How To Swap Top-Level Forms

The Problem

I recently had a requirement where I needed to change the top-level form. The problem is, the ApplicationContext hooks the form's Close event, so that when you use the Close method to close the current form, the application exits. This is no good!

The Solution

The solution is to implement a specialized ApplicationContext that allows the application to close the current top-level form and replace it with a different one. The implementation is quite straightforward:

using System;
using System.Windows.Forms;

namespace ApplicationContextDemo
{
  public class MainFormManager : ApplicationContext
  {
    protected bool exitAppOnClose;

    public Form CurrentForm
    {
      get {return MainForm;}
      set
      {
        if (MainForm != null)
        {
          // close the current form, but don't exit the application
          exitAppOnClose=false;
          MainForm.Close();
          exitAppOnClose=true;
        }
        // switch to the new form
        MainForm=value;
        MainForm.Show();
      }
    }

    public MainFormManager()
    {
      exitAppOnClose=true;
    }

    // when a form is closed, don't exit the application if this is a swap.
    protected override void OnMainFormClosed(object sender, EventArgs e)
    {
      if (exitAppOnClose)
      {
        base.OnMainFormClosed(sender, e);
      }
    }
  }
}

In the above code, assigning the CurrentForm property to a form blocks the OnMainFormClosed method from its usual operation, which is to call ExitThreadCore.

Your main application would look something like this:

using System;
using System.Windows.Forms;

namespace ApplicationContextDemo
{
  public class App
  {
    private static MainFormManager mainFormManager;

    public static MainFormManager MainFormManager
    {
      get {return mainFormManager;}
    }

    public App()
    {
      mainFormManager=new MainFormManager();

      mainFormManager.CurrentForm=new Form1();
      Application.Run(mainFormManager);
    }

    [STAThread]
    static void Main() 
    {
      new App();
    }
  }
}

The above code instantiates the first form, and instead of the typical Application.Run(new Form1) method, the specialized application context is provided instead.

To swap a form, simply assign a new form to the CurrentForm property; for example:

App.MainFormManager.CurrentForm=new Form1();

I've provided a demonstration application that illustrates swapping three different top-level forms.

Conclusion

Simple, but useful when you need this kind of functionality!

References

Use the ApplicationContext Class to Fully Encapsulate Splash Screen Functionality



Downloads

Comments

  • Nice tip, Marc!

    Posted by Tom Archer on 07/14/2004 12:27am

    Definitely a '5'

    Reply
Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • Where the business performance of their mobile app portfolios are concerned, most companies are flying blind. While traditional application portfolios are held to all kinds of ROI measure, the investment plan for mobile apps -- increasingly the more crucial bet -- is made by guesswork and dart-throwing. This interactive e-book investigates how mobile is driving the need for app and portfolio measures unlike any we saw in the days of web. Good mobile analytics must deliver leading indicators of user experience …

  • Live Event Date: July 30, 2014 @ 11:00 a.m. ET / 8:00 a.m. PT You may already know about some of the benefits of Bluemix, IBM's open platform for developing and deploying mobile and web applications. Check out this upcoming eSeminar that focuses on building an Android application using the MobileData service, with a walk-through of the real process and workflow used to build and link the MobileData service within your application. Join IBM's subject matter experts as they show you the way to build a base …

Most Popular Programming Stories

More for Developers

Latest Developer Headlines

RSS Feeds