How To Swap Top-Level Forms

Monday Jul 12th 2004 by Marc Clifton
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Learn to implement a specialized ApplicationContext that allows the application to close the current top-level form and replace it with a different one.

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

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