A beginner's tutorial for understanding Asynchronous execution of SQL statements using a simple application.

Introduction:

This tip aims at understanding the fundamental concept of asynchronous execution, i.e., how to use worker thread in collaboration with ADO.NET's BeginExecute & EndExecute feature to avoid UI freeze.

Background:

Below are the 2 main issues that arise when your application is intended to deal with huge data:
  • SQL Server takes significant time to process (long running SQL statements) which leads to blocking the execution of your .NET code
  • Main thread or UI thread also gets blocked till the response from the SQL Server.
These issues are the serious issues while building interactive applications. User patience is an unpredictable parameter and user’s reaction against long waiting screen is uncertain. At-least UI shouldn't freeze to engage the user and make him wait for the result.
Since, transactional SQL statements will definitely take time to process the things, the quickest solution sought is on the application programming level. Also, it is known that MS SQL server takes each & every call as synchronous, even if you change the connection string property AsynchronousProcessing to true. It is client application (C#, .NET) which gets affected. So, below are some widely used solutions.
  1. Cancellation Token mechanism - so that user can cancel ongoing longer execution if they are unwilling to wait
  2. Callback mechanism - so that UI thread can't get blocked
Cancellation Mechanism
It has many limitations, for example, a fired SQL command can be cancelled by firing SqlCommand.Cancel() but for this, that command object must be persist. SqlReader's Close or Read is designed to ignore this Cancel() call if this call is not placed before them and so on.
Callback Mechanism
It is a great programming style that solves many issues and helps you to build more interactive UI based applications. Instead of holding execution on a long running statement(s), it allows you to carry on to the next line of code. In this post, we will implement this functionality to get execute SQL query asynchronously.

How to implement:

In our sample project which is a WPF application, we will use Callback mechanism. Let's add the following controls on the Window:
  1. Button   ( btnExecuteQuery )
  2. ListBox  ( listboxResult )
  3. TextBox ( textboxQuery )
Idea is to fire asynchronous call to MS SQL for the T-SQL statement written in textboxQuery on btnExecuteQuery click event.

For executing MS SQL statements in asynchronous, we have to:
  • Set the Asynchronous Processing property of ConnectionString to true
  • Use command.BeginExecuteNonQuery() and command.EndExecuteNonQuery()
At connection level, a specification is required in connection string for asynchronous Processing. Example: AsynchronousProcessing = true

SqlConnectionStringBuilder connectionBuilder = new SqlConnectionStringBuilder( "Network Address=localhost;Initial Catalog=DB_EINV;Integrated Security=true;" )
  {
     ConnectTimeout = 4000,
     AsynchronousProcessing = true
  };
 
SqlConnection conn = new SqlConnection(connectionBuilder.ConnectionString);

At SqlCommand execution level, instead of ExecuteReader(), we need to use BeginExecuteReader() and EndExecuteReader() methods to achieve asynchronous execution. BeginExecuteReader takes 2 arguments:
  1. A callback procedure/method
  2. User-state (called 'StateObject' which holds the status of an asynchronous operation)
Example:
//The actual T-SQL execution happens in a separate work thread.
cmd.BeginExecuteReader(new AsyncCallback(MyCallbackFunction), cmd);

Generally, state-object is the object which will be required in the callback method to process the things. Here, we are passing SqlCommand object because our SQL query will be executed in different functions inside different worker threads. Our SqlCommand object is already initialized and ready to use by callback function.

Let’s focus on our callback function, which has been passed to AsyncCallback() inside BeginExecuteReader(). It has syntax as below:
private void MyCallbackFunction(IAsyncResult asyncResult)
 {
           
 }

Please note IAsyncResult type, which is required to hold the status of an asynchronous operation.

To un-box the AsynState back to the SqlCommand, we need to typecast as below:
//un-box the AsynState back to the SqlCommand
SqlCommand cmd = (SqlCommand)asyncResult.AsyncState;
SqlDataReader reader = cmd.EndExecuteReader(asyncResult);

So far, we have understand the required logic & way. Let's wire the things together.
Our Button Click Event Handler Code would look like:
private void btnExecuteQuery_Click(object sender, RoutedEventArgs e)
{
    SqlConnectionStringBuilder connectionBuilder
        = new SqlConnectionStringBuilder("Network Address=localhost;
        Initial Catalog=DemoDatabase; Integrated Security=true;")
    {
        ConnectTimeout = 4000,
        AsynchronousProcessing = true
    };
  
    SqlConnection conn = new SqlConnection(connectionBuilder.ConnectionString);
    SqlCommand cmd = new SqlCommand(textboxQuery.Text, conn);
    try
    {
        conn.Open();
  
        //The actual T-SQL execution happens in a separate work thread.
        cmd.BeginExecuteReader(new AsyncCallback(MyCallbackFunction), cmd);
    }
    catch (SqlException se)
    {
        //ToDo : Swallow exception log
    }
    catch (Exception ex)
    {
        //ToDo : Swallow exception log
    }
}

And the callback function looks like:
private void MyCallbackFunction(IAsyncResult result)
{
 try
  {
    //un-box the AsynState back to the SqlCommand
    SqlCommand cmd = (SqlCommand)result.AsyncState;
    SqlDataReader reader = cmd.EndExecuteReader(result);
    while (reader.Read())
     {
       Dispatcher.BeginInvoke( new delegateAddTextToListbox(AddTextToListbox),
       reader.GetString(0));
     }
 
    if (cmd.Connection.State.Equals(ConnectionState.Open))
     {
       cmd.Connection.Close();
     }
  }
 catch (Exception ex)
  {
   //ToDo : Swallow exception log
  }
}

Great! We have implemented it! But wait, we need few more tackle. Since, our sample project is a Windows based WPF application, form’s controls & method are not accessible from other than main UI thread. Thus, accessing Windows Form’s controls listboxResult will produce exception (InvalidOperationException) - { "The calling thread cannot access this object because a different thread owns it." }

So, we need a different approach to access Windows Form’s controls & associated methods and i.e., Delegate. We will shift listboxResult accessing code inside a function and we will call that function through delegate.

Let’s create a delegate which can point out any function that will accept one string argument and return void.

Next is to create a method that will accept string and add that to Windows Form’s control listbox:
private void AddTextToListbox(string Text)
{
  listboxResult.Items.Add(Text);
}

Now, we can use delegate in our callback function to add the result into listbox as:
while (reader.Read())
 {
    Dispatcher.BeginInvoke(new delegateAddTextToListbox(AddTextToListbox), reader.GetString(0));
 }

Done! Now, worker thread will be able to add the result to the controls by method through delegate call. Dispatcher.BeginInvoke() is used to execute delegate asynchronously with the specified arguments. This method is under System.Windows.Threading namespace.

Summary:

C# 5.0 introduces a new asynchronous programming model, i.e., async & await. It no longer requires pair codes like BeginExecute & EndExecute. A task is declared for asynchronous programming and the rest is done automatically by runtime. For example:
var task = await cmd.ExecuteReaderAsyc();

And method containing these task based statement(s) must must be decorated with async. For example:
private async void MyAsyncFunction() { ... }

Future:

Since C# 5.0 has introduced a new approach using async await & task based programming, so many people will think that this tip is outdated. But the idea behind writing this tip is to target those developers who are still developing or maintaining applications in old .NET Frameworks. Also, it will help to understand the basic mechanism behind the modern Task based asynchronous approach.