vb.net Task.Factory multiple tasks needed? - vb.net

Question

Is this asynchronous programming correct ?
Since this is my first time using TAP, I want to make sure I do it correctly from the beginning.
I want to fill a table from a ODBC database and afterwards read some files and extract values out of it, without freezing my UI.
Why do I need to run OdbcDataAdapter and the file reading as tasks if I run the whole Function as a task in my UI Sub ? Otherwise it blocks my UI. thread.
UI Code
Private Async Sub frmOfsList_Shown(sender As Object, e As EventArgs) Handles MyBase.Show
Dim sw As New Stopwatch 'query time
sw.Start()
DataGridView1.Visible = False
Label2.Visible = False
DataGridView1.DataSource = Await OFS.GetJobList 'async method
sw.Stop()
Label2.Text = "Query time: " & sw.Elapsed.TotalSeconds & "s"
For i As Integer = 0 To DataGridView1.Rows.Count - 1 'color days until prodution date
If DataGridView1.Rows(i).Cells(3).Value < 0 Then
DataGridView1.Rows(i).Cells(3).Style.ForeColor = Color.Red
Else
DataGridView1.Rows(i).Cells(3).Style.ForeColor = Color.Green
End If
Next
DataGridView1.Visible = True 'show grid
DataGridView1.ClearSelection()
Label2.Visible = True
End Sub
Async Function
Public Shared Async Function GetJobList() As Task(Of DataTable)
Dim dq As Char = """"
Dim con As OdbcConnection = New OdbcConnection(constr)
con.Open()
'get data from OFS
Dim cmd As String = "SELECT p1.ProductionOrder, p1.Project, p1.ProductionDate, p1.Item, p1.Revision, p1.PlannedQty FROM " &
dq & "OFS460" & dq & "." & dq & "dbo" & dq & "." & dq & "tblProductionOrders" & dq & " p INNER JOIN " & dq & "OFS460" & dq & "." & dq & "dbo" &
dq & "." & dq & "tblProductionOrders" & dq & " p1 ON p.ProductionOrder = p1.ProductionOrder WHERE (p.Task=2820 AND p.StatusID=4) AND (p1.Task=2830 AND (p1.StatusID=1 OR p1.StatusID=2 OR p1.StatusID=3)) ORDER BY p1.ProductionDate"
Dim adapter As OdbcDataAdapter = New OdbcDataAdapter(cmd, con)
Dim datatable As New DataTable("JobList")
'fil table with job data async
Await Task.Factory.StartNew(Sub()
adapter.Fill(datatable)
End Sub)
'add columns to table
datatable.Columns.Add("Length", GetType(Double))
datatable.Columns.Add("Outside Dia", GetType(Double))
Dim proddate As DateTime
datatable.Columns.Add("Days until").SetOrdinal(3)
'calculate days
For j As Integer = 0 To datatable.Rows.Count - 1
proddate = datatable(j)(2)
datatable.Rows(j)(3) = proddate.Subtract(DateTime.Now).Days
Next
'Get length and diameter for each part
Dim searchpath As String = My.Settings.g250path
Await Task.Factory.StartNew(Sub()
Dim files As String()
Dim filetext As String
For i As Integer = 0 To datatable.Rows.Count - 1
files = System.IO.Directory.GetFiles(searchpath, "*" & datatable.Rows(i)("Item") & "*") 'get file by item#
If files.Length > 0 Then
filetext = System.IO.File.ReadAllText(files(0)) 'read file
datatable.Rows(i)("Length") = ProgramManager.GetValue(filetext, "I_R872", 7).ToString 'extract values
datatable.Rows(i)("Outside Dia") = ProgramManager.GetValue(filetext, "I_R877", 7).ToString
End If
Next i
End Sub)
Return datatable
End Function

Answer 1

You should not use Task.Factory.StartNew with Async-Await. You should use Task.Run, instead.
And you only need to get out of the UI thread once for the "heavy work" and return when done.
Try this:
Public Shared Function GetJobList() As DataTable
Dim dq As Char = """"
Dim con As OdbcConnection = New OdbcConnection(constr)
con.Open()
'get data from OFS
Dim cmd As String = "SELECT p1.ProductionOrder, p1.Project, p1.ProductionDate, p1.Item, p1.Revision, p1.PlannedQty FROM ""OFS460"".""dbo"".""tblProductionOrders"" p INNER JOIN ""OFS460"".""dbo"".""tblProductionOrders"" p1 ON p.ProductionOrder = p1.ProductionOrder WHERE (p.Task=2820 AND p.StatusID=4) AND (p1.Task=2830 AND (p1.StatusID=1 OR p1.StatusID=2 OR p1.StatusID=3)) ORDER BY p1.ProductionDate"
Dim adapter As OdbcDataAdapter = New OdbcDataAdapter(cmd, con)
Dim datatable As New DataTable("JobList")
'fil table with job data async
adapter.Fill(datatable)
'add columns to table
datatable.Columns.Add("Length", GetType(Double))
datatable.Columns.Add("Outside Dia", GetType(Double))
Dim proddate As DateTime
datatable.Columns.Add("Days until").SetOrdinal(3)
'calculate days
For j As Integer = 0 To datatable.Rows.Count - 1
proddate = datatable(j)(2)
datatable.Rows(j)(3) = proddate.Subtract(DateTime.Now).Days
Next
'Get length and diameter for each part
Dim searchpath As String = My.Settings.g250path
Dim files As String()
Dim filetext As String
For i As Integer = 0 To datatable.Rows.Count - 1
files = System.IO.Directory.GetFiles(searchpath, "*" & datatable.Rows(i)("Item") & "*") 'get file by item#
If files.Length > 0 Then
filetext = System.IO.File.ReadAllText(files(0)) 'read file
datatable.Rows(i)("Length") = ProgramManager.GetValue(filetext, "I_R872", 7).ToString 'extract values
datatable.Rows(i)("Outside Dia") = ProgramManager.GetValue(filetext, "I_R877", 7).ToString
End If
Next i
Return datatable
End Function
And invoke it like this:
DataGridView1.DataSource = Await Task.Run(AddressOf OFS.GetJobList1)
This way, the OFS.GetJobList1 function will be scheduled to execute on a thread pool thread and, when completed, the execution will resume on the caller sub/function and the return value of Task.Run (which is the return value of OFS.GetJobList1 wrapped with a Task(Of DataTable)) will be unwrapped and assigned to DataGridView1.DataSource.