Pulling data into a grid view using 1 method and getting null ref exception when saving changes

Question

Good day everyone,

Firstly I would like to thank anyone in advance who takes the time to look through this lengthy post.

Context: I am building a simple GUI interface using windows forms and c sharp that connects to a MySql database to perform CRUD operations.

Problem: I am trying to implement a method that would take the necessary MySql code elements as arguments and pull a table into a data grid view. I have produced 2 methods - 1 that takes arguments and the other one which is hard coded to pull the data.

After pulling the data I am able to insert/amend any records in the database with a SAVE button method. The issue is that when I use the method that takes the arguments to pull the data into the grid view I am not able to SAVE my changes as I get a null reference error. However when using the hard coded method I don't get any null errors and all works fine. For some reason when the sqlDataand dataTable variables are passed to the SAVE button method AFTER the grid view gets filled with the method based on arguments they end up as null. Any experts that have experience building these applications have any advice please?

Hard coded method for pulling data into grid view:

 private void fill_grid_view1(string sequelQueryString)
    {
        using (MySqlConnection mysqlConn = new MySqlConnection(db_connection))
        {
            mysqlConn.Open();
            sqlData = new MySqlDataAdapter(sequelQueryString, mysqlConn);
            sqlCommandBuilder = new MySqlCommandBuilder(sqlData);

            dataTable = new DataTable();
            sqlData.Fill(dataTable);

            dataGridView1.DataSource = dataTable;
        }   
    }

The SAVE button methods:

private void bttnSave_Click(object sender, EventArgs e)
    {

        save_changes_refresh(sqlData, dataTable);

    }

private void save_changes_refresh(MySqlDataAdapter given_data_adapter, DataTable given_data_table) 

    {
        try
        {
            given_data_adapter.Update(given_data_table);

            select_both_user_tweet();

            MessageBox.Show("Your changes have been saved successfully!"); //
        }
        catch (Exception errobj)
        {
            MessageBox.Show(errobj.Message.ToString());
        }

    }

The method I want to use to pull data into grid view based on given arguments:

 private void fill_given_grid_view (string sequelQueryString, MySqlDataAdapter given_data_adapter, DataTable given_data_table, DataGridView given_grid_view,
                                        MySqlCommandBuilder given_builder)

    {
        using (MySqlConnection mysqlConn = new MySqlConnection(db_connection))
        {
            mysqlConn.Open();
            given_data_adapter = new MySqlDataAdapter(sequelQueryString, mysqlConn);
            given_builder = new MySqlCommandBuilder(given_data_adapter);

            given_data_table = new DataTable();
            given_data_adapter.Fill(given_data_table);

            given_grid_view.DataSource = given_data_table;
        }
    }

All the new method does is pull data based on arguments so that if I had let's say 5 dataGridView elements I wouldn't need to hard code all five pull methods separately like I did in the first code snippet. And it works but it doesn't let me save any changes as mentioned above because of the sqlData and dataTable variables end up as null when I try to execute the save method.

Method that passes the needed parameters to fill_given_grid_view:

private void view_users_Click(object sender, EventArgs e) 
    {
        fill_given_grid_view("SELECT * FROM new_schema.user", sqlData, dataTable, dataGridView1, sqlCommandBuilder);

    }

EDIT: I've read the possible duplicate thread and it is useful however I struggle to understand why essentially using 2 methods that do the same thing one of them drops the sqlData and dataTable variables to null and the hard coded method from the first snippet does not drop the variables and keeps the needed values to pass into the SAVE method.

Answer 1

Based on @Reniuz suggestions the fill_given_grid_view and save_changes_refresh methods had to be rewritten to take in dataGridView and sequelQueryString as arguments. sqlData and dataTable variables are not used anywhere as input to a method. Code examples below:

Method to pull data into a grid view based on input:

        private void fill_given_grid_view (string sequelQueryString, DataGridView given_grid_view) /* master method that takes an SQL query and grid view as input 
                                                                                                  and displays a table accordingly */

    {
        using (MySqlConnection mysqlConn = new MySqlConnection(db_connection)) // using stored connection params
        {
            mysqlConn.Open();
            sqlData = new MySqlDataAdapter(sequelQueryString, mysqlConn);
            sqlCommandBuilder = new MySqlCommandBuilder(sqlData);

            dataTable = new DataTable(); // new dataTable created, filled based on given query and set as the DataSource of the grid given as input to the method
            sqlData.Fill(dataTable);

            given_grid_view.DataSource = dataTable;
        }
    }

SAVE method rewritten with grid view and sequel string as arguments:

private void save_changes(string sequelQueryString, DataGridView given_grid_view) /* master save method that initializes a new Data Adapter based on given sequel string 
                                                                                         that saves any changes inputted into the given grid view */

    {
        using (MySqlConnection mysqlConn = new MySqlConnection(db_connection))
        {
            mysqlConn.Open();
            sqlData = new MySqlDataAdapter(sequelQueryString, mysqlConn);
            sqlCommandBuilder = new MySqlCommandBuilder(sqlData);

            var current_data_table = (DataTable)given_grid_view.DataSource; // if any changes occur in given grid view db table is updated when a "SAVE" button is clicked
            sqlData.Update(current_data_table);
        }
    }