EventArgs e)
{
    Search searchbox = new Search();
    searchbox.btnSearchClicked += new EventHandler(SearchClicked);
}

This is all fine and dandy. However, in SearchClicked, you create a new instance of Search like so:

Search content = new Search();

This is a separate object to the one you created in formMain_Load and you never register the event to this object. It looks like what you want to do is share the Search instance from formMain_Load with the SearchClicked method. To do this, create a property in your codebehind:

public partial class formMain : Form
{
    private Search _searchbox;
    ...
}

Then, in formMain_Load:

private void formMain_Load(object sender, EventArgs e)
{
    _searchbox = new Search();
    _searchbox.btnSearchClicked += new EventHandler(SearchClicked);
}

Now, you can reuse this object with the event registered in SearchClicked by changing this:

Search content = new Search();

To this:

Search content = _searchbox;

You should find that the exception goes away. Hopefully, this will have provided a little more insight and will help you to understand the cause of the error and how to circumvent it.

Answer 3

That means that you haven't got an instance of the type you want to use.

public event EventHandler btnSearchClicked; is just a reference for the "real" object you want to use.

It's like you trying to open a door of a house you only have a blueprint. This isn't really possible (at least not in our universe). You will first need to build the house and then try to enter it. Something like this is the case with your problem.

You will have to read a few tutorials about C#

Edit:

The thing about null is that there is nothing the reference you have is pointing too. If you haven't created anything then there isn't anything to reference...

Because you are trying to use something that doesn't exists (is null) you are getting an exception.

Answer 4

You are creating a new instance of your user control in SearchClicked method and you are not registering the event against it.

Search content = new Search();

Also its better if you check whether any control has register your event before raising it like:

private void btnsearch_Click(object sender, EventArgs e)
{
 if(btnSearchClicked != null)
       btnSearchClicked(sender, e);
}