Patterns exist for a reason. Patterns are something we can get familiar with, consequently making like-things anticipatable and easier to use. I am not just talking about patterns like the Gang of Four GoF patterns; I mean simple ways that things work too.
Now, don't get me wrong. Re-inventing the wheel makes sense if you go from a rock wheel to a rubber one, but like things are easier to adopt when they are new if they are used like similar things that exist. For example, if I have a Menu and it has a collection of element-like sub-menus, I am going to look automatically for a collection property and an add method. Easy.
The MenuStrip is like a menu, but if you want to add a sub-menu it's not as easy as invoking Add with an instance of a menu item. It's sort of one off. In this article, I will show you how to dynamically add ToolStripMenuItems to a MenuStrip. If you know how to do that, I encourage you to read the article anyway. After the half—it is football season after all—the dynamic menu is added to a Command class, making the use of this technique a matter of importing the command class in any future application. It's also cooler and better housekeeping. If you already know how to do all of these things, I won't mind if you wait for the next article. ~wink~
Creating a Recent Submenu with the MenuStrip
Many applications have dynamic menus. Even Visual Studio tracks open files and projects in "recents" menus. The basic behavior is to open a file and create a dynamic sub-menu with the name of the file and add that menu to the MenuStrip. The code in Listing 1 is plain old vanilla VB.NET code behind a simple form with a File menu, an Open sub-menu, and a Recent Files sub-menu (see Figure 1). When you Open the file, it is read into a TextBox and a dynamic menu is created.
Figure 1: A simple text browser containing a multi-line TextBox and a MenuStrip.
Listing 1: Click Open, select a file, and create a dynamic sub-menu off of the Recent Files menu.
Imports System.IO Imports System.Diagnostics Public Class Form1 Private Sub OpenToolStripMenuItem_Click(ByVal sender _ As System.Object, _ ByVal e As System.EventArgs) _ Handles OpenToolStripMenuItem.Click OpenFileDialog1.InitialDirectory = "C:\TEMP" OpenFileDialog1.Filter = "Text Files (*.txt)|*.txt" OpenFileDialog1.Title = "Open File" OpenFileDialog1.ShowDialog() End Sub Private Function OpenFile(ByVal filename As String) As Integer Debug.Assert(File.Exists(filename)) Dim data As String = File.ReadAllText(filename) TextBox1.Text = data.Replace("\r", " ") Return 0 End Function Private Sub RecentsClick(ByVal sender As Object, _ ByVal e As EventArgs) If (TypeOf sender Is ToolStripMenuItem = False) Then Return Dim menu As ToolStripMenuItem = CType(sender, _ ToolStripMenuItem) OpenFile(menu.Text) End Sub Private Sub AddToRecents(ByVal filename As String) Dim menu As ToolStripMenuItem = _ RecentFilesToolStripMenuItem.DropDownItems.Add(filename) AddHandler menu.Click, AddressOf RecentsClick End Sub Private Sub OpenFileDialog1_FileOk(ByVal sender As System.Object, _ ByVal e As System.ComponentModel.CancelEventArgs) _ Handles OpenFileDialog1.FileOk Dim dialog As OpenFileDialog = DirectCast(sender, OpenFileDialog) OpenFile(dialog.FileName) AddToRecents(dialog.FileName) End Sub End Class
In the code, the OpenToolStripMenuItem_Click event responds when you click the File|Open menu. The click event sets some properties of an OpenFileDialog control and calls ShowDialog. You can check the return value of ShowDialog or handle FileOK in an event of the OpenFileDialog, which is show as the last method of Listing 1.
If the user clicks OK, OpenFileDialog1_FileOK runs. FileOK casts the sender using DirectCast to an OpenFileDialog object (rather than using the OpenFileDialog1 object directly), making the code more portable. The OpenFile method is called and AddToRecents is called.
OpenFile uses basic File I/O to read the text file with the shared method File.ReadAllText. String.Replace strips the carriage return characters. (You can skip that line if you like.) Finally, the text is put in the TextBox.
Note: By the way, when you write code this straightforward using named methods like OpenFile and AddToRecents, you don't need comments. Tell them I said it was OK.
AddToRecents adds the filename string as the text for the dynamic menu. Notice that you have to refer to the parent menu's DropDownItems property—not as intuitive as just Add—and call DropDownItems.Add method. Add returns the new ToolStripMenuItem, and you attach an event handler.
The last bit is the RecentsClick that responds when you click on the new dynamic menu. The code does a simple sanity check to see whether the sender is a ToolStripMenuItem. (You could safely skip that check here because your code is doing the wire-up of the event, so you know it's a ToolStripMenuItem. By the way, this form of the If conditional is called a sentinel; it's shorter and sweeter than an If Then End If, but does result in multiple exit points.) Finally, you reuse OpenFile, which is exactly why you use named methods rather than writing all code directly in the event handler itself.
Of course, you don't have to agree with me on style, but my style for me is a very fast way to program. It reduces superfluous comments, and promotes reuse.
Refactoring the Code into a Command Class
Now, in all but trivial samples I might start with code like Listing 1 (probably not, but I might). What I would end up with, though, is a Command class for the Open File with Recents behavior embedded in it. Why? Good question. Encapsulating this behavior in a class means it's portable at the class level, it keeps my Form code simpler, and it promotes reuse in other ways. For example, with a Command class I could add new ways to invoke the behavior like through a button, a wizard, auto-pilot mode, or perhaps as a controllable API (think OLE Automation-type control).
Listing 2 shows the Form code Refactored. The code, combined with Listing 3, does the same thing but now most of the behaviors are moved to an external command class.
Listing 2: Form1 revised to use a Command class.
Imports System.IO Imports System.Diagnostics Public Class Form1 Private _openFileCommand As OpenFileCommand Private Sub OpenToolStripMenuItem_Click(ByVal sender _ As System.Object, _ ByVal e As System.EventArgs) Handles OpenToolStripMenuItem.Click _openFileCommand._Do() End Sub Private Function OpenFile(ByVal filename As String) As Integer Debug.Assert(File.Exists(filename)) Dim data As String = File.ReadAllText(filename) TextBox1.Text = data.Replace("\r", " ") Return 0 End Function Private Sub FileOK(ByVal sender As Object, ByVal e As FileEventArgs) OpenFile(e.FileName) End Sub Private Sub Form1_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles MyBase.Load _openFileCommand = New OpenFileCommand() AddHandler _openFileCommand.FileOKClick, AddressOf FileOK AddHandler _openFileCommand.RecentsClicked, AddressOf FileOK _openFileCommand.RecentsMenu = RecentFilesToolStripMenuItem End Sub End Class
The two things that happen in the new form are the initialization of the command object with event wire-ups and the OpenFile method and that is it. Listing 3 contains the command class.