9.2. Canceling an Asynchronous Query


Given a query running that runs asynchronously on a background thread, you want to give the user the option to cancel the query if it is taking too long.


Abort the background thread and clean up in an exception handler.

The sample code contains two event handlers and a single method:

Start Button.Click

Checks whether there is an existing background thread loading the DataSet. If the DataSet is not being loaded, a new thread is created invoking the AsyncFillDataSet( ) method to fill a DataSet. Otherwise, a message is displayed stating that the DataSet is currently being filled.

Cancel Button.Click

Aborts the background thread filling the DataSet.

AsyncFillDataSet( )

This method loads a DataSet with the Orders and Order Details tables from the Northwind database. The method displays a message when the method has started and when it has completed. The method also traps the ThreadAbortException to handle the situation where the fill on the background thread is canceled.

The C# code is shown in Example 9-2.

Example 9-2. File: AsynchronousFillCancelForm.cs

// Namespaces, variables, and constants using System; using System.Configuration; using System.Threading; using System.Data; using System.Data.SqlClient; // Table name constants private const String ORDERS_TABLE = "Orders"; private const String ORDERDETAILS_TABLE = "OrderDetails"; // Relation name constants private const String ORDERS_ORDERDETAILS_RELATION = "Orders_OrderDetails_Relation"; ...

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