Subclassed System Menu

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Introduction

The reason for this article is to show a method of subclassing an application's system menu, so that extra menu items can be added and click events can be easily handled. This is my first article submitted to CodeProject and all suggestions, criticisms etc. are welcome.

Background

This all came about after working on an application that didn't have a standard UI, e.g. menus. I needed to show an About dialog for the application, but I didn't have the usual menus and there didn't seem a natural place from which to launch the dialog, so I created this. I have tried to make it as generic as possible so that it adds a menu separator, then a user defined menu item.

When the object is created, it will retrieve the handle for the parent window and append two new menu items, the first being a separator and the second being "About...". Once this has been completed, an overridden WndProc is used to receive the WM_SYSCOMMAND (&H112) message. If the message is invoked by the user selecting the "About..." menu item, the LaunchDialog event is raised.

Approach

The object inherits from System.Windows.Forms.NativeWindow to receive the window messages. Also the IDisposible interface is implemented, so we can release our reference to the window handle when the object is destroyed or goes out of scope.

Using the code

The implementation of the object is straight forward and uses a couple of Win32 API calls, e.g. GetSystemMenu and AppendMenu. The object uses standard VB.NET events to notify the parent form that the new system menu item has been clicked. There are many different ways that the event notifications can be implemented including using an interface. There are many other ways but this way does the job.

The code is very easy to use, you just need to declare a module level variable using WithEvents.

Private WithEvents mobjSubclassedSystemMenu As SubclassedSystemMenu

Then instantiate the declared object, passing in two parameters, the first is the parent window handle and the second being the text for the new menu item.

mobjSubclassedSystemMenu = New _ 
  SubclassedSystemMenu(Me.Handle.ToInt32, "&About...")

Next you need to implement the SubclassedSystemMenu event (LaunchDialog).

Private Sub mobjSubclassedSystemMenu_LaunchDialog() _
                         Handles mobjSubclassedSystemMenu.LaunchDialog
    Dim frmNew As New frmAbout

    frmNew.ShowDialog(Me)
End Sub

And that's it really!

History

01-31-2004: Initial version.

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