private IDbCommand _activeSqlCommand;
private CancellationTokenSource _cts;
private async void btnExecute_Click(object sender, RoutedEventArgs e)
    // The _activeSqlCommand and _cts should be null here.
    // Otherwise, you may end up with multiple concurrent executions.
    Debug.Assert(_activeSqlCommand == null);
    Debug.Assert(_cts == null);
    var sqlQuery = "A very long and complicated SQL query...";
    var localSqlCommand = new SqlCommand(sqlQuery, _connection);
    var localCts = new CancellationTokenSource();
    _activeSqlCommand = localSqlCommand;
    _cts = localCts;
    btnExecute.IsEnabled = false;
    btnCancel.IsEnabled = true;
        DataTable dataTable = await AsCancelable(Task.Run(() =>
            var dt = new DataTable();
            using (SqlDataAdapter a = new SqlDataAdapter(localSqlCommand))
                a.Fill(dt);
            return dt;
        }, localCts.Token), localCts.Token);
        // Here use the dataTable to update the UI
    catch (OperationCanceledException) { } // Ignore
    catch (SqlException ex) when (ex.ErrorCode == CANCEL_ERROR_CODE) { } // Ignore
    finally
        btnCancel.IsEnabled = false;
        btnExecute.IsEnabled = true;
        // The _activeSqlCommand and _cts should still have the local values here.
        Debug.Assert(_activeSqlCommand == localSqlCommand);
        Debug.Assert(_cts == localCts);
        _activeSqlCommand = null;
        _cts = null;
        localCts.Dispose();
private void btnCancel_Click(object sender, RoutedEventArgs e)
    _activeSqlCommand?.Cancel();
    _cts?.Cancel();