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Building an Extendable Windows Forms Application

The final step is to create a new Windows Forms application (MyExtendableApp) that allows the

user to select a snap-in using a standard Windows Open dialog box. Next, set a reference to the

CommonSnappableTypes.dll assembly, but not the CSharpSnapIn.dll or VbNetSnapIn.dll code

libraries. Remember that the whole goal of this application is to make use of late binding and

reflection to determine the “snapability” of independent binaries created by third-party vendors.

Again, I won’t bother to examine all the details of Windows Forms development at this point

in the text. However, assuming you have placed a MenuStrip component onto the forms designer,

define a topmost menu item named File that provides a single submenu named Snap In Module. As

well, the main window will contain a ListBox type (which I renamed as lstLoadedSnapIns) that will

be used to display the names of each snap-in loaded by the user. Figure 16-11 shows the final GUI.

Figure 16-11.

GUI for MyExtendableApp

The code that handles the Click event for the File ➤ Snap In Module menu item (which may

be created simply by double-clicking the menu item from the design-time editor) displays a File

Open dialog box and extracts the path to the selected file. Assuming the user did not select the

CommonSnappableTypes.dll assembly (as this is purely infrastructure), the path is then sent into a

helper function named LoadExternalModule() for processing. This method will return false when

it is unable to find a class implementing IAppFunctionality:

private void snapInModuleToolStripMenuItem_Click(object sender,

EventArgs e)

{