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Next, the application needs to be modified to pass the parameters along to ExampleForm. Listing 3 shows how this can be done for new instances by overriding the OnCreateMainForm() method in the base class. This method copies the command-line arguments to a local variable, creates a new ExampleForm, then calls the Activate method overload that we added in order to pass the parameters to it.

If the application is already running, however, OnCreateMainForm() won't be called, so another method is required. When Windows attempts to start an application that is already running, an event named "StartupNextInstance" fires. Listing 4 is the code for a custom event handler that does essentially the same thing as the OnCreateMain-Form() override, but for a running application.

Finally, Listing 5 shows the default constructor for Program, where the StartupNextInstance event handler is wired up, and where we set Program to run as one instance. If you don't set the "IsSingleInstance" property to "True," the program will launch a new form each time it is activated. This completes the demo application.

ClickOnce Configuration
Next, the application needs to be configured for ClickOnce publishing. Right click on the project in the Solution Explorer pane and choose Properties. Select the Publish tab and click the Options button. Click on the checkbox labeled, "Allow URL parameters to be passed to application." That's all that needs to be done - all other ClickOnce options can be left at their default settings.

Publishing the Application
In Visual Studio, click on the link labeled "Publish ClickOnceWindowsFormsApplication" in the Build menu. If you have IIS running on your machine, you can publish to the default localhost. If not, you will need to publish the application to an IIS server. After choosing the location, click the button labeled "Next >."

The next form lets you choose whether or not a start menu shortcut will be created for this application. It does not matter for our purposes here, the example will work either way. Click "Next >" to the confirmation screen, then "Finish" to publish the application to the server.

Web Application Links
The last piece is to create the hyperlinks in your Web page or Web application. This should be fairly straightforward. Create a simple HTML page with two anchors in it, one to pass a single parameter and another to pass two parameters. Listing 6 shows the code for the HTML file. This particular HTML file can be run in any browser, and does not need to be served up by IIS.

Now click on the first hyperlink and see what happens. ClickOnce should prompt you to download and install the application. When installation is complete, the form will open displaying the parameter values from the hyperlink If you switch back to the HTML page in the browser and click on the other hyperlink, a ClickOnce window will display briefly, checking for new updates, then the Windows Form application will gain focus and display the parameter values from the second querystring.

Conclusion
Naturally, this process is extremely simplified. It does not address the important issues of authentication and authorization, for example, which need to be addressed for any application using this pattern. Also, this solution provides only one-way communication, with messages originating at the Web application and being received by the Windows Forms application. If the Web application needs to respond to changes, refreshes will need to be initiated.

However, this pattern provides a way to launch or refresh a Windows Forms application running on a user's desktop with data from a legacy Web application, something that has been extremely difficult and risky in the past. Expanding on this pattern, systems can be engineered so that complex run-time data is transmitted quickly from Web to windows, thereby integrating the applications and increasing user productivity.

Resources

• ClickOnce FAQ: http://msdn.microsoft.com/smartclient/community/wffaq/wf20.aspx#shnwqp2b

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