Using .NET 4.5 Task Async and Await with WPF and SQL Server

Most of the examples I’ve seen of ¬†using the new async, await and Task methods in .NET 4.5 have been aimed at Windows 8 (Metro or “UI Style”) apps. This is understandable, given Microsoft’s push towards both a new OS and to populate the Windows 8 Store. But for some (many?) developers there is also a need to support both Windows 7 and Windows 8 Desktop. Notably missing from the WinRT scenario is the ability to directly connect to SQL Server. I (mostly) get the reasons for that, although it seems against Microsoft’s interests to cede that space to SQLite instead of a sandboxed, light version of SQL Server.

Anyway, I put together a sample WPF application (download below) that supports asynchronous connections to SQL Server. You can cancel or restart the demo query, which populates a ListView with data from the AdventureWorks DB Production.WorkOrder table, which had a decent number of records for testing. I really wanted to simulate some real world conditions, such as server-side delays, so that’s included too. One of my beefs with many of the samples I come across is that they never include decent error handling, especially to accomodate network latency or timeouts. This sample also includes the¬†Microsoft.Data.ConnectionUI SQL Server connection dialog box.

One advantage of making a WPF app asynchronous is it no longer locks up while running a process. Try dragging the window around while the ListView is being populated with data. Yay, no UI-thread blocking!

Clicking the Start button triggers the StartTest method. Notice that in the query, I’ve embedded an artificial server-side delay of 3 seconds – WAITFOR DELAY ’00:00:03′; – which will simulate a slow server return. Also, a queryTimeout has been set to 120 seconds. If you set the WAITFOR delay longer than that, you will see an error occurs and is handled. I did my best to include specific SQL Server error handling so we get actual error details, rather than just “some crap happened and that’s all we know.”


private async void StartTest(object sender, RoutedEventArgs e)
{
try
{
 if (_connectionString == String.Empty)
 {
 var scsb = new SqlConnectionStringBuilder(Settings.Default.DBConnString);

_connectionString = scsb.ConnectionString;

if (_connectionString == String.Empty)
 {
 throw new Exception("Please use Change DB to first select a server and database.");
 }
 }

if (DbInfo.DbName == String.Empty)
 {
 throw new Exception("Please use Change DB to first select a database from the server.");
 }

// Note: If the start test button was disabled, this would be unnecessary
 _cts.Cancel();

// Re-initialize this. Otherwise, StartTest won't work.
 _cts = new CancellationTokenSource();

DataItemsList.Clear();

// Note the simulated delay, which will happen on the DB server. You can vary this to see its effect.
 // Try dragging the window around after hitting the start test button.
 const string query =
 @"WAITFOR DELAY '00:00:03'; SELECT * FROM [Production].[WorkOrder];";

// Note that setting the WAITFOR delay to exceed this, eg. 00:02:01, will trigger an exception
 int queryTimeout = 120;

RequestStatus = "started";

await ProcessQueryCancelleable(_connectionString, query, queryTimeout, _cts.Token);
}
catch (Exception ex)
{
 MessageBox.Show(ex.Message + " (from StartTest)");

Debug.WriteLine(ex.Message);
}
}

The ProcessQueryCancelleable method. Note the use of the CancellationToken…’cause users might actually want to cancel a long-running query. Inside this method is another simulated delay (await Task.Delay(TimeSpan.FromMilliseconds(1));):

/// <summary>
/// Runs a cancelleable query
/// </summary>
/// <param name="connectionString">SQL Server connection string</param>
/// <param name="query">SQL query</param>
/// <param name="timeout">SqlDataReader timeout (maximum time in seconds allowed to run query). Note: Try varying this in conjunction
/// with the WAITFOR DELAY in the SQL query, e.g. make it shorter than the WAITFOR or maybe a second longer</param>
/// <param name="cancellationToken">Allows cancellation of this operation</param>
/// <returns></returns>
private async Task ProcessQueryCancelleable(string connectionString, string query, int timeout,
 CancellationToken cancellationToken)
{
 // await Task.Delay(TimeSpan.FromSeconds(5));

try
 {
 // Keep sqlConnection wrapped in using statement so disposal is handled automatically
 using (var sqlConnection = new SqlConnection(connectionString))
 {
 await sqlConnection.OpenAsync(cancellationToken);

using (SqlCommand cmd = sqlConnection.CreateCommand())
 {
 cmd.CommandTimeout = timeout;

cmd.CommandText = query;

using (SqlDataReader dataReader = await cmd.ExecuteReaderAsync(cancellationToken))
 {
 while (!dataReader.IsClosed)
 {
 // While the dataReader has not reached its end keep adding rows to the DataItemsList
 while (await dataReader.ReadAsync(cancellationToken))
 {
 // Process dataReader row
 WorkOrder workOrder = GetWorkOrderFromReader(dataReader);

DataItemsList.Add(workOrder);

// Another simulated delay...this one internal
 await Task.Delay(TimeSpan.FromMilliseconds(1));
 }

if (!dataReader.NextResult())
 {
 dataReader.Close();
 }
 }

Debug.WriteLine("done with reader");

RequestStatus = "finished";
 }
 }
 }
 }
 catch (SqlException sqlEx)
 {
 Debug.WriteLine(sqlEx.Message);
 }
}

Download C# Project (an MDF with just the WorkOrders table is included)