SqlCommand.BeginExecuteXmlReader Method (AsyncCallback, Object)

 

Initiates the asynchronous execution of the Transact-SQL statement or stored procedure that is described by this SqlCommand and returns results as an XmlReader object, using a callback procedure.

Namespace:   System.Data.SqlClient
Assembly:  System.Data (in System.Data.dll)

<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading := True)>
Public Function BeginExecuteXmlReader (
	callback As AsyncCallback,
	stateObject As Object
) As IAsyncResult

Parameters

callback
Type: System.AsyncCallback

An AsyncCallback delegate that is invoked when the command's execution has completed. Pass null (Nothing in Microsoft Visual Basic) to indicate that no callback is required.

stateObject
Type: System.Object

A user-defined state object that is passed to the callback procedure. Retrieve this object from within the callback procedure using the AsyncState property.

Return Value

Type: System.IAsyncResult

An IAsyncResult that can be used to poll, wait for results, or both; this value is also needed when the EndExecuteXmlReader is called, which returns the results of the command as XML.

Exception Condition
InvalidCastException

A SqlDbType other than Binary or VarBinary was used when Value was set to Stream. For more information about streaming, see SqlClient Streaming Support.

A SqlDbType other than Char, NChar, NVarChar, VarChar, or Xml was used when Value was set to TextReader.

A SqlDbType other than Xml was used when Value was set to XmlReader.

SqlException

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.

InvalidOperationException

The name/value pair "Asynchronous Processing=true" was not included within the connection string defining the connection for this SqlCommand.

The SqlConnection closed or dropped during a streaming operation. For more information about streaming, see SqlClient Streaming Support.

IOException

An error occurred in a Stream, XmlReader or TextReader object during a streaming operation. For more information about streaming, see SqlClient Streaming Support.

ObjectDisposedException

The Stream, XmlReader or TextReader object was closed during a streaming operation. For more information about streaming, see SqlClient Streaming Support.

The BeginExecuteXmlReader method starts the process of asynchronously executing a Transact-SQL statement or stored procedure that returns rows as XML, so that other tasks can run concurrently while the statement is executing. When the statement has completed, developers must call the EndExecuteXmlReader method to finish the operation and retrieve the requested XML data. The BeginExecuteXmlReader method returns immediately, but until the code executes the corresponding EndExecuteXmlReader method call, it must not execute any other calls that start a synchronous or asynchronous execution against the same SqlCommand object. Calling the EndExecuteXmlReader before the command's execution is completed causes the SqlCommand object to block until the execution is finished.

The CommandText property ordinarily specifies a Transact-SQL statement with a valid FOR XML clause. However, CommandText can also specify a statement that returns data that contains valid XML. This method can also be used to retrieve a single-row, single-column result set. In this case, if more than one row is returned, the EndExecuteXmlReader method attaches the XmlReader to the value on the first row, and discards the rest of the result set.

A typical BeginExecuteXmlReader query can be formatted as in the following C# example:

SqlCommand command = new SqlCommand("SELECT ContactID, FirstName, LastName FROM Contact FOR XML AUTO, XMLDATA", SqlConn);

This method can also be used to retrieve a single-row, single-column result set. In this case, if more than one row is returned, the EndExecuteXmlReader method attaches the XmlReader to the value on the first row, and discards the rest of the result set.

The multiple active result set (MARS) feature lets multiple actions use the same connection.

The callback parameter lets you specify an AsyncCallback delegate that is called when the statement has completed. You can call the EndExecuteXmlReader 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 is 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.

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 will return 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 following Windows application demonstrates the use of the BeginExecuteXmlReader method, executing a Transact-SQL statement that includes a delay of a few seconds (emulating a long-running command). 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 EndExecuteXmlReader method corresponding to the initial call to BeginExecuteXmlReader.

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 ListBox 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.

Imports System.Data.SqlClient
Imports System.Xml

Public Class Form1
    ' Add this code to the form's class:
    ' You need these delegates in order to display text from a thread
    ' other than the form's thread. See the HandleCallback
    ' procedure for more information.
    Private Delegate Sub DisplayInfoDelegate(ByVal Text As String)
    Private Delegate Sub DisplayReaderDelegate(ByVal reader As XmlReader)

    Private isExecuting As Boolean

    ' This example maintains the connection object 
    ' externally, so that it is available for closing.
    Private connection As SqlConnection

    Private Function GetConnectionString() As String
        ' To avoid storing the connection string in your code,            
        ' you can retrieve it from a configuration file. 

