我必须执行多个 SQL 查询并且我想为此使用线程,因为必须每 1 秒执行一次此查询以更新或获取数据库的一些不同值。
当我尝试执行我的代码时,出现以下错误:
WindowsApplication1.vshost.exe Error: 0 : There is already an open DataReader associated with this Connection which must be closed first.
我在 stackoverflow 上读到,连接应该始终尽可能晚地打开并尽快关闭。
这个问题有解决办法吗?
这是我的代码:
Imports MySql.Data.MySqlClient
Imports System.Threading
Public Class Form1
Private server As String = Nothing
Private pass As String = Nothing
Private user As String = Nothing
Private port As String = Nothing
Private db As String = Nothing
Dim Thread1 As Thread
Dim Thread2 As Thread
Dim con As New MySqlConnection
Dim cmd As New MySqlCommand
Dim reader As MySqlDataReader
Public Sub New()
Me.server = "localhost"
Me.user = "root"
Me.pass = ""
Me.port = "3306"
Me.db = "diagnosedb"
cmd.Connection = con
con.ConnectionString = "Server = " & Me.server & ";
Port = " & Me.port & ";
Database = " & Me.db & ";
Uid = " & Me.user & ";
Pwd = " & Me.pass & ";"
Thread1 = New Thread(AddressOf Querie1)
Thread2 = New Thread(AddressOf Querie2)
Thread1.Start()
Thread2.Start()
End Sub
Private Sub Querie1()
cmd.CommandText = "UPDATE teileliste
SET verschleis = 500
WHERE ID = 1;"
Try
con.Open()
cmd.ExecuteNonQuery()
Catch ex As Exception
MsgBox(ex.Message)
Finally
con.Close()
End Try
End Sub
Private Sub Querie2()
cmd.CommandText = "UPDATE teileliste
SET verschleis = 0
WHERE ID = 20;"
Try
con.Open()
cmd.ExecuteNonQuery()
Catch ex As Exception
MsgBox(ex.Message)
Finally
con.Close()
End Try
End Sub
End Class
最佳答案
在 Querie1
和 Querie2
方法中创建并关闭一个新连接。您不能在一个连接上进行多个并发操作。 (即使可以,您也在两种方法中关闭连接。一种方法可能会在另一种方法使用它时尝试关闭它。)
.NET Framework 擅长管理连接。当您在代码中打开和关闭它们时,在幕后它实际上是在短时间内保持连接打开。这称为线程池。当您关闭连接时,它实际上暂时保持打开状态。当您打开一个新连接时,它实际上可能会为您提供相同的连接。
大多数时候我们不需要关注它。我们所做的就是创建一个尽可能接近我们打开和使用它的新连接,然后尽快关闭(处置)它。
关于mysql - SQL 查询和线程,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/48227088/