如何运行.Net：SQLCommand在其自己的线程上具有允许用户在运行中安全取消它的能力

Question

My team and I have been banging our heads around this all week. Appreciate any help. We are using VB.Net. We have a form where the user will use a SQLCommand to execute a stored procedure that can take several minutes to run. We would like the user to be able to continue working in the application while it's running or to be able to cancel the request in the middle of processing. We have looked into threading and background workers, and asynchronous processing. We cannot find a solution that will allow us to cancel the running of the stored procedure and thread safely without errors. Does anyone know how to do this safely. Here's what we have so far. *some details removed for security and size

Private Sub frmAdhocRptGenerator_Load(sender As Object, e As EventArgs) Handles Me.Load
    BackGroundWorkerAdhoc.WorkerReportsProgress = True
    BackGroundWorkerAdhoc.WorkerSupportsCancellation = True
End Sub

Private Sub btnExecute_Click(sender As Object, e As EventArgs) Handles btnExecute.Click
        If Not BackGroundWorkerAdhoc.IsBusy = True Then BackGroundWorkerAdhoc.RunWorkerAsync()
End Sub

Private Sub BackGroundWorkerAdhoc_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackGroundWorkerAdhoc.DoWork
    Try
        If BackGroundWorkerAdhoc.CancellationPending = True Then
            e.Cancel = True
            Exit Sub
        End If
        Load_n_ExecuteProc(tmpConn)
    Catch ex As Exception
        ErrorHandler("frmAdhocRptGenerator.subBackGroundWorkerAdhoc_DoWork", ex.ToString)
        tmpConn.Close()
    End Try
End Sub

Private Function Load_n_ExecuteProc(theConn As SqlConnection) As Boolean
    If BackGroundWorkerAdhoc.CancellationPending = True Then
        Return False
        Exit Function
    End If
    Try
        Dim RunCompleted As Boolean = False

        cmdPROVIDER_FINDER = New SqlCommand("SP$PROVIDER_FINDER", tmpConn)
                cmdPROVIDER_FINDER.CommandType = CommandType.StoredProcedure
                cmdPROVIDER_FINDER.CommandTimeout = 10000
                Dim rst As Integer = cmdPROVIDER_FINDER.ExecuteNonQuery()
                RunCompleted = rst > -1
        Return RunCompleted
    Catch ex As Exception
        ErrorHandler("frmAdhocRptGenerator.Load_n_ExecuteProc", ex.ToString)
        Return False
    End Try
End Function


Private Sub BackGroundWorkerAdhoc_RunWorkerCompleted(sender As Object, e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackGroundWorkerAdhoc.RunWorkerCompleted
    Try
        If Success = True Then
            Dim filename As String = "AdhocResults" & Now.Month.ToString & "-" & Now.Day.ToString & "-" & Now.Year.ToString
            Export2Excel("SELECT * FROM ##PF_PROVIDERS" & currUser.UserID, filename, True)
        End If
        tmpConn.Close()
        Success = False
    Catch ex As Exception
        ErrorHandler("frmAdhocRptGenerator.BackGroundWorkerAdhoc_RunWorkerCompleted", ex.ToString)
    Finally
        tmpConn.Close()
    End Try
End Sub


Private Sub btnCancel_Click(sender As Object, e As EventArgs) Handles btnCancel.Click
    If BackGroundWorkerAdhoc.IsBusy = True Then
        BackGroundWorkerAdhoc.CancelAsync()
        cmdPROVIDER_FINDER.Cancel()
        tmpConn.Close()
        tmpConn = Nothing
    End If
    btnCancel.Enabled = False
End Sub

Answer 1

Background worker cancellation isn't a magic method. All it does is sets .CancellationPending to true. It's up to your code to actually do the cancellation in a safe way and then exit the DoWork handler.

Your existing code for cancellation can only work if your user cancels before the thread starts, that's unlikely so you can remove it.

If BackGroundWorkerAdhoc.CancellationPending = True Then
    Return False
    Exit Function
End If

Instead issue an SqlCommand.Cancel call from your UI thread. Then the command will terminate and then you'll exit DoWork as normal. If you're going to be terminating an sql command prematurely I'd recommend doing it on a throw away SqlConnection hence why I've created a new one.

Private Sub BackGroundWorkerAdhoc_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackGroundWorkerAdhoc.DoWork
    Try
        Using myNewConn As New SqlConnection(...)
            Load_n_ExecuteProc(myNewConn)
        End Using
    Catch ex As Exception
        ErrorHandler("frmAdhocRptGenerator.subBackGroundWorkerAdhoc_DoWork", ex.ToString)
    End Try
End Sub

Private Function Load_n_ExecuteProc(theConn As SqlConnection) As Boolean
    Try
        cmdPROVIDER_FINDER = New SqlCommand("SP$PROVIDER_FINDER", tmpConn)
        cmdPROVIDER_FINDER.CommandType = CommandType.StoredProcedure
        cmdPROVIDER_FINDER.CommandTimeout = 10000
        Dim rst As Integer = cmdPROVIDER_FINDER.ExecuteNonQuery()
        return = rst > -1
    Catch ex As Exception
        ErrorHandler("frmAdhocRptGenerator.Load_n_ExecuteProc", ex.ToString)            
    Finally
        cmdPROVIDER_FINDER = Nothing
    End Try
End Function

Public Sub CancelSqlCommand()
    Dim cmd = cmdPROVIDER_FINDER
    If cmd isnot Nothing cmd.Cancel()
End Sub