How to use async methods to load Db Data and keep UI responsive

Question

I made a somewhat large application that works well, except for its UI (winforms) freezes when using webclient to retrieve data from web (link is um... not the fastest), or

Answer 1

One way to leave the UI free is to use a Task. The proposed solution sort of thwarts the point of a BackGroundWorker with a do nothing loop to wait for it to complete. The code below makes a few other noteworthy changes:

' note the Async modifier
Private Async Sub btnDoIt_Click(...
    Dim sql = "SELECT * FROM RandomData"
    dtSample = Await Task(Of DataTable).Run(Function() LoadDataTable(sql))
    dgv2.DataSource = dtSample
End Sub

Private Function LoadDataTable(sql As String) As DataTable
    ' NO UI/Control references allowed
    Dim dt = New DataTable
    Using dbcon As New OleDbConnection(ACEConnStr)
        Using cmd As New OleDbCommand(sql, dbcon)
            dbcon.Open()
            dt.Load(cmd.ExecuteReader())
        End Using
    End Using
    Return dt
End Function

I dont have the exact conditions mentioned in the OP, but I do have an Access table with 500k rows. This taxes OleDB enough that it can take 6-10 seconds to load which is plenty long enough to tell if the UI remains responsive. It does.

1. Don't forget to use Asynch on whatever method will be awaiting the Task to complete.
2. The LoadTable method is set up to load any table from a valid query, so other things can use it. If/when queries change, just make changes to the things calling it.
3. DB Provider objects such as Connections and DBCommand objects ought to be disposed when you are done with the to release resources and prevent leaks. The Using blocks do this for us.
4. There is no need for a do-nothing/DoEvents loop, nor an event for a completed notice. The Await code will wait for the load method to complete so that any other things you need to do can be done afterwards.

A Task can often be simpler to use than a BackGroundWorker.

Resources

• Task Class
• Asynchronous Programming with Async and Await (Visual Basic)
• Using Statement