Mysql vb.net登录访问控制

我制作了一个mysql sigin表单，我试图使用我的用户表来控制登录访问，但是每当我输入它登录的任意字母时，都没有显示错误，即它是一个无效的用户名

这就是我到目前为止所做的：

   Imports MySql.Data.MySqlClient

    Public Class frmLogin
        Private Sub cmdCancel_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdCancel.Click
            Application.Exit()
        End Sub

        Private Sub cmdLogin_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdLogin.Click
            Dim conn As New MySqlConnection
            Dim myCommand As New MySqlCommand

            Dim myConnString As String
            Dim UserID As String

            myConnString = "server=" & My.Settings.HostIP & ";" _
                & "user id=" & My.Settings.Username & ";" _
                & "password=" & My.Settings.Password & ";" _
                & "database=attendance"

            conn.ConnectionString = myConnString

            Try
                conn.Open()

                myCommand.Connection = conn
                myCommand.CommandText = "SELECT user_bannerid FROM user WHERE BINARY username = ?Username"
                myCommand.Parameters.Add("?Username", txtUsername.Text)

                UserID = myCommand.ExecuteScalar

                conn.Close()

                Dim mainForm As New frmMain
                mainForm.UserID = UserID
                mainForm.connectionString = myConnString
                mainForm.Show()

                Me.Hide()
                Me.Close()
            Catch myerror As MySqlException
                MessageBox.Show("Invalid User. Please Enter Your Correct Username")
                conn.Dispose()
            End Try
        End Sub

        Private Sub frmLogin_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
            Me.AcceptButton = cmdLogin
            Me.CancelButton = cmdCancel
        End Sub
    End Class

---

这是因为在关闭表单之前没有测试userid的值

您还需要实现using语句，以确保所有一次性物品都得到正确处置

以下是解决这两个问题的重写：

    Try
        Using conn As New MySqlConnection
            Using myCommand As New MySqlCommand
                Dim myConnString As String
                Dim UserID As String

                myConnString = "server=" & My.Settings.HostIP & ";" _
                    & "user id=" & My.Settings.Username & ";" _
                    & "password=" & My.Settings.Password & ";" _
                    & "database=attendance"

                conn.ConnectionString = myConnString

                conn.Open()

                myCommand.Connection = conn
                myCommand.CommandText = "SELECT user_bannerid FROM user WHERE BINARY username = ?Username"
                myCommand.Parameters.Add("?Username", txtUsername.Text)

                Dim oUserID As Object

                oUserID = myCommand.ExecuteScalar

                conn.Close()

                If oUserID IsNot DBNull.Value AndAlso Not String.IsNullOrEmpty(oUserID) Then
                    Dim mainForm As New frmMain
                    mainForm.UserID = UserID
                    mainForm.connectionString = myConnString
                    mainForm.Show()

                    Me.Hide()
                    Me.Close()
                Else
                    MessageBox.Show("Invalid User. Please Enter Your Correct Username")
                End If
            End Using
        End Using
    Catch myerror As MySqlException
        MessageBox.Show("SQL Error" & myerror.ToString())
    End Try