How to send a notification to the user that the file is in use by another program

[nqioweryuadfge]

How do you send a notification to the user that the program cannot Autosave a file, while it is in use on another program? It cannot be saved even if there is a timer on the program on the RichTextBox_TextChanged event(). Here is the function:

Private Sub AutosaveFile()

        Dim path As String = (My.Computer.FileSystem.SpecialDirectories.Desktop & "\Common-Mistakes-Backup.rtf")
 
            ' This text is added only once to the file.
            If File.Exists(path) = False Then
 
            Using fs As FileStream = File.Create(My.Computer.FileSystem.SpecialDirectories.Desktop & "\Common-Mistakes-Backup.rtf")
                fs.Close()
                ' Save the contents of the RichTextBox into the file.
                RichTextBox1.SaveFile(path, RichTextBoxStreamType.RichText)
                Timer1.Start()

            End Using

            End If
 
        Try
            ' Delete the file if it exists. 
            If File.Exists(path) = True Then

                ' Save the contents of the RichTextBox into the file.
                    RichTextBox1.SaveFile(path, RichTextBoxStreamType.RichText)

                    Timer1.Start()

            End If

        Catch ex As Exception
            Console.WriteLine(ex.ToString())
        End Try
 
    End Sub