Wednesday, September 3, 2008

Asynchronous Data Loading using TableAdapter with Cancel Feature

Level: Intermediate

Knowledge Required:
  • BackgroundWorker Control
  • DataGridView Control
  • Data Binding
  • TableAdapter
In my previous post

Implement SQLCommand Cancel with Threading

I have discussed how to implement the SQLCommand Cancel using Threading. In this post we shall see, how we can implement the same feature in a TableAdapter.

As we know that TableAdapter is used as a middle level man standing between Physical Database and Typed DataSet. Which gets the data from Physical Database and loads into Typed DataSet. Sometimes this loading of data takes time (for example due to complex query). Therefore we can provide the user an ability to cancel the on going command by clicking on Cancel Button.

I have created a Test Database (TestDB) and have added a table (tbl_Student). Also I have added a Stored Procedure (GetStudent). Take a look at this Stored Procedure:

CREATE PROCEDURE [dbo].[GetStudent] 

-- here we are simulating a very long query
-- which takes 3 seconds to complete

WAITFOR DELAY '00:00:03';

FROM tbl_Student;

In this procedure I have added a WAITFOR DELAY SQL Statement. Which simulates a long query that takes time 3 seconds. We will call this procedure from our TableAdapter to fill the DataTable.

Now we will add the SQLCommand Cancel stuff in our TableAdapter. This can be achieved by creating a Partial Class of TableAdapter,

Namespace TestDataSetTableAdapters
Partial Class StudentTableAdapter
Public Sub CancelSelect()
End Sub
End Class
End Namespace

As you see I have given the name of method CancelSelect which actually cancels the SelectCommand of Adapter. Note that I haven’t implemented any checking here (for example is command executing or NOT). This is upto the user of TableAdapter class. He/She will be responsible to check before executing this method.

Here we have done with our TableAdapter. Now next thing is to implement the loading in the different Thread. For this purpose I have used the BackgroundWorker control which is quite simple. The overall idea is,

  • Create 2 buttons
    • LoadButton
    • CancelButton
  • When LoadButton is clicked
    • Execute the BackgroundWorker
  • In BackgroundWorker DoWork Event Handler
    • Note somewhere that Execution is started
    • Execute the Fill() Method of TableAdapter
  • When CancelButton is clicked
    • Check if execution is started then
      • Execute the CancelSelect() method of TableAdapter

Update Sep-4-2008:
Here is the code of Form1
Public Class Form1

    ' ErrorText will contain the error information if occurred
    ' while loading data in different thread
    Private _ErrorText As String
    ' this boolean variable will be true when command is executed
    Private _CommandExecuted As Boolean

    Private Sub btnLoad_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnLoad.Click
        ' first we will disable the Load button so it should NOT be clicked again
        Me.btnLoad.Enabled = False
        ' then we will enable the Cancel button so the command can be canceled
        Me.btnCancel.Enabled = True
        ' set the status text
        Call Me.SetStatusText("Loading...")

        ' clear the Error Text
        Me._ErrorText = ""

        ' here we have first clear the currently loaded data
        ' from DataSet, because the Adapter will fill the DataSet
        ' in different thread, therefore it will create problems
        ' while displaying the same data (which is being modified
        ' in different thread) in DataGridView

        ' start the asynchronous task
    End Sub

    Private Sub BackgroundWorker1_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
            ' first set the executed flag
            Me._CommandExecuted = True
            ' execute the command
        Catch ex As Exception
            _ErrorText = ex.Message
        End Try
    End Sub

    Private Sub BackgroundWorker1_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
        ' ok our data is loaded now
        ' so enable the Load button
        Me.btnLoad.Enabled = True
        ' disable the Cancel button
        Me.btnCancel.Enabled = False
        ' if NO error was occurred in thread
        If Me._ErrorText = "" Then
            Me.SetStatusText(Me.TestDataSet.Student.Count & " Records loaded")
        Else ' else (some error was occurred)
        End If
        ' since data is changed in different thread
        ' therefore by reseting the Bindings the DataGridView
        ' will refresh itself and will display all the data
    End Sub

    Private Sub btnCancel_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCancel.Click
        ' if command is executed then
        If Me._CommandExecuted Then
            ' cancel it
        End If
    End Sub

    Private Sub SetStatusText(ByVal sText As String)
        Me.lblStatus.Text = sText
    End Sub
End Class


Note: The zip file contains TestDBScript.sql T-SQL Script which will,
1) Create a database "TestDB"
2) Create a table "tbl_Student" in that database
3) Create a Stored Procedure "GetStudent"
4) Put some data in tbl_Student

Please execute this script before running the project.


Troy Johnson said...

You may want to consider trapping e.Cancel in both DoWork and RunWorkerCompleted if you are allowing cancellations. You can see what I mean at:

Dwight Miller said...

When I try to fill the tableadapter from within the background worker, I get a thread error. I have to use a delegate to attach the tableadapter back to the binding source.