System.InvalidOperationException：＆＃39;连接必须有效并且打开

Question

我试图将值添加到数据库中，但每次我尝试添加一些内容时，我都会在ExecuteNonQuery（）中收到错误消息＆＃34; Connection必须有效并且打开。＆＃34 ;而且我不知道该怎么做

Imports MySql.Data.MySqlClient

Public Class Form1
    Dim ServerString As String = "Server=localhost;User Id=root;Password=root;Database=youtube"
    Dim SQLConnection As MySqlConnection = New MySqlConnection

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load


        SQLConnection.ConnectionString = ServerString

        Try
            If SQLConnection.State = ConnectionState.Closed Then

                MsgBox("YAY! you are now connected")
            Else

                MsgBox("LOL Nope! you are not connected")
            End If
        Catch ex As Exception
            MsgBox(ex.ToString)

        End Try
    End Sub

    Private Sub Form1_Unload(sender As Object, e As EventArgs) Handles MyBase.FormClosing
        SQLConnection.Close()
        SQLConnection.Dispose()
    End Sub

    Public Sub SaveNames(ByRef SQLStatement As String)
        Dim cmd As MySqlCommand = New MySqlCommand

        With cmd
            .CommandText = SQLStatement
            .CommandType = CommandType.Text
            .Connection = SQLConnection
            .ExecuteNonQuery() // keeps on saying System.InvalidOperationException: 'Connection must be valid and open.'

        End With

        SQLConnection.Close()
        MsgBox("YAY, you are now connected")
        SQLConnection.Dispose()

    End Sub

    Private Sub cmdSave_Click(sender As Object, e As EventArgs) Handles cmdSave.Click
        Dim SQLStatement As String = "INSERT INTO people(name) VALUES ('" & txtName.Text & "')"

        SaveNames(SQLStatement)

    End Sub
End Class

Answer 1

    Imports MySql.Data.MySqlClient

Public Class Form1
    Dim ServerString As String = "Server=localhost;User Id=root;Password=root;Database=youtube"
    Dim SQLConnection As MySqlConnection(ServerString)

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

        Try
            SQLConnection.Open()
            If SQLConnection.State != ConnectionState.Closed Then
                MsgBox("LOL Nope! you are not connected")
            Else
                MsgBox("YAY! you are now connected")
            End If
        Catch ex As Exception
            MsgBox(ex.ToString)

        End Try
        SQLConnection.Close()
        SQLConnection.Dispose()
    End Sub

    Public Sub SaveNames(ByRef SQLStatement As String)
        Dim cmd As MySqlCommand = New MySqlCommand(SQLStatement,SQLConnection)
        cmd.ExecuteNonQuery()

        MsgBox("YAY, you are now connected")
        SQLConnection.Close()
        SQLConnection.Dispose()

    End Sub

    Private Sub cmdSave_Click(sender As Object, e As EventArgs) Handles cmdSave.Click
        Dim SQLStatement As String = "INSERT INTO people(name) VALUES ('" & txtName.Text & "')"

        SaveNames(SQLStatement)

    End Sub
End Class

尝试一下，让我知道它是否有效，我没有测试它但它应该工作。 您希望在进行调用的方法中打开和关闭连接。 您不希望在表单首次加载时打开连接，然后将其保持打开状态直到表单关闭，这是一种不好的做法。

了解更多信息 https://support.microsoft.com/en-us/help/301075/how-to-connect-to-a-database-and-run-a-command-by-using-ado.net-and-visual-basic-.net

另请参阅使用数据库访问时，这将关闭并处理您的连接而无需添加代码。 https://docs.microsoft.com/en-us/dotnet/articles/visual-basic/language-reference/statements/using-statement

在控制台应用程序而不是WinForm应用程序中尝试它，这将在添加UI问题之前从示例中获得一些复杂性。