如果电子邮件/密码输入错误，无法显示错误消息

如果电子邮件/密码输入错误，我希望出现一个消息框。我正在使用 else 语句，但如果登录不正确，它不会显示消息。

MyConn = New OleDbConnection 
        MyConn.ConnectionString = connString 
        MyConn.Open() 

        str1 = ("SELECT * FROM [UserData] WHERE [Username] = '" & TxtUserName.Text & "' AND [Password] = '" & TxtPassword.Text & "'") 
        Dim cmd1 As OleDbCommand = New OleDbCommand(str1, MyConn)
        dr = cmd1.ExecuteReader 
        While dr.Read() 
            userFound = True 
            TxtPassword.Text = dr("Username").ToString 
            TxtUserName.Text = dr("Password").ToString 
            FirstNameToPass = dr("First Name").ToString 
            LastNameToPass = dr("Last Name").ToString 
            AddressToPass = dr("Address").ToString 
            EmailToPass = dr("Email").ToString 
            If userFound = True Then
                UserAccountView.Show() 
                Me.Hide() 
                TxtPassword.Clear() 
                TxtUserName.Clear() 
            Else
                MsgBox("Login is incorrect")
            End If
        End While
        MyConn.Close() 
    End If

reader.Read()如果行存在则返回 true，如果结果集为空则返回 false。在您的情况下，如果没有这样的用户（提供的用户名和密码），程序将不会进入循环并且不会到达If userFound = True block.

所以，跳过while循环并编写如下内容：

If dr.HasRows
    dr.Read()
    TxtPassword.Text = dr("Username").ToString 
    TxtUserName.Text = dr("Password").ToString 
    FirstNameToPass = dr("First Name").ToString 
    LastNameToPass = dr("Last Name").ToString 
    AddressToPass = dr("Address").ToString 
    EmailToPass = dr("Email").ToString 

    UserAccountView.Show() 
    Me.Hide() 
    TxtPassword.Clear() 
    TxtUserName.Clear() 
Else
    MsgBox("Login is incorrect")
End If

edit:

还有一件事：永远不要连接字符串来创建命令文本。相反，使用参数：

str1 = ("SELECT * FROM [UserData] WHERE [Username] = ? AND [Password] = ?") 
Dim cmd1 As OleDbCommand = New OleDbCommand(str1, MyConn)
cmd1.Parameters.Add(New OleDbParameter("Username", CType(TxtUserName.Text, String))) 
cmd1.Parameters.Add(New OleDbParameter("Password", CType(TxtPassword.Text, String)))