        ' If you have not included "Asynchronous Processing=true" in the
        ' connection string, the command is not able
        ' to execute asynchronously.
        Return "Data Source=(local);Integrated Security=true;" & _
          "Initial Catalog=AdventureWorks; Asynchronous Processing=true"
    End Function

    Private Sub DisplayStatus(ByVal Text As String)
        Me.Label1.Text = Text
    End Sub

    Private Sub ClearProductInfo()
        ' Clear the list box.
        Me.ListBox1.Items.Clear()
    End Sub

    Private Sub DisplayProductInfo(ByVal reader As XmlReader)
        ' Display the data within the reader.
        While reader.Read()
            ' Skip past items that are not from the correct table.
            If reader.LocalName.ToString = "Production.Product" Then
                Me.ListBox1.Items.Add(String.Format("{0}: {1:C}", _
                    reader("Name"), CSng(reader("ListPrice"))))
            End If
        End While
        DisplayStatus("Ready")
    End Sub

    Private Sub Form1_FormClosing(ByVal sender As Object, _
        ByVal e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
        If isExecuting Then
            MessageBox.Show(Me, "Cannot close the form until " & _
                "the pending asynchronous command has completed. Please wait...")
            e.Cancel = True
        End If
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles Button1.Click
        If isExecuting Then
            MessageBox.Show(Me, "Already executing. Please wait until the current query " & _
                "has completed.")
        Else
            Dim command As SqlCommand
            Try
                ClearProductInfo()
                DisplayStatus("Connecting...")
                connection = New SqlConnection(GetConnectionString())
                ' To emulate a long-running query, wait for 
                ' a few seconds before working with the data.
                Dim commandText As String = _
                    "WAITFOR DELAY '00:00:03';" & _
                    "SELECT Name, ListPrice " & _
                    "FROM Production.Product WHERE ListPrice < 100 " & _
                    "FOR XML AUTO, XMLDATA"

                command = New SqlCommand(commandText, connection)
                connection.Open()

                DisplayStatus("Executing...")
                isExecuting = True
                ' Although it is not required that you pass the 
                ' SqlCommand object as the second parameter in the 
                ' BeginExecuteXmlReader call, doing so makes it easier
                ' to call EndExecuteXmlReader in the callback procedure.
                Dim callback As New AsyncCallback(AddressOf HandleCallback)
                command.BeginExecuteXmlReader(callback, command)

            Catch ex As Exception
                isExecuting = False
                DisplayStatus(String.Format("Ready (last error: {0})", ex.Message))
                If connection IsNot Nothing Then
                    connection.Close()
                End If
            End Try
        End If
    End Sub

    Private Sub HandleCallback(ByVal result As IAsyncResult)
        Try
            ' Retrieve the original command object, passed
            ' to this procedure in the AsyncState property
            ' of the IAsyncResult parameter.
            Dim command As SqlCommand = CType(result.AsyncState, SqlCommand)
            Dim reader As XmlReader = command.EndExecuteXmlReader(result)

            ' You may not interact with the form and its contents
            ' from a different thread, and this callback procedure
            ' is all but guaranteed to be running from a different thread
            ' than the form. 

            ' Instead, you must call the procedure from the form's thread.
            ' One simple way to accomplish this is to call the Invoke
            ' method of the form, which calls the delegate you supply
            ' from the form's thread. 
            Dim del As New DisplayReaderDelegate(AddressOf DisplayProductInfo)
            Me.Invoke(del, reader)

        Catch ex As Exception
            ' Because you are now running code in a separate thread, 
            ' if you do not handle the exception here, none of your other
            ' code catches the exception. Because none of 
            ' your code is on the call stack in this thread, there is nothing
            ' higher up the stack to catch the exception if you do not 
            ' handle it here. You can either log the exception or 
            ' invoke a delegate (as in the non-error case in this 
            ' example) to display the error on the form. In no case
            ' can you simply display the error without executing a delegate
            ' as in the Try block here. 

            ' You can create the delegate instance as you 
            ' invoke it, like this:
            Me.Invoke(New DisplayInfoDelegate(AddressOf DisplayStatus), _
                String.Format("Ready(last error: {0}", ex.Message))
        Finally
            isExecuting = False
            If connection IsNot Nothing Then
                connection.Close()
            End If
        End Try
    End Sub
End Class

.NET Framework
Available since 2.0
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