这个 DataReader.HasRows 属性检查必须在 DataReader.Read() 方法，该方法应包含在 While 循环因为 DataReader 是只向前的流（在之后立即检查行可用性） ExecuteReader() ). 以下示例是正确的方法：

Try
    cmd = New Odbc.OdbcCommand("SELECT * FROM tb_user WHERE username = @UserName AND password = @Password", conn)
    cmd.Parameters.AddWithValue("@UserName", txt_username.Text)
    cmd.Parameters.AddWithValue("@Password", txt_password.Text)
    Using rd As OdbcDataReader = cmd.ExecuteReader()
    If rd.HasRows = True Then
        While rd.Read()
            txt_akses.Text = rd!hak_akses
            MsgBox("Welcome '" & rd!nama & "'", MsgBoxStyle.Exclamation, "Information")
            MenuCustomer.txt_user.Text = rd!username
            cmd2 = New Odbc.OdbcCommand("SELECT * FROM tb_permainan WHERE status = 1", conn)
            Using rd2 As OdbcDataReader = cmd2.ExecuteReader()
            If rd2.HasRows = True Then
            While rd2.Read()
                If txt_akses.Text = "Customer" Then
                    If rd.HasRows = False Then
                        txt_username.Text = ""
                        txt_password.Text = ""
                        txt_akses.Text = ""
                        MenuCustomer.Show()
                    Else
                        MsgBox("Game is in progress. Please login for a while", MsgBoxStyle.Exclamation, "Information")
                    End If
                Else
                    MenuStaff.Show()
                End If
            End While
            End If
            End Using
        End While
    Else
        txt_username.Text = ""
        txt_password.Text = ""
        txt_akses.Text = ""
        txt_username.Select()
        MsgBox("Incorrect username or password", MsgBoxStyle.Exclamation, "Information")
    End If
    End Using
Catch ex As Exception
    MsgBox(ex.Message)
End Try

请注意，您应该使用不同的 DataReader 示例，因为在当前代码中 DataReader 赋值覆盖第一个。还可以在查询中使用参数来防止sql注入尝试。

我已经修改了我的代码这样，它仍然显示错误

Using rd As OdbcDataReader = cmd.ExecuteReader
            If rd.HasRows = True Then
                While rd.Read()
                    txt_akses.Text = rd!hak_akses
                    MsgBox("Welcome '" & rd!nama & "'", MsgBoxStyle.Exclamation, "Information")
                    MenuCustomer.txt_user.Text = rd!username
                    If txt_akses.Text = "Customer" Then
                        cmd2 = New OdbcCommand("SELECT * FROM tb_permainan WHERE status = 1", conn)
                        Using rd2 As OdbcDataReader = cmd2.ExecuteReader
                            If rd2.HasRows = True Then
                                MsgBox("Permainan sedang berlangsung. Mohon Untuk login sesaat lagi", MsgBoxStyle.Exclamation, "Information")
                            Else
                                While rd2.Read()
                                    txt_username.Text = ""
                                    txt_password.Text = ""
                                    txt_akses.Text = ""
                                    MenuCustomer.Show()
                                End While
                            End If
                        End Using
                    Else
                        MenuStaff.Show()
                    End If
                End While
            Else
                txt_username.Text = ""
                txt_password.Text = ""
                txt_akses.Text = ""
                txt_username.Select()
                MsgBox("Username atau password salah", MsgBoxStyle.Exclamation, "Information")
            End If
        End Using

显示的错误是循环。。