Overview

The singleton design pattern is focused on having one—and only one—instance of an object. It instills encapsulated control of the object from a common place. Most commonly, singletons don't allow any parameters to be specified when creating the instance - as otherwise a second request for an instance but with a different parameter could be problematic! The class implemented using this pattern is responsible for keeping track of its sole instance rather than relying on global variables to single instances of objects.

This article deals only with the situation where no parameters are required. The following sample application depicts how to implement the singleton design pattern in Windows Application using C#.

Software Requirements

Microsoft Visual C# 2005 Express Edition (or better)

Create the Child form

Add a new Windows Application project (with name as SingletonDesignPattern) and to it add a form as frmchild and paste the following code in it as shown below:

Listing 1

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
 
namespace SingletonDesignPattern
{
  public partial class frmChild: Form
  {
    private static int iCount = 0;
 
    public frmChild()
    {
      InitializeComponent();
      iCount = iCount + 1;
    }
 
    public static int Count
    {
      get
      {
        return iCount;
      }
    }
  }
}

In listing 1, firstly a static variable iCount is declared which keeps the count of the number of instances of the child form. Then in the form constructor, the iCount variable is incremented every time an instance of the form is created.

Next, through the Count Public property, the count of the number of instances of the form is returned to the calling form.

Finally, in the frmChild_FormClosing event, the iCount variable is set to zero when the frmchild dialog is closed. This is done to allow the user to open the frmchild dialog again from the Main form if the frmchild dialog is not open.

Create the Parent form

Add one more form called frmParent to the above windows application project. To this form, add a Menustrip control with the name menuToolStripMenuItem. Add a menuitem with name of ChildForm to the Menustrip control. The screen should look as below:

Figure 1

Paste the following code in the click event of the ChildForm MenuItem.

Listing 2

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
 
namespace SingletonDesignPattern
{
  public partial class frmParent: Form
  {
    public frmParent()
    {
      InitializeComponent();
    }
 
    private void childFormToolStripMenuItem_Click(objectsender, EventArgs e)
    {
      frmChild objform = new frmChild();
 
      if (frmChild.Count == 1)
        objform.Show();
    }
  }
}

In Listing 2, first an instance of frmChild is created and then it is checked whether this is the first instance of frmChild.  If yes, it displays the frmChild dialog. If there is already an instance of thefrmchild dialog open, other instances of frmChild dialog will not be opened.

Now run the Main form and click the childform menuitem upon which frmChild dialog is displayed as shown below.

Figure 2

Again click the childform menuitem many times, but this time additional instances of the frmChild dialog will be not opened. Notice that at any one time, we are able to open Childform dialog only once from the Main form.

Downloads

[Download Sample]

Conclusion

The Singleton design pattern has proven to be a useful pattern in many programs. Now, the code in this code snippet shows how to implement that same pattern in a .NET Windows Application. There are various different ways of implementing the singleton pattern in C#.

Resources

Session Based Singleton

Singleton Design Pattern at dofactory.com