mysql - 修复VB.NET错误 "Connection must be valid and open"

Question

我在创建的登录表单中收到错误，内容是“连接必须有效且打开，有人可以帮助我吗？这是代码。我无法理解此错误是什么，我需要您的帮助.

Imports MySql.Data.MySqlClient
Public Class LoginForm1

    Dim Username As String
    Dim Password As String
    Public StudentName As String
    'MySql
    Dim connection As New MySqlConnection("Server=server;User ID=id;Password=pass;Database=data;SSLMode=None")

    Private Sub OK_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles OK.Click

        Username = UsernameTextBox.Text
        Password = PasswordTextBox.Text
        StudentName = NameTextBox.Text

        Dim command As New MySqlCommand("SELECT `Username`, `Password`, `FirstName` FROM `people` WHERE `Username` = @username AND `Password` = @password And `FirstName` = @name", connection)
        command.Parameters.Add("@username", MySqlDbType.VarChar).Value = Username
        command.Parameters.Add("@password", MySqlDbType.VarChar).Value = Password
        command.Parameters.Add("@name", MySqlDbType.VarChar).Value = StudentName


        Dim adapter As New MySqlDataAdapter(command)
        Dim table As New DataTable()
        Dim reader As MySqlDataReader = command.ExecuteReader

        adapter.Fill(table)

        If table.Rows.Count = 0 Then
            MessageBox.Show("Invalid Username Or Password", "Invalid Login", MessageBoxButtons.OK, MessageBoxIcon.Information)
        Else

            MessageBox.Show("Logged In", "Logged in Successfully", MessageBoxButtons.OK, MessageBoxIcon.Information)
            MainMenuForm.Show()
            Me.Close()
        End If
    End Sub

    Private Sub Cancel_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Cancel.Click
        Me.Close()
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim webAddress As String = "http://windowsadvancedstudy.com"
        Process.Start(webAddress)
    End Sub
End Class

Answer 1

如果您使用 adapter.Fill，则无需声明 reader。删除这一行:

Dim reader As MySqlDataReader = command.ExecuteReader

正是上面的行引发了异常。因为reader需要一个开放的连接。

并添加行:

command.CommandType = CommandType.Text