Question

VB.NET 2.0：

我正在从VB.NET应用程序向SQL服务器发出备份命令。我正在捕获它发出的消息并将它们附加到多行文本框中。

但是，我要做的是允许应用程序继续响应GUI控件事件（主要是为了让用户可以调整输出窗口的大小或取消备份）。所以我使用BeginExecuteReader并在Application.DoEvents()的循环中旋转。但似乎只要备份开始发出其打印语句，我就会得到IAsyncResult.IsCompleted = True并且它会下降到EndExecuteReader，现在GUI再次被锁定。如何让它保持循环直到备份命令完成，但仍然获取那些输出语句并保持GUI响应？感谢。

这是我的代码：

' Enable retrieve print statements from the server'
AddHandler oConn.InfoMessage, AddressOf LogToBufferHandler

strSQL &= vbNewLine & "begin try"
strSQL &= vbNewLine & " declare @BackupName as varchar(255)"
strSQL &= vbNewLine & " declare @BackupDesc as varchar(255)"
strSQL &= vbNewLine & " declare @backupTime as varchar(50)"
strSQL &= vbNewLine & " set @backupTime = (select convert(datetime, getdate(), 100))"
strSQL &= vbNewLine & " set @BackupName = (SELECT '[' + db_name() + '] Full Backup')"
strSQL &= vbNewLine & " set @BackupDesc = (SELECT 'Automated full backup of [' + db_name() + '] on ' + @backupTime + '.')"
strSQL &= vbNewLine & " "
strSQL &= vbNewLine & " BACKUP DATABASE [#Database#]"
strSQL &= vbNewLine & " TO DISK = @BackupFullPath"
strSQL &= vbNewLine & " WITH stats,"
strSQL &= vbNewLine & "      NAME = @BackupName,"
strSQL &= vbNewLine & "      DESCRIPTION = @BackupDesc;"
strSQL &= vbNewLine & " select [IsSuccessful] = 1"
strSQL &= vbNewLine & " end try"
strSQL &= vbNewLine & " begin catch"
strSQL &= vbNewLine & " SELECT [IsSuccessful] = 0"
strSQL &= vbNewLine & " end catch"

'Workaround: Backup Database requires the name of the object, not a string'
'            and I dont want to use dynamic SQL.'
strSQL = strSQL.Replace("#Database#", sb.InitialCatalog)

oConn.Open()
oCmd = New SqlCommand()
oCmd.Connection = oConn
oCmd.CommandText = strSQL
oCmd.CommandType = CommandType.Text
oCmd.Parameters.AddWithValue("@BackupFullPath", backupFullPath)
oCmd.CommandTimeout = 60 * 5

'Spin until complete, cancel, or timeout'
Dim result As IAsyncResult = oCmd.BeginExecuteReader(CommandBehavior.CloseConnection)
While Not result.IsCompleted
    Application.DoEvents()
    If blnCancel Then
        oCmd.Cancel()
    End If
    System.Threading.Thread.Sleep(50)
End While
Try
    oDataReader = oCmd.EndExecuteReader(result)
    oDataTable.Load(oDataReader)

    'Get results'
    ' (unfourtunately, you cannot do BeginExecuteScalar ASync in .Net 2.0,'
    ' so we are using a DataTable first column, row)'
    If oDataTable IsNot Nothing _
    AndAlso oDataTable.Rows.Count > 0 _
    AndAlso oDataTable.Columns.Contains("IsSuccessful") _
    AndAlso oDataTable.Rows(0).Item("IsSuccessful") = 1 Then
        eBackupResult = BackupStatus.Succeeded
        returnPath = backupFullPath 'Only set return path if the backup succeeded'
    Else
        eBackupResult = BackupStatus.Failed
    End If
Catch ex As Exception
    If Not ex.Message.Contains("cancelled by user") Then Throw ex
    eBackupResult = BackupStatus.Canceled
End Try

Answer 1

轮询循环（DoEvents）被认为是邪恶的，原因有很多。这可能是您切换到BackgroundWorker并放弃令人讨厌的Begin-poll-End方案的最简单方法。

如果你想保留它，这就是错误：接受读者可以很快但这并不意味着所有结果都已到达（让我说明一下：如果你查询了1TB的数据怎么办？ - 读取的是一个漫长的过程）。您也需要以异步（轮询）方式Read。现在事情变得无法控制。放弃它。

换句话说，oDataTable.Load阻塞，很难解决。

Answer 2

您可以使用TASKS。任务在概念上类似于线程。通过将代码放入任务中，主线程可以继续运行（这是获取用户输入并响应调整大小命令的线程）。

这篇文章更多地讨论了任务，并有一个VB.NET示例。

http://www.dotnetcurry.com/ShowArticle.aspx?ID=491

玩得开心！

Answer 3

BackGroundWorker和ReportProgress

BackgroundWorker Class

在SQL备份期间保持GUI响应

3 个答案: