Can I make a Form a singleton?

Question

I have a Visual C# 2010 application, and it has one main form called MainWnd with other tool windows and dialogs. I want the other tool windows to be able to 'talk' to the main form, and call its methods. But that requires an instance of MainWnd, and since there will only be one of these forms created at any given time there is no reason while I should enumerate through all instances of MainWnd or look for the first one. So I want my main application form MainWnd to be a singleton so other windows can easily call code from it.

Here is the code of the main form that I would like to make a singleton:

using System;
using System.ComponentModel;
using System.Windows.Forms;

namespace MyLittleApp
{
    public partial class MainWnd : Form
    {
        public MainWnd()
        {
            InitializeComponent();
        }

        public void SayHello()
        {
            MessageBox.Show("Hello World!");

            // In reality, code that manipulates controls on the form
            // would go here. So this method cannot simply be made static.
        }
    }
}

I am looking to be able to call SayHello() from another form, simply by writing:

MainWnd.SayHello();

How could I accomplish this?

Can you not have it as a static public method and then do what you are saying ? — Ravi Y, Jan 14 '13 at 05:26
Since it is called MainWnd i expect, that the form opens the other forms. In this case you could use an event on your sub-forms, and register an event-receiver from the MainWnd there. — TGlatzer, Jan 14 '13 at 05:27
@ryadavilli As the comment in the code I posted says, the method manipulates objects in the form, so it must have an instance, therefor it can't just simply be static. — Brandon Miller, Jan 14 '13 at 05:30

Chamila Chulatunga · Accepted Answer · 2013-01-14T05:34:52.877

You could probably find a way to make the main window a singleton, however that's not the best way to achieve the outcome you want, nor is it really an appropriate situation in which to use the singleton pattern.

If all of the other tool windows/ dialogs are encapsulated within the main window, then a much better pattern to use for communication would be events.

Have the inner windows/dialogs raise events to represent a 'request' for the main window to do something. Have the main window subscribe to these events, and do the work via the event handlers.

By avoiding the singleton approach, you avoid the difficulties of testing the singleton, as well as avoiding extensive explicit circular references, where not only does the main window have references to the encapsulated windows/dialogs, but they in turn have explicit references back to the main window.

+1 Definitely way better than what I had in mind. Thank you for opening my eyes to a much better design pattern :) — Brandon Miller, Jan 14 '13 at 05:40

score 1 · Answer 2 · answered Jan 14 '13 at 05:31

See below.

using System;
using System.ComponentModel;
using System.Windows.Forms;

namespace MyLittleApp
{
    public partial class MainWnd : Form
    {
       public static MainWnd Instance;
        public MainWnd()
        {
            Instance = this;
            InitializeComponent();
        }

        public void SayHello()
        {
            MessageBox.Show("Hello World!");

            // In reality, code that manipulates controls on the form
            // would go here. So this method cannot simply be made static.
        }
    }
}

You can now use it anywhere in your code by calling MainWnd.Instance All its members are also available to the instance.

score 0 · Answer 3 · answered Jan 14 '13 at 05:26

You can certainly do this.

 public MainWnd Instance = new MainWnd();

Then access as MainWnd.Instance.SayHello().

Replace following calls

MainWind instance = new MainWnd();

To

 MainWnd instance = MainWnd.Instance;

I am not sure how Visual Studio designer would react after making the constructor as private though. But if it does not allow, it will be Visual Studio issue, rather than language/compiler issue.

Can I make a Form a singleton?

3 Answers3

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