SqlCommand.BeginExecuteReader Method (AsyncCallback, Object)
Initiates the asynchronous execution of the Transact-SQL statement or stored procedure that is described by this SqlCommand and retrieves one or more result sets from the server, given a callback procedure and state information.
Assembly: System.Data (in System.Data.dll)
[<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)>] member BeginExecuteReader : callback:AsyncCallback * stateObject:Object -> IAsyncResult
- Type: System.AsyncCallback
An AsyncCallback delegate that is invoked when the command's execution has completed. Pass a null reference (Nothing in Visual Basic) (Nothing in Microsoft Visual Basic) to indicate that no callback is required.
Any error that occurred while executing the command text.
A timeout occurred during a streaming operation. For more information about streaming, see SqlClient Streaming Support.
The name/value pair "Asynchronous Processing=true" was not included within the connection string defining the connection for this SqlCommand.
The BeginExecuteReader method starts the process of asynchronously executing a Transact-SQL statement or stored procedure that returns rows, so that other tasks can run concurrently while the statement is executing. When the statement has completed, developers must call the EndExecuteReader method to finish the operation and retrieve the SqlDataReader returned by the command. The BeginExecuteReader method returns immediately, but until the code executes the corresponding EndExecuteReader method call, it must not execute any other calls that start a synchronous or asynchronous execution against the same SqlCommand object. Calling the EndExecuteReader before the command's execution is completed cause the SqlCommand object to block until the execution is finished.
The callback parameter lets you specify an AsyncCallback delegate that is called when the statement has completed. You can call the EndExecuteReader method from within this delegate procedure, or from any other location within your application. In addition, you can pass any object in the stateObject parameter, and your callback procedure can retrieve this information using the AsyncState property.
Note that the command text and parameters are sent to the server synchronously. If a large command or many parameters are sent, this method may block during writes. After the command is sent, the method returns immediately without waiting for an answer from the server--that is, reads are asynchronous. Although command execution is asynchronous, value fetching is still synchronous. This means that calls to Read may block if more data is required and the underlying network's read operation blocks.
Because the callback procedure executes from within a background thread supplied by the Microsoft .NET runtime, it is very important that you take a rigorous approach to handling cross-thread interactions from within your applications. For example, you must not interact with a form's contents from within your callback procedure; should you have to update the form, you must switch back to the form's thread in order to do your work. The example in this topic demonstrates this behavior.
All errors that occur during the execution of the operation are thrown as exceptions in the callback procedure. You must handle the exception in the callback procedure, not in the main application. See the example in this topic for additional information on handling exceptions in the callback procedure.
If you use ExecuteReader or BeginExecuteReader to access XML data, SQL Server returns any XML results greater than 2,033 characters in length in multiple rows of 2,033 characters each. To avoid this behavior, use ExecuteXmlReader or BeginExecuteXmlReader to read FOR XML queries. For more information, see article Q310378, "PRB: XML Data Is Truncated When You Use SqlDataReader," in the Microsoft Knowledge Base at http://support.microsoft.com.
The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.
The following Windows application demonstrates the use of the BeginExecuteReader method, executing a Transact-SQL statement that includes a delay of a few seconds (emulating a long-running command). Because the sample executes the command asynchronously, the form remains responsive while awaiting the results. This example passes the executing SqlCommand object as the stateObject parameter; doing so makes it simple to retrieve the SqlCommand object from within the callback procedure, so that the code can call the EndExecuteReader method corresponding to the initial call to BeginExecuteReader.
This example demonstrates many important techniques. This includes calling a method that interacts with the form from a separate thread. In addition, this example demonstrates how you must block users from executing a command multiple times concurrently, and how you must make sure that the form does not close before the callback procedure is called.
To set up this example, create a new Windows application. Put a Button control, a DataGridView control, and a Label control on the form (accepting the default name for each control). Add the following code to the form's class, modifying the connection string as needed for your environment.