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Simple Singleton Forms

, 15 Sep 2003
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Simple way to work with singleton forms

Introduction

When I started with VB.NET a few years ago, one of the things I missed was how to deal with forms where you only want one instance open at the same time. I read a lot of articles concerning Singleton forms etc, which was all complicated and the result was not easy to use. At the end I found a very simple way to do this. Maybe too easy !

Solution

The trick is realy very simple. I just make a class with Public Shared variables with as type the Forms itself.

Public Class clsGlobals
#Region "Singleton Forms"
    Public Shared frmStockSelection As frmStockSelection
    Public Shared frmSettings As frmSettings
#End Region
End Class

These public shared variables will be usable eveywhere in your code. You do not create a new instance from this class. Call the variable by using the class name as shown in the next bit of code.

The code to open the Singleton Form is also very simple. The only thing to do is to check if the variable is not nothing, if not create a new instance. You have to use Focus to get your form to the foreground in case an instance was already existing but maybe hidden behind other windows.

If clsGlobals.frmSettings Is Nothing Then
    clsGlobals.frmSettings = New frmSettings
End If
clsGlobals.frmSettings.Show()
clsGlobals.frmSettings.Focus()

One more thing we have to do to make this work, the variable must be set back to nothing in case we close the form. This we can do in the Dispose sub of the form. After Dispose we can safely set the Form to nothing.

'Form overrides dispose to clean up the component list.
Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
    If disposing Then
        If Not (components Is Nothing) Then
            components.Dispose()
        End If
    End If
    MyBase.Dispose(disposing)
    clsGlobals.frmSettings = Nothing
End Sub

Another option is to keep the Form open in memory. This is very handy for Forms, like a little calculator for example where you don't want to start with a clear form every time.
The way to do this is to avoid the close the form and hide it. e.Cancel = True will stop the form from closing.

Private Sub frm_Closing(ByVal sender As Object, _
  ByVal e As System.ComponentModel.CancelEventArgs) Handles MyBase.Closing
    Me.Hide()
    e.Cancel = True
End Sub

Maybe the purist readers will not 100% agree with this approach, but for me it works very well. 

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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About the Author

Peter Verijke
Founder Computech bvba
Belgium Belgium
I´m a freelance ICT consultant and Technical Project Manager for the last 15 years already.
Before that, I was a Software Developer for the Pharmaceutical and Petrochemical industry.
I developed a very wide knowledge in the ICT world due to my intrest.
This includes Networking (Lan, Wan), Routing Switching Bridging, etc...
Windows environment, which has no secrets for me.
Developing: To many languages to mension here.
Of course, the latest one on the list is dot net.

Comments and Discussions

 
GeneralAlso check this out (MDI forms)! PinmemberSire40414-Dec-04 8:56 
GeneralSingletons PinmemberAdam Byrne22-Sep-03 23:11 
GeneralRe: Singletons Pinmemberashleyb18-Jan-04 11:33 
GeneralRe: Singletons PinmemberAdam Byrne19-Jan-04 6:31 
GeneralRe: Singletons Pinmemberashleyb19-Jan-04 8:18 
GeneralRe: Singletons PinmemberAdam Byrne20-Jan-04 0:20 
Yeah, it's not very efficient at all. Your best bet would probably be to leave the form classes as they are (don't singletonify them) and just keep track of their instanciation.
 
Hmm... Actually, what you could do is add a static property to the form class that is used to get the single instance of the form. This property would test the IsDisposed attribute of the class. Here's a thread-safe "Singleton" form...
 
	public class Singleton : System.Windows.Forms.Form
	{
		private System.ComponentModel.Container components = null;
 
		static Singleton instance = null;
		static readonly object padlock = new object();
 
		public static Singleton Instance
		{
			get
			{
				if (instance == null || instance.IsDisposed)
				{
					lock (padlock)
					{
						if (instance == null || instance.IsDisposed)
							instance = new Singleton();
					}
				}
				return instance;
			}
		}
 
		private Singleton()
		{
			InitializeComponent();
		}
 
		protected override void Dispose( bool disposing )
		{
			if( disposing )
			{
				if(components != null)
				{
					components.Dispose();
				}
			}
			base.Dispose( disposing );
		}
 

		#region Windows Form Designer generated code
 
		private void InitializeComponent()
		{
			this.components = new System.ComponentModel.Container();
			this.Size = new System.Drawing.Size(300,300);
			this.Text = "Singleton";
		}
 
		#endregion
	}
 
But, you see, it's not really a Singleton, because if the current instance is disposed, it will be reinstanciated when you access it. If you make sure you refer to the instance only with Singleton.Instance, you'd get away with it. But if you do something like:
 
Singleton s1 = Singleton.Instance;
s1.Dispose();
 
Singleton s2 = Singleton.Instance;
// now you have two seperate objects on the heap: s1, and s2
 
you have two Singleton objects, which sort of defeats the purpose (even though only more than one instance can exist iff only one of the instances is not disposed and the rest are disposed, so only one useful object exists).
 
So, with a form like the above, if you refer to the instance only by Singleton.Instance, you should be laughing.
 
For more information on implementing thread-safe Singleton classes, go to http://www.yoda.arachsys.com/csharp/singleton.html[^]
 
Hope this helps.
 
-adam
GeneralRe: Singletons Pinmemberashleyb20-Jan-04 9:23 
AnswerRe: Singletons Pinmembervdewisme6-May-09 23:11 
GeneralRe: Singletons PinmemberPeter Verijke7-May-09 11:00 
GeneralRe: Singletons Pinmembervdewisme7-May-09 21:46 

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