Question

I have written this code to read thousands of rows from Excel files and load them in a DataGridView.

But the problem I am facing is, no matter which file I load, the DataGridView is showing the rows from the first file only and the _list is never cleared.

public class MyForm : Form
{
    private List<Student> _list = null;

    private void LoadFile_Click(object sender, EventArgs e)
    {
        try
        {
            if (_list != null)
            {
                _list.Clear();
            }

            openFileDialog1.ShowDialog();

            _connStr = MakeConnectionString.GetConnectionString(openFileDialog1.FileName);

            if (!string.IsNullOrEmpty(_connStr))
            {
                backgroundWorker1.RunWorkerAsync();
            }
        }
        catch
        {
            MessageBox.Show("Application is busy with the first task!", "Busy...", MessageBoxButtons.OK, MessageBoxIcon.Warning);
        }
    }

    private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
    {
        if (backgroundWorker1.CancellationPending)
        {
            e.Cancel = true;
            return;
        }

        IDataReader read = StudentDA.GetReader(_connStr);
        List<Student> localList = null;

        if (_list != null)
        {
            _list.Clear();
        }
        _list = StudentMapper.GetStudents(read);


        localList = new List<Student>(_list);

        dataGridView1.Invoke(new MethodInvoker(delegate
        {
            dataGridView1.Rows.Clear();
        }));

        foreach (Student std in localList)
        {
            dataGridView1.Invoke(new MethodInvoker(delegate
            {
                dataGridView1.Rows.Add(std.SerialNo, std.RollNo);
            }));
        }
    }
}

Answer 1

Try creating a new BackgroundWorker object each time , you load new data.

you are not chaning the _connection object

static _connection = null;

if(_connection == null)
  {

  }

this will work only works for 1st time and next time when you change the file this connection is not getting changed.