' 导入命名空间
Imports System.Data.SqlClient

Module Module1
    Sub Main()
        ' This is a simple example that demonstrates the usage of the 
        ' BeginExecuteReader functionality.
        ' The WAITFOR statement simply adds enough time to prove the 
        ' asynchronous nature of the command.
        Dim commandText As String = _
         "WAITFOR DELAY '00:00:03';" & _
         "SELECT LastName, FirstName FROM Person.Contact " & _
         "WHERE LastName LIKE 'M%'"

        RunCommandAsynchronously(commandText, GetConnectionString())

        Console.WriteLine("Press ENTER to continue.")
        Console.ReadLine()
    End Sub

    Private Sub RunCommandAsynchronously( _
     ByVal commandText As String, ByVal connectionString As String)

        ' Given command text and connection string, asynchronously execute
        ' the specified command against the connection. For this example,
        ' the code displays an indicator as it is working, verifying the 
        ' asynchronous behavior. 
        Using connection As New SqlConnection(connectionString)
            Try
                Dim command As New SqlCommand(commandText, connection)

                connection.Open()
                Dim result As IAsyncResult = command.BeginExecuteReader()

                ' Although it is not necessary, the following procedure
                ' displays a counter in the console window, indicating that 
                ' the main thread is not blocked while awaiting the command 
                ' results.
                Dim count As Integer
                While Not result.IsCompleted
                    count += 1
                    Console.WriteLine("Waiting ({0})", count)
                    ' Wait for 1/10 second, so the counter
                    ' does not consume all available resources 
                    ' on the main thread.
                    Threading.Thread.Sleep(100)
                End While

                ' Once the IAsyncResult object signals that it is done
                ' waiting for results, you can retrieve the results.
                Using reader As SqlDataReader = command.EndExecuteReader(result)
                    DisplayResults(reader)
                End Using
            Catch ex As SqlException
                Console.WriteLine("Error ({0}): {1}", ex.Number, ex.Message)
            Catch ex As InvalidOperationException
                Console.WriteLine("Error: {0}", ex.Message)
            Catch ex As Exception
                ' You might want to pass these errors
                ' back out to the caller.
                Console.WriteLine("Error: {0}", ex.Message)
            End Try
        End Using
    End Sub

    Private Sub DisplayResults(ByVal reader As SqlDataReader)
        ' Display the data within the reader.
        While reader.Read()
            ' Display all the columns.
            For i As Integer = 0 To reader.FieldCount - 1
                Console.Write("{0} ", reader.GetValue(i))
            Next
            Console.WriteLine()
        End While
    End Sub

    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
End Module

' 导入命名空间
Imports System.Data.SqlClient

Module Module1
    Sub Main()
        ' This example is not terribly useful, but it proves a point.
        ' The WAITFOR statement simply adds enough time to prove the 
        ' asynchronous nature of the command.
        Dim commandText As String = _
         "WAITFOR DELAY '00:00:03';" & _
         "SELECT ProductID, Name FROM Production.Product WHERE ListPrice < 100"

        RunCommandAsynchronously(commandText, GetConnectionString())

        Console.WriteLine("Press ENTER to continue.")
        Console.ReadLine()
    End Sub

    Private Sub RunCommandAsynchronously( _
     ByVal commandText As String, ByVal connectionString As String)

        ' Given command text and connection string, asynchronously execute
        ' the specified command against the connection. For this example,
        ' the code displays an indicator as it is working, verifying the 
        ' asynchronous behavior. 
        Try
            ' The code does not need to handle closing the connection explicitly--
            ' the use of the CommandBehavior.CloseConnection option takes care
            ' of that for you. 
            Dim connection As New SqlConnection(connectionString)
            Dim command As New SqlCommand(commandText, connection)

            connection.Open()
            Dim result As IAsyncResult = _
              command.BeginExecuteReader(CommandBehavior.CloseConnection)

            ' Although it is not necessary, the following code
            ' displays a counter in the console window, indicating that 
            ' the main thread is not blocked while awaiting the command 
            ' results.
            Dim count As Integer = 0
            While Not result.IsCompleted
                count += 1
                Console.WriteLine("Waiting ({0})", count)
                ' Wait for 1/10 second, so the counter
                ' does not consume all available resources 
                ' on the main thread.
                Threading.Thread.Sleep(100)
            End While

            ' The "using" statement closes the SqlDataReader when it is 
            ' done executing.
            Using reader As SqlDataReader = command.EndExecuteReader(result)
                DisplayResults(reader)
            End Using
        Catch ex As SqlException
            Console.WriteLine("Error ({0}): {1}", ex.Number, ex.Message)
        Catch ex As InvalidOperationException
            Console.WriteLine("Error: {0}", ex.Message)
        Catch ex As Exception
            ' You might want to pass these errors
            ' back out to the caller.
            Console.WriteLine("Error: {0}", ex.Message)
        End Try
    End Sub

    Private Sub DisplayResults(ByVal reader As SqlDataReader)
        ' Display the data within the reader.
        While reader.Read()
            ' Display all the columns. 
            For i As Integer = 0 To reader.FieldCount - 1
                Console.Write("{0} ", reader.GetValue(i))
            Next
            Console.WriteLine()
        End While
    End Sub

    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
End Module

' 导入命名空间
Imports System.Data.SqlClient

Public Class Form1
    ' Add this code to the form's class:
    ' You need this delegate in order to fill the grid from
    ' a thread other than the form's thread. See the HandleCallback
    ' procedure for more information.
    Private Delegate Sub FillGridDelegate(ByVal reader As SqlDataReader)

    ' You need this delegate to update the status bar.
    Private Delegate Sub DisplayStatusDelegate(ByVal Text As String)

    ' This flag ensures that the user does not attempt
    ' to restart the command or close the form while the 
    ' asynchronous command is executing.
    Private isExecuting As Boolean

    ' Because the overloaded version of BeginExecuteReader
    ' demonstrated here does not allow you to have the connection
    ' closed automatically, this example maintains the 
    ' connection object externally, so that it is available for closing.
    Private connection As SqlConnection

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

    Private Sub FillGrid(ByVal reader As SqlDataReader)
        Try
            Dim table As New DataTable
            table.Load(reader)
            Me.DataGridView1.DataSource = table
            DisplayStatus("Ready")

        Catch ex As Exception
            ' Because you are guaranteed this procedure
            ' is running from within the form's thread,
            ' it can directly interact with members of the form.
            DisplayStatus(String.Format("Ready (last attempt failed: {0})", ex.Message))
        Finally
            ' Do not forget to close the connection, as well.
            If Not reader Is Nothing Then
                reader.Close()
            End If
            If Not connection Is Nothing Then
                connection.Close()
            End If
        End Try
    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 SqlDataReader = command.EndExecuteReader(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. Therefore you cannot simply call code that 
            ' fills the grid, like this:
            ' FillGrid(reader)

            ' 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 FillGridDelegate(AddressOf FillGrid)
            Me.Invoke(del, reader)
            ' Do not close the reader here, because it is being used in 
            ' a separate thread. Instead, have the procedure you have
            ' called close the reader once it is done with it.

        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 there is none of 
            ' your code 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 DisplayStatusDelegate(AddressOf DisplayStatus), _
             "Error: " & ex.Message)
        Finally
            isExecuting = False
        End Try
    End Sub

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

        ' If you do not include the Asynchronous Processing=true name/value pair,
        ' you wo not be able to execute the command asynchronously.

        Return "Data Source=(local);Integrated Security=true;" & _
        "Initial Catalog=AdventureWorks; Asynchronous Processing=true"
    End Function

    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
                DisplayStatus("Connecting...")
                connection = New SqlConnection(GetConnectionString())
                ' To emulate a long-running query, wait for 
                ' a few seconds before retrieving the real data.
                command = New SqlCommand( _
                 "WAITFOR DELAY '0:0:5';" & _
                 "SELECT ProductID, Name, ListPrice, Weight FROM Production.Product", _
                 connection)
                connection.Open()

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

            Catch ex As Exception
                DisplayStatus("Error: " & ex.Message)
                If connection IsNot Nothing Then
                    connection.Close()
                End If
            End Try
        End If
    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
End Class

' 导入命名空间
Imports System.Data.SqlClient

Public Class Form1
    ' Add this code to the form's class:
    ' You this delegate in order to fill the grid from
    ' a thread other than the form's thread. See the HandleCallback
    ' procedure for more information.
    Private Delegate Sub FillGridDelegate(ByVal reader As SqlDataReader)

    ' You need this delegate to update the status bar.
    Private Delegate Sub DisplayStatusDelegate(ByVal Text As String)

    ' This flag ensures that the user does not attempt
    ' to restart the command or close the form while the 
    ' asynchronous command is executing.
    Private isExecuting As Boolean

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

    Private Sub FillGrid(ByVal reader As SqlDataReader)
        Try
            Dim table As New DataTable
            table.Load(reader)
            Me.DataGridView1.DataSource = table
            DisplayStatus("Ready")

        Catch ex As Exception
            ' Because you are guaranteed this procedure
            ' is running from within the form's thread,
            ' it can directly interact with members of the form.
            DisplayStatus(String.Format("Ready (last attempt failed: {0})", ex.Message))
        Finally
            ' Closing the reader also closes the connection,
            ' because this reader was created using the 
            ' CommandBehavior.CloseConnection value.
            If reader IsNot Nothing Then
                reader.Close()
            End If
        End Try
    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 SqlDataReader = command.EndExecuteReader(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. Therefore you cannot simply call code that 
            ' fills the grid, like this:
            ' FillGrid(reader)

            ' 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 FillGridDelegate(AddressOf FillGrid)
            Me.Invoke(del, reader)

            ' Do not close the reader here, because it is being used in 
            ' a separate thread. Instead, have the procedure you have
            ' called close the reader once it is done with it.

        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 there is none of 
            ' your code 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 DisplayStatusDelegate(AddressOf DisplayStatus), _
             "Error: " & ex.Message)
        Finally
            isExecuting = False
        End Try
    End Sub

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

        ' If you do not include the Asynchronous Processing=true name/value pair,
        ' you wo not be able to execute the command asynchronously.

        Return "Data Source=(local);Integrated Security=true;" & _
        "Initial Catalog=AdventureWorks; Asynchronous Processing=true"
    End Function

    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 connection As SqlConnection
            Dim command As SqlCommand
            Try
                DisplayStatus("Connecting...")
                connection = New SqlConnection(GetConnectionString())
                ' To emulate a long-running query, wait for 
                ' a few seconds before retrieving the real data.
                command = New SqlCommand( _
                 "WAITFOR DELAY '0:0:5';" & _
                 "SELECT ProductID, Name, ListPrice, Weight FROM Production.Product", _
                 connection)
                connection.Open()

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

            Catch ex As Exception
                DisplayStatus("Error: " & ex.Message)
                If connection IsNot Nothing Then
                    connection.Close()
                End If
            End Try
        End If
    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
End Class