Asynchronous Processing of Database Records on a WinForm UI Using Multithreading

• Add code for the DoWork, ProgressChanged and RunWorkerCompleted events of the BackgroundWorker control:

  private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
  {
  MyWorkerClass obj = (MyWorkerClass)e.Argument;
  LoadData(obj);
  }

  Here, the LoadData method is called to load data from the database:
  
  private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
  {
  if (!backgroundWorker1.CancellationPending)
  {
  MyWorkerClass obj = (MyWorkerClass)e.UserState;
  cbEmployee.Items.Add(obj.EmployeeName);
  cbEmployee.SelectedIndex = e.ProgressPercentage;
  lblEmployeeName.Text = obj.EmployeeName;
  lblTotalRows.Text ="Rows processed : " +e.ProgressPercentage.ToString();
  }
  }

  The ProgressChanged event is fired when the ReportProgress method is called. Here, it is called from the LoadData method for every record processed. The ReportProgress method passes the MyWorkerClass object for every record. This object contains EmployeeName that is shown in the lblEmpoyeeName Label control and it is added in the ComboBox item collection.

  ProgressChangedEventArgs.ProgressPercentage returns an integer as percentage that is passed in ReportProgress method.
  
  Note: We cannot access a UI control directly from the DoWork event of a BackgroundWorker. So we use a class object for communication between the DoWork and ProgressChanged events like our MyWorkerClass.
  
  private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
  {
  lblEmployeeName.Text = "All data loaded!";
  btnLoadData.Enabled = true;
  btnLoadData.Text = "Load Data";
  }

  The RunWorkerCompleted event is fired after the BackgroundWorker has completed its task or cancelled or an exception has occurred.
  

• Code for LoadData method:
  
  public void LoadData(MyWorkerClass obj)
  {
  try
  {
  using (con = new SqlConnection(ConString))
  {
  CmdString = "SELECT EmployeeID, FirstName + ' ' + LastName AS Name FROM Employees";
  cmd = new SqlCommand(CmdString, con);
  con.Open();
  reader = cmd.ExecuteReader();
  int i = 0;
  while (reader.Read())
  {
  obj.EmployeeName = reader["Name"].ToString();
  obj.EmployeeID =int.Parse(reader["EmployeeID"].ToString());
  backgroundWorker1.ReportProgress(i, obj);
  i++;
  Thread.Sleep(10);
  }
  con.Close();
  }
  }
  catch (Exception ex)
  {
  MessageBox.Show(ex.Message);
  }
  }

Here we are simply accessing data from the Employees table using a DataReader. We are using the ReportProgress method to report progress of every record accessed from the database and passing a counter for the ProgressPercentage and an object of MyWorkerClass with current employee's Name and EmployeeID. The ReportProgress method will raise the ProgressChanged event where EmployeeName is accessed via the passed object of the MyWorkerClass. The Thread.Sleep method is called to pause the background thread for 10 ms so that the current processing record is visible in the ComboBox and Label control on our UI.

Finally, when we click on the LoadData button, we can see the Employees being added to the ComboBox and lblEmployeeName Label. In the other Label we can see the number of rows processed